Methods and Apparatus for Implementing a High Speed Serial Link and Serial Data Bus

1. Field of the Invention

The invention relates to methods and apparatus for serial transfer of binary data among digital devices. More particularly, the invention relates to a self-timed serial bus having efficient software implementation. As used herein, "serial link" means a point-to-point data link and "serial bus" means either a point-to-point or multi-user serial bus.

2. State of the Art

Known serial communications bus techniques are all based on a clock pulse data scheme. These techniques may provide clock and data signals on separate lines or provide a data signal which is encoded with clock information. Alternatively, data may be transmitted asynchronously with a clock signal being generated locally by the receiver. All techniques, however, require that the receiver view data at a specific instant in time, i.e. at the clock edge.

Many microprocessors are provided with integral (dedicated) serial data ports. However, in many applications, additional serial ports are required. The additional ports can be effected through additional external circuitry or the logic of the port can be implemented in the processor using software and any free I/O pin(s) of the processor. While the reading of serial data at specific instances is not a significant problem for dedicated devices, it poses difficult problems for software implementation in the logic of a microprocessor.

Virtually all microprocessors are required to service a number of independent tasks in an apparently concurrent manner. This is sometimes referred to as multi-tasking. In reality, a microprocessor can only operate on one task at a time and multi-tasking is accomplished by programming the microprocessor to switch between or among different tasks very rapidly so that there is an appearance of concurrent task processing. The switching among different tasks is most often in response to external events (e.g. the press of a key on a keyboard) which are random in nature. Therefore, it is problematic to program the microprocessor so that it is always available at the instance of a clock transition to read serially transmitted data. This problem is usually solved in one of two ways: through the use of polling, or through the use of interrupts.

In polling, the microprocessor repeatedly reads an I/O pin through which the clock signal is received to determine if it has changed state. Since the data must be read at an exact instance of clock signal transition, the microprocessor must poll the I/O pin at a rate which is significantly faster than the frequency of the clock signal. If the microprocessor misses a clock transition data will be lost. Serial communications clock signals are typically in the tens or hundreds of kilohertz. Therefore, the microprocessor must devote a significant number of its cycles to the task of polling the clock signal.

The use of interrupts is similar to polling but utilizes additional hardware (e.g. a separate interrupt pin, internal timers, etc.) in order to enable more efficient software. While the use of interrupts relieves the processor of some of the time wasted during polling, it still interferes with the fundamental operation of the processor, and considerable time must be spent by programmers ensuring that the scheduling of interrupts does not cause other other parts of the software to malfunction. As the rate of serial data transfer increases, the challenge in scheduling interrupts becomes progressively more difficult.

Therefore, while soft implementation of a serial port in a microprocessor is known and is widely used, the hard timing of serial data communications complicates the design of system software and places unwanted constraints on the processor.

It is therefore an object of the invention to provide methods and apparatus for implementing a self-timed serial link and serial bus.

It is also an object of the invention to provide methods and apparatus for implementing a serial link and serial bus which has an efficient software implementation.

It is another object of the invention to provide methods and apparatus for implementing a serial link and serial bus which has a robust feedback mechanism to prevent data loss.

It is also an object of the invention to provide methods and apparatus for implementing a serial link and serial bus which is suitable for high speed data transfer among digital devices.

It is another object of the invention to provide methods and apparatus for implementing a serial link and serial bus which does not require substantial hardware.

It is still another object of the invention to provide methods and apparatus for implementing a serial link and serial bus which utilizes a small number of lines and processor pins.

It is yet another object of the invention to provide methods and apparatus for implementing a serial link and serial bus which does not require interrupts.

It is also an object of the invention to provide methods and apparatus for implementing a serial link and serial bus which simplifies scheduling of microprocessor code for handling the communications port(s).

It is still another object of the invention to provide methods and apparatus for implementing a serial link and serial bus which can be implemented with the logic of a microprocessor using short code and any port pin(s).

It is also an object of the invention to provide methods and apparatus for implementing a serial link and serial bus which can be implemented with the logic of a microprocessor using code which is non-intrusive relative to system software.

In accord with these objects which will be discussed in detail below, the methods and apparatus of the present invention provide a point-to-point serial communications bus using one, two, or three lines, and a multi-user serial communications bus using three lines. All provided buses are easily implemented in the system software of virtually any microprocessor and make use of virtually any one, two or three available pins of the processor. The point-to-point bus allows two processors (bus users) to communicate via one, two, or three lines coupled to any I/O pins of each processor. In the two-line embodiment, the bus has four possible states. The transmitting processor transmits a binary ZERO by asserting line 0 (preferably by taking it low) and holding the line asserted until the receiving processor acknowledges the transmission by asserting line 1 (preferably by taking it low). When the transmitting processor sees the acknowledgment (i.e. that line 1 is low), line 0 is de-asserted. When the receiver sees that line 0 is no longer asserted, it de-asserts line 1. Similarly, a transmitting processor sends a binary ONE by asserting line 1 and holding the line asserted until the receiving processor acknowledges the transmission by asserting line 0. When the transmitting processor sees the acknowledgment, line 1 is de-asserted. When the receiver sees that line 1 is no longer asserted, it de-asserts line 0. In other words, when transmitting a ZERO, line 0 is a data line and line 1 is an acknowledge line. When transmitting a ONE, line 1 is a data line and line 0 is and acknowledge line. Therefore, every transmitted bit is accompanied by a full handshake in which the receipt of the bit is acknowledged and the acknowledgment is acknowledged, i.e. each state of the bus is held until the full handshake is completed. Moreover, if either the transmitting processor or the receiving processor needs to perform an interrupt service routine during a transfer of data, no data will be lost. Even with this frequent handshaking protocol, the simplicity of the software implementation of the bus presently allows for data transfer at rates up to approximately 200 Kbit/sec using standard 8-bit microprocessors such as the 8051, 6805, or PIC. Higher speeds are possible with high speed microprocessors. The point-to-point bus according to the invention may also be implemented using a single line where a data transmission is effected by a change in voltage and an acknowledge transmission is effected by a change in current. In either of the point-to-point bus embodiments, an additional control line may be used to signal the start of a data frame.

The multi-user bus allows an unlimited number of processors to communicate via three lines coupled to any three I/O pins of each processor. Since the multi-user bus uses three lines, the bus has eight possible states. Six states are used for data transmission. One state is used for a bus reset. One state is reserved for future use, but is recognized by software as a second reset condition. The initial state of the bus has line 0 and line 1 (data lines D0 and D1, respectively) unasserted (pulled high) and line 2 (acknowledge line ACK) asserted (pulled low) by all bus users. A transmitting processor transmits a binary ZERO by asserting line 0 (D0), i.e. by pulling line 0 low. When a receiver senses that line 0 (D0) is low, the receiver also asserts (pulls low) line 0 (D0) and releases line 2 (ACK). Since line 2 (ACK) is being pulled low by all bus users during the initial state of the bus, line 2 will not go high until all users have released line 2, i.e. when all bus users acknowledge receipt of the transmitted ZERO. When all bus users release line 2, the function of all the lines is "positively" rotated so that line 0 (now low) becomes the acknowledge (ACK) line, line 1 becomes the ZERO data line (D0), and line 2 (now high) becomes the ONE data line (D1). Further transmissions of binary ZEROS result in the same behavior of the D0, D1, and ACK lines followed by further rotation in this positive direction. Transmission of a binary ONE is effected by asserting the D1 line (whether that be physical line 0, 1, or 2), and acknowledgement from receivers is effected by asserting the D1 line and releasing the ACK line. When all receivers have released the ACK line, the function of all physical lines is rotated in the "negative" direction.

The methods of the invention include providing each processor with communications software by which data is transmitted and received one bit at a time by asserting one or more of the lines of the bus according to a fixed protocol.

Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures.

FIG. 1 is a schematic block diagram of a pair of microprocessors coupled to each other by a point-to-point two line serial bus according to the invention;

FIG. 1a is schematic block diagram of a typical port pin circuit;

FIG. 2 is a timing diagram of the states of the point-top-point bus of FIG. 1;

FIG. 3 is a partial schematic block diagram of a pair of digital devices coupled to each other by a point-to-point one line serial bus according to the invention;

FIG. 4 is a schematic block diagram of a pair of microprocessors coupled to each other by a point-to-point three line serial bus according to the invention;

FIG. 5 is a timing diagram of a frame start sequence using the point-to-point bus of FIG. 4;

FIG. 6 is a schematic block diagram of four microprocessors coupled to a multi-user serial bus according to the invention;

FIG. 7 is a timing diagram of the states of the multi-user bus of FIG. 6; and

FIG. 8 is a state diagram of stable and meta-stable states of the multi-user bus of FIG. 6.

Referring now to FIG. 1, a first embodiment of a point-to-point serial bus 10 according to the invention includes a first communications line 12 ("line 0") and a second communications line 14 ("line 1"). A pair of microprocessors 16, 18 are coupled to each other via the two-line serial bus 10. More specifically, the fist and second communications lines 12, 14 are coupled to respective I/O pins 20, 22 of microprocessor 16 and to respective I/O pins 24, 26 of microprocessor 18. As will be appreciated by those skilled in the art from the following description, any two I/O pins of the microprocessors 16, 18 may be used to couple the processors to the bus 10. As seen in FIG. 1a, a microprocessor I/O pin, e.g. 20, is typically coupled through a current source I to a positive voltage rail +V.sub.DD and through an open drain transistor 20a to ground GND. The pin 20 is also coupled to the D-input of a D-latch 20b which is clocked by the clock CLK of the microprocessor. The state (high or low) of the pin 20 is sensed and latched by the latch 20b and provided at the Q-output of the latch for use by the logic of the processor. The state of the pin 20 is pulled low when a voltage is applied to the gate G of the transistor 20a and is released and pulled high when the voltage is removed from the gate. The pin 20, therefore, functions as a bit-wide bidirectional I/O pore.

Turning now to FIGS. 1 and 2, according to the methods of the invention, the initial state of the bus (e.g. at time t0) is such that both line 0 and line 1 are "unasserted". Preferably, both line 0 and line 1 are normally pulled "high" by a voltage source (not shown) when unasserted. Either processor 16, 18 transmits a ZERO bit or a ONE bit by asserting (pulling low) line 0 or line 1, respectively via an open drain transistor at the I/O pin of the processor. For example, as shown at time t1 in FIG. 2, a ZERO bit is transmitted by pulling line 0 low. In response to a bit transmission such as shown at t1, the receiving processor pulls the remaining unasserted line low. For example, in response to the asserted line 0, the receiving processor asserts line 1 at t2. This acknowledges receipt of the transmitted ZERO bit. Upon acknowledgement by the receiving processor, the transmitting processor de-asserts line 0, as shown for example at time t3 in FIG. 2. This acknowledges receipt of the acknowledgement. Upon de-assertion of line 0 by the transmitting processor, the receiving processor deasserts line 1, as shown for example at t4. The transmission of a ONE bit is effected by asserting line 1. For example, as shown at time t5 in FIG. 2, a ONE bit is transmitted by pulling line 1 low. In response to the asserted line 1, the receiving processor asserts line 0 at t6. This acknowledges receipt of the transmitted ONE bit. Upon acknowledgement by the receiving processor, the transmitting processor de-asserts line 1, as shown for example at time t7 in FIG. 2. This acknowledges receipt of the acknowledgement. Upon de-assertion of line 1 by the transmitting processor, the receiving processor deasserts line 0, as shown for example at t8. It will therefore be appreciated that the transmission of each bit of data from one microprocessor to the other is followed by a full handshake in which the receipt of the bit is acknowledged and the acknowledgement is acknowledged before the two lines are returned to their initial state. Those skilled in the art will also appreciate that when a ZERO bit is being transmitted, line 0 acts as a data line and line 1 acts as an acknowledgment line and that when a ONE bit is being transmitted, line 1 acts as a data line and line 0 acts as an acknowledgment line. It will further be appreciated that each state of the bus is maintained until acknowledged. That is, the transmitting processor maintains the assertion of the data line until it receives acknowledgment and the receiving processor maintains the assertion of the acknowledgment line until the data line is de-asserted by the transmitting processor. Therefore, if either the transmitting processor or the receiving processor needs to perform an interrupt service routine during communication, no data will be lost. In addition, software implementation of the above described protocol is simple and efficient.

An example of software (written in the C language) used to implement this method of the invention is shown in listings 1 and 2 below where the code in listing 1 implements the transmission of a byte "c", the code in listing 2 implements the reception of a byte "rxByte", and the code in listing 3 represents a portion of the processor's main task handler.

As seen in Code Listing 1, a byte "c" is parsed into eight bits for individual transmission via line 0 (referred to a DACK0 in the code) or line 1 (referred to as DACK1 in the code). When it is determined by the "if" statement that the bit to be transmitted is a ONE, the line DACK1 is asserted by pulling it low in the DACK1=0 statement. It is held low until the DACK0 line is seen to be pulled low in the "while (DACK0)" statement. DACK1 is then released with the statement DACK1=1. Further execution of the code awaits the return of DACK0 to the unasserted high level as shown by the statement "while (|DACK0)". When it is determined by the "else" statement that the bit to be transmitted is a ZERO, the line DACK0 is asserted by pulling it low in the DACK0=0 statement. It is held low until the DACK1 line is seen to be pulled low in the "while (DACK1)" statement. DACK0 is then released with the statement DACK0=1. Further execution of the code awaits the return of DACK1 to the unasserted high level as shown by the statement "while (|DACK1)". After a bit of the byte "c" is successfully transmitted, the next bit is obtained for transmission via the statement "cc<<=1" until eight bits have been transmitted as specified by the statement "d=8".

As seen in Code Listing 2, the receiver determines whether the transmitted bit is a ZERO or a ONE, by determining if line 0 has been taken low, that determination being made with the "if (|DACK0)" statement. If the bit is a ZERO, a zero bit is shifted into the register rxByte with the "rxByte<<=1" statement and the DACK1 line is asserted in acknowledgment with the "DACK1=0" statement. The acknowledgment is held low until the return of DACK0 to the unasserted high level as shown by the statement "while (|DACK0)". The acknowledgement is then released with the statement "DACK1=1". If the bit is a ONE, a one bit is shifted into the register rxByte with the "rxByte=(rxByte<<1).vertline.1" statement and the DACK0 line is asserted in acknowledgment with the "DACK0=0" statement. The acknowledgment is held low until the return of DACK1 to the unasserted high level as shown by the statement "while (|DACK1)". The acknowledgement is then released with the statement "DACK0=1". The code repeats for eight bits after which the accumulated byte rxByte is returned to the receiving processor.

It will be understood that both Code Listings 1 and 2 will be implemented as callable modules within the main program of each processor. Moreover, in the case of Code Listing 2, the main task loop of the processor program will include periodic monitoring of the condition of lines 0 and 1 as shown in Code Listing 3.

Those skilled in the art will appreciate that the code examples shown in listings 1-3 may be implemented in virtually any programming language and incorporated as part of system software in virtually any microprocessor. Moreover, it will further be appreciated that, in some applications, an application specific integrated circuit (ASIC) can be used rather than a microprocessor to effect the serial communications bus according to the invention. Furthermore, it will be appreciated that the above-described embodiment of the invention provides for point-to-point communication between two devices with a present data transfer rate of up to approximately 200 Kbps; and higher rates can be expected with fatser microprocessors.

The four state bus described above may also be implemented on a single communication line, if desired. FIG. 3 illustrates a point-to-point data link 100 which uses a single communications line 112. A pair of digital devices 116, 118 are coupled to the line 112 by respective I/O ports 120, 124. As shown in FIG. 3, the circuitry supporting the I/O port 120 of device 116 is identical to the circuitry supporting the I/O port 124 of device 118. However, as will be understood from the following, it is not necessary that the circuitry of the devices 116 and 118 be identical. The circuits shown are exemplary and it will also be understood that the devices 116, 118 will also require additional circuitry which is well known to those skilled in the art.

Turning now to FIG. 3, the I/O circuitry of each device 116 (118) includes a relatively high value resistor 128 (130) which couples the respective port 120 (124) to ground. A relatively low value resistor 132 (134) with an associated current sensing comparator 136 (138) is coupled in series between the port 120 (124) and the co-coupled drains of two switching transistors 140, 142 (144, 146). A voltage sensing (window) comparator 148 (150) has one input coupled to the co-coupled drains of the two switching transistors 140, 142 (144, 146) and another input coupled to ground. (Those skilled in the art will appreciate that a window comparatort may actually comprise two comparators and have several outputs. The comparators are shown here schematically for simplicity.) The switching transistor 140 (144) is shown as a p-channel MOSFET transistor and is coupled by its source to a positive voltage +V. The gate of transistor 140 (144) is driven by the output of a NAND gate 152 (154) which has two inputs 152a (DATA), 152b (ENABLE) (154a, 154b). The switching transistor 142 (146) is shown as an n-channel MOSFET transistor and is coupled by its source to a negative voltage -V. The gate of transistor 142 (146) is driven by the output of art AND gate 156 (158) having one inverted input 156a (158a) (DATA) and one non-inverted input 156b (158b) (ENABLE). One of the inputs 152a (154a) of the NAND gate 152 (154) is coupled to the inverted input 156a (158a) of the AND gate 156 (158) and the other input 152b (154b) of the NAND gate 152 (154) is coupled to the non-inverted input 156b (158b) of the AND gate 156 (158). According to the methods of the invention, the DATA inputs 152a, 156a (154a, 158a) of the gates 152, 156 (154, 158) are used to transmit data and acknowledgements while the ENABLE inputs 152b, 156b (154b, 158b) of the gates 152, 156 (154, 158) are used to enable transmission of data and acknowledgements. Additionally, the output of the voltage comparator 148 (150) is used to receive data while the output of the current sensing comparator 136 (138) is used to receive acknowledgments. These outputs are hereinafter referred to as RxDATA and ACK.

The I/O circuitry shown in FIG. 3 operates as follows: Initially, there is no current or voltage on the bus 112. All of the transistors are off and the DATA and ENABLE inputs are LOW. The device 116 transmits a binary ONE by asserting both the ENABLE and DATA inputs HIGH. This results in LOW outputs from both gates 152 and 156 which are applied to the gates of transistors 140, 142 respectively. Since the transistor 140 is a p-channel transistor, the LOW signal applied to its gate turns it on. Since the transistor 142 is an n-channel transistor, the LOW signal applied to its gate turns it off. This results in a signal at pin 120 having positive voltage and relatively low current. The positive voltage is carried by the line 112 to the device 118. The device 118 senses the transmission of a binary ONE when the RxDATA output of the voltage comparator 150 indicates the positive voltage. In response to the receipt of a binary ONE, the device 118 transmits an ACKNOWLEDGEMENT by asserting the ENABLE input of gates 154, 158 HIGH and the DATA input of gates 154, 158 LOW. This results in the turning on of transistor 146 which thereby couples the bus to a negative voltage (-V). This causes a relatively high current (compared to the previous state of the bus) to pass through the bus from +V to -V via transistor 140, resistors 132 and 134, and transistor 146, and this current is sensed by the comparator 136. In response, the comparator 136 provides an acknowledge indication ACK for the device 116. When the device 116 senses the ACK signal, it responds by bringing the DATA and ENABLE inputs LOW. This turns off transistor 140 and drops the current on the bus which is pulled to a negative voltage. The drop in current is sensed by the current sensor 138 which indicates to the device 118 that the ACK is no longer present. The device 118 responds by bringing the ENABLE input low, thereby turning off transistor 146 and returning the bus 112 to its initial state.

Those skilled in the art will appreciate that the transmission of a binary ZERO by the device 116 is effected by asserting the ENABLE input of gates 152, 156 HIGH and the DATA input of gates 152, 156 LOW. This turns on transistor 142 which provides a negative voltage low current signal on the bus. The device 118 senses the voltage at comparator 150 and transmits an ACK by bringing the DATA and ENABLE inputs of gates 154, 158 HIGH. This turns on transistor 144 which couples a positive voltage to the bus and thus raises the current flowing through the bus. The current is sensed by comparator 136 which provides the ACK signal to device 116. In response to the ACK signal, the device 116 brings the ENABLE input LOW, turning off transistor 142 and dropping the current on the bus. The absence of current is detected by comparator 138 and the device 118 responds by bringing the DATA and ENABLE inputs LOW. The bus then returns to its idle state. It will be appreciated that the bus described in FIG. 3 has four states: positive voltage with low current, negative voltage with low current, no voltage or floating voltage with no current, and a high current state.

Both of the above-described point-to-point data transfer methods provide for a bit-by-bit acknowledgement and maintain the state of the bus until each bit is acknowledged. Thus, the data transfer is self-timed and there is no need for interrupts to prevent loss of data. The software implementation of the serial link is simple and robust. However, in addition to the protocols described above, it is useful in serial data transmission to "frame" the data stream. Framing is typically accomplished by flagging the start and stop of a group of bits. Using the protocols described above, it is possible to flag the start and stop of a frame with an otherwise illegal state transition. For example, in the two wire bus described above, a transition from LOW LOW to HIGH HIGH or from HIGH HIGH to LOW LOW could be used, but the bus would have to be kept in this state for an undetermined length of time so that the receiver is guaranteed to recognize it. Although this method of framing is relatively simple to implement, it can significantly reduce the data transmission rate as frame starts are frequently occurring events. An alternative approach is to provide an additional line on the bus for use in indicating the start of a frame. FIGS. 4 and 5 illustrate how the two wire bus system of FIGS. 1 and 2 may be modified to accomplish efficient framing of data.

Turning now to FIGS. 4 and 5, a point-to-point serial bus 200 according to the invention includes a first communications line 212 ("line 0"), a second communications line 214 ("line 1"), and a third communications line 215 (the "control line"). A pair of microprocessors 216, 218 are coupled to each other via the three-line serial bus 200. More specifically, the first, second, and third communications lines 212, 214, and 215 are coupled to respective I/O pins 220, 222, 223 of microprocessor 216 and to respective I/O pins 224, 226, 227 of microprocessor 218. As will be appreciated by those skilled in the art, any three I/O pins of the microprocessors 216, 218 may be used to couple the processors to the bus 200. The bus 200 operates in substantially the same manner as the bus 10 described above. However, as illustrated in FIG. 5, the start of a frame is signalled by taking line 0 and line 1 from the HIGH HIGH idle state of t0 to the state LOW LOW at time t1. Both lines are held in this state until the receiver takes the control line LOW at t2. The receiver holds the control line LOW until the transmitter releases line 0 and line 1 at t3. When the receiver sees that lines 0 and 1 are no longer pulled LOW, it releases the control line at t4, after which the transmitter may begin data transmission as described above with reference to FIGS. 1 and 2. It will be appreciated that this protocol for flagging the start of a frame is self-timing like the protocol for transmitting data described above. Therefore, communication is never unduly delayed and may proceed at a rate as fast as the processors will allow. According to the presently preferred embodiment of the invention, a frame is eight bits (one byte), and the end of the frame is known by the receiver when eight bits are received. During the transmission of the frame, the control line is preferably left HIGH. If the receiver detects an error or the transmitter decides to abort the transmission, either can pull the control line low for a defined time (e.g. one millisecond) to signal a reset and resend.

Turning now to FIG. 6, a multi-user bus 300 according to the invention utilizes three lines 312, 314, 316 ("line 0", "line 1", and "line 2") each of which is coupled to a respective I/O pin of a microprocessor. For example, microprocessor 318 is coupled to the bus 300 via I/O pins 320, 322, 324 which are coupled to lines 312, 314, 316, respectively. Microprocessor 326 is coupled to the bus 300 via I/O pins 328, 330, 332 which are coupled to lines 312, 314, 316, respectively. Microprocessor 334 is coupled to the bus 300 via I/O pins 336, 338, 340 which are coupled to lines 312, 314, 316, respectively. Microprocessor 342 is coupled to the bus 300 via I/O pins 344, 346, 348 which are coupled to lines 312, 314, 316, respectively. According to the methods of the invention, the physical lines 312, 314, and 316 each function as a ZERO data line, a ONE data line, and an ACKnowledge line. Which function each physical line performs depends, as described in detail below, on the state of the bus 300.

The multi-user bus methods according to the invention will be better understood with reference to FIG. 7. The initial state of the bus is illustrated at time t0 in FIG. 7 where line 0 functions as the ZERO data line (D0), line 1 functions as the ONE data line (D1), and line 2 functions as the acknowledge line (ACK). In this initial state, the data lines D0 and D1 are pulled high (unasserted) and the acknowledge line ACK is pulled low by all the bus users.

The transmission of a ZERO bit by a bus user is effected by asserting the D0 line and the transmission of a ONE bit is effected by asserting the D1 line in the initial state of the bus, therefore, the transmission of a ZERO bit by a transmitting bus user is effected by asserting line 0 as shown at time t1 in FIG. 7. In response to the transmission of a ZERO bit, all receiving bus users also assert the D0 line and also release the ACK line. It will be understood, however, that since the ACK line is initially pulled low by all of the bus users, it will not go high until all receiving bus users have released the ACK line. The transmitting bus user releases the ACK line at the start of transmission. When acknowledgement has been noted by all of the bus users, the ACK line goes high as shown at time t2 in FIG. 7. Upon the ACK line going high, the function of each of the physical lines line 0, line 1, and line 2 rotates so that the lines which are now high (line 2 and line 3) become the data lines (D0 and D1 respectively) and the line which is now low (line 0) becomes the acknowledgement line (ACK). This state of the bus is shown in FIG. 7 during the time between t2 and t3. A next ZERO bit transmission, will therefore be effected by asserting line 1, as shown in FIG. 7 at time t3, since line 1 now functions as the D0 line. Acknowledgement by all bus users will result in line 0 going high, as shown in FIG. 7 at time t4, since line 0 now functions as the ACK line. Following the acknowledgement at time t4, the function of each of the physical lines line 0, line 1, and line 2 rotates so that the lines which are now high (line 2 and line 0) become the data lines (D0 and D1 respectively) and the line which is now low (line 1) becomes the acknowledgement line (ACK). This state of the bus is shown in FIG. 7 during the time between t4 and t5. As described thus far, following each ZERO bit transmission and acknowledgement, the function of each physical line is rotated clockwise or in a positive direction. For example, following the acknowledgment at t2 in FIG. 7, the function D0 is rotated from line 0 to line 1, the function D1 is rotated from line 1 to line 2, etc.

As mentioned above, the transmission of a ONE bit is effected by asserting the D1 line, which, as described above, may be any of the physical lines depending on the state of the bus. For example, from the bus state shown between times t4 and t5, a ONE bit is transmitted by asserting physical line 0 (which in this state has the function D1) as shown in FIG. 7 at time t5. The transmission of a ONE bit is followed by each of the receiving bus users asserting the D1 line and releasing the ACK line. When all bus users have released the ACK line, as shown in FIG. 7 at time t6, the function of the physical lines is rotated so that the lines which are now high (line 1 and line 2) become the data lines (D0 and D1 respectively) and the line which is now low (line 0) becomes the acknowledgement line (ACK). It will be appreciated that the rotation of physical line functions following the acknowledgement of a ONE bit transmission is counterclockwise or negative. For example, in the function rotation following the acknowledgement at t6 in FIG. 7, the function D0 is rotated from line 1 to line 0, the function D1 is rotated from line 2 to line 1, etc..

During the times, between data transmission and full acknowledgment, e.g. between times t1 and t2, between times t3 and t4, and between times t5 and t6, the bus may be said to be "meta-stable". According to the presently preferred embodiment of the invention, bus users are prevented from transmitting during these meta-stable bus states. It will be understood that the meta-stable states are indicated when any two lines are low and one line is high.

It will be appreciated that each bus user in the multi-user embodiment of the invention must track the state of the bus so that each bus user knows which physical line has which function. Also, it will be appreciated that under some circumstances it will be necessary to reset the bus to its initial state where line 0 is the D0 line, line 1 is the D1 line, and where line 2 is the ACK line. According to a preferred embodiment of the invention, the bus is reset by taking all three lines low as shown at time t7 in FIG. 7. While it is possible that any bus user could be permitted to reset the bus, it is more typical that the decision to reset the bus will be the province of a master bus user. When the bus users sense that all lines have been taken low, the functionality of the physical lines is restored to the initial state as shown at time t8 in FIG. 7.

Exemplary code for implementing the multi-user bus according to the invention are shown in Code Listings 4 and 5. In order to better understand the code in listings 4 and 5, however, it is useful to consider the state diagram in FIG. 8. For the purpose of illustrating the code listings, the designations SB0, SB1, and SBA are used in FIG. 8 and in the code listings to indicate the physical lines of the bus where SB0 is line 0, SB1 is line 1, and SBA is line 2. FIG. 8 shows the three stable states of the bus in numbered ovals 3, 5, and 6 and the meta-stable states of the bus in numbered hexagons 1, 2, and 4. The outer arrows in FIG. 8 illustrate bus state transitions upon the transmission and acknowledgement of a ONE bit and the inner arrows show the transitions following the transmission and acknowledgement of a ZERO bit.

Turning now to FIG. 8, the initial state of the bus is shown at the oval numbered 3 where the SB0 line has the D0 function, the SB1 line has the D1 function and the SBA line has the ACK function. Upon transmitting a binary ONE, the bus enters a meta-stable state shown at the hexagon numbered 1. Upon acknowledgement of the transmitted ONE by all bus users, the bus enters the stable state shown at the oval numbered 5 wherein the SB0 line has the D1 function, the SB1 line has the ACK function and the SBA line has the D0 function. From the state 5, the transmission of a binary ONE causes the bus to a meta-stable state shown at the hexagon numbered 4. Upon acknowledgement of the transmitted ONE by all bus users, the bus enters the stable state shown at the oval numbered 6 wherein the SB0 line has the ACK function, the SB1 line has the D0 function and the SBA line has the D1 function. Further transmissions of binary ONE change the state of the bus in the counterclockwise direction through the states shown in FIG. 8. From the initial state of the bus at the oval numbered 3, the transmission of a binary ZERO places the bus in a meta-stable state shown at the hexagon numbered 2. Upon acknowledgement of the transmitted ZERO by all bus users, the bus enters the stable state shown at the oval numbered 6 wherein the SB0 line has the ACK function, the SB1 line has the D0 function and the SBA line has the D1 function. Further transmissions of binary ZERO change the state of the bus in the clockwise direction through the states shown in FIG. 8.

Code Listing 4 is illustrative of software used to transmit data on the bus and Code Listing 5 is illustrative of software used to receive data from the bus. Referring now to Code Listing 4, each bit of a byte "c" is transmitted Sequentially with the MSB first. The transmitting bus user checks the stable state of the bus by reading each line and asserts the appropriate line SB0, SB1, or SBA depending on the bus state. The state of the bus is indicated in the code listing as "case 3", "case 5", and "case 6" which refer to the states 3, 5, and 6 shown in FIG. 8 discussed above.

Referring now to Code Listing 5, the receiving bus users accumulate bits to receive "rxByte" and identify the stable and meta-stable states of the bus by reading the lines. The meta-stable states of the bus are indicated in Code Listing 5 as "case 1", "case 2", and "case 4" which correspond to the meta-stable states 1, 2, and 4 shown in FIG. 8 and discussed above. Depending on the state of the bus, the receivers assert and release the appropriate lines in accord with the methods descirbed above. It should be noted that in each meta-stable state, each receiver waits for all of the receivers to release the ACK line before incrementing to the next stable state. This is shown in the code with the "while (|SBx)" statement.

There have been described and illustrated herein several embodiments of a self-timed high speed serial data bus which is easily implemented in software. While particular embodiments of the invention have been described, it is not intended that the invention be limited thereto, as it is intended that the invention be as broad in scope as the art will allow and that the specification be read likewise. Thus, while particular code listings have been disclosed, it will be appreciated that other code could be utilized to effect the methods of the invention. Also, while the methods of the invention have been shown as a static communication protocol, it will be recognized that additional time-out code could be incorporated into the invention. Moreover, while the invention has been disclosed as having microprocessors coupled to the data bus, it will be understood that other programmable devices can utilize the methods and apparatus of the invention. It will also be understood that circuitry other than the circuits shown may be used to assert and detect signals according to the methods described herein. Moreover, features of the disclosed embodiments may be mixed to provide other embodiments. For example, the single-line voltage-current embodiment may be combined with a control line or a separate ACK line to serve multiple users and/or to provide rapid framing signals. Also, the three-line multiple user embodiment may be reset after each acknowledgement so that the functions of the physical lines remains the same throughout data transmission. Furthermore, which assertion of certain signals has been disclosed a high or low or positive or negative, it will be understood that low may be substituted for high and negative may be substituted for positive and vice versa while obtaining substantially the same results. It will therefore be appreciated by those skilled in the art that yet other modifications could be made to the provided invention without deviating from its spirit and scope as so claimed.