Method and apparatus for encoding a bus to minimize simultaneous switching outputs effect

[0002] Embodiments of the present invention relate to data busses. In particular, the present invention relates to a method and apparatus for minimizing the effects generated by simultaneous switching of outputs in a bus.

[0003] A bus is a collection of wires, interfaces, and control elements through which information is transmitted between components in a computer system. A bus may connect to components such as a processor, cache memory, or Random Access Memory (RAM) and may connect to subsystems or devices such as a disk drive, input device, or output device. The component transmitting information over a bus may be referred to as the “transmitting node” and the component receiving information at the output of the bus may be referred to as the “receiving node.”

[0004] Data is typically transmitted over a bus in digital format with each wire or signal in the bus transmitting a single bit of information (i.e., binary value). During normal operation, each signal is either transmitting a voltage in a high range, which may represent the value of “one,” or transmitting a voltage in a low range, which may represent the value of “zero.” A signal may be said to be in a “high state” when it is driving a high voltage range and may be said to be in a “low state” when it is driving a low voltage range. A collection of signals in the bus may be referred to as a “databus.” The individual states of the signals in a databus taken together define a set of states which the bus may be in. For example, when a bus is driving the value of FFFF it is one state, and when the bus is driving the value F0FF it is in another state.

[0005] During normal operation of the bus, signals are repeatedly switched between states as dictated by the content of the data being transferred. The state of the bus is sampled by a receiving node at time intervals, or cycles, which may be defined by a clock or strobe. Any signal in the bus may be changed from one cycle to the next, and it is not uncommon for the values being driven on each of the signals in the bus to switch states for consecutive cycles. For example, a large number of signals in a bus will change from one state to another from one cycle to the next (i.e., simultaneously) if a large number of the digital values being transmitted over the bus changes from one clock interval to the next. In a high speed bus the signals may be switched between states at a high rate, such as for example 1 million times per second.

[0006] A power supply or “rail” (which may be designated VCC) supplies the power to drive a signal from a low state to a high state. To go from a high state to a low state, the capacitance is drained from the signal to a ground. The switching of states of a large number of signals between cycles is known as simultaneous switching and causes a condition known as the simultaneous switching outputs (“SSO”) effect. For example, when a large number of signals change from a high state to a low state during a clock cycle, this may cause the ground to rise in voltage. Because the ground has risen in voltage, it may take longer to drain the charge from the signals and, thus, longer to change the state of the signals. Similarly, when a large number of signals change from a low state to a high state during a clock cycle, this may cause the VCC to be lowered in voltage. In general, a large change in current relative to time (di/dt) produces supply offsets that may affect input buffers and adjacent drivers. A lower power supply voltage or higher ground voltage may slow outputs. A large di/dt can also cause errors on data strobes and may put noise on adjacent signals.

[0007] The SSO effect may be difficult to find, understand and correct and is particularly a problem in high speed busses or busses with a large width. The SSO effect can be minimized by distributing multiple grounds for each bus, for example two or three grounds for every bus pair. However, the use of multiple grounds and supply pins substantially increases the cost of the bus and the board area used.

[0008] FIG. 1 is a partial block diagram of a system including an apparatus to encode a bus to minimize simultaneous switching according to an embodiment of the present invention.

[0009] FIG. 2 is a partial block diagram of a bus according to an embodiment of the present invention.

[0010] FIG. 3 is a table that contains sample bus states according to an embodiment of the present invention.

[0011] FIG. 4 is a flow diagram of a method of encoding a bus to minimize simultaneous switching according to an embodiment of the present invention.

[0012] Embodiments of the present invention relate to a method and apparatus for encoding a bus to minimize the impact of simultaneous switching effect by eliminating the cases where the entire bus draws current from VCC or drain to ground in a single cycle. In an embodiment, a databus is divided into right and left halves. Before data is driven down the bus for each cycle, the number of “ones” for each half the databus is counted and the half that has the most “ones” is inverted. In a further embodiment, a control bit is sent with the databus and indicates to the receiving agent which half of the databus is inverted. According to embodiments of this invention, one half of the databus will always be inverted. When embodiments such as these are used, the extreme cases are removed from the transition distribution (e.g., the bus is not will not transition from all “ones” to all “zeros” or vice versa). According to an embodiment, the maximum number of “ones” that can be driven is one-half the databus.

[0013] An embodiment of the invention includes an encoder that has a component to count the number of “ones” in each part of the databus and a component to compare the magnitudes of the counts. In a further embodiment, the result of the magnitude comparison determines which part of the databus is inverted and determines the value transmitted on the control signal. In a still further embodiment, a predictive method is used to avoid serializing the control bit with parity generation.

[0014] FIG. 1 is a partial block diagram of a system 100 including an apparatus to encode a bus to minimize simultaneous switching according to an embodiment of the present invention. System 100 includes a transmitting node 110 which is coupled to a receiving node 120 through a bus 130. A node may be an architectural unit that includes a single component, such as a processor, memory device, or input/output device, or may include multiple components. Thus, each node in system 100 may be a single processor or may contain multiple processors as well as other resources such as memory devices. The term “coupled” encompasses a direct connection, an indirect connection, an indirect communication, etc.

[0015] Bus 130 may be a uni-directional bus or a bi-directional bus. For the purposes of illustration, bus 130 is shown as a uni-directional bus that transmits information from transmitting node 110 to receiving node 120. Bus 120 includes data lines 131 and a decode control line 132 which are coupled to transmitting node 110 and receiving node 120. In an embodiment, each of the lines in bus 130 is a signal that may transmit a single bit of binary information. The “data lines” 131 in bus 130 may include one or more strobe signals, parity lines, and control information. Data lines 131 may be referred to as a databus. Though not shown in FIG. 1, system 100 may also include a system clock which is coupled to the transmitting node 110 and receiving node 120. As shown in FIG. 1, bus 130 has eight data lines. In other embodiments, bus 130 may have any number of data lines, such as for example sixteen, twenty, etc.

[0016] As shown in FIG. 1, transmitting node 110 contains an encoder circuit 115 and receiving node 120 contains a decoder circuit 125. In an embodiment of the invention, decode control line 132 is a simultaneous switching decode control line, encoder circuit 115 is a simultaneous switching encoder circuit, and decoder circuit 125 is a simultaneous switching decoder circuit. In an embodiment, encoder circuit 115 may encode one-half of the bit values to be transmitted across data lines 131 in a cycle before driving the bit values over the databus so that maximum number of “ones” that can be driven is equal to or less than one-half the number of signals in the databus. In an embodiment, encoder circuit 115 contains components to determine the number of “one” values in each half of the databus and compare the results of this determination. In this embodiment, encoder circuit 115 may determine the number of “one” values in each half of the databus and compare the results of said determination. In an embodiment, encoder circuit 115 inverts the values sent on a part of the of data lines 131. In a further embodiment, encoder circuit 115 ensures that one-half the lines in the databus are inverted. In an embodiment, decoder circuit 125 decodes (i.e., re-inverts) the inverted values. In an embodiment, bus 130 is a bi-directional bus, receiving node 120 contains a encoder circuit and transmitting node 110 contains a decoder circuit. Details of “one” embodiment of an encoder circuit and a decoder circuit are discussed below with reference to FIG. 2.

[0017] FIG. 2 is a partial block diagram of a bus 200 according to an embodiment of the present invention. Bus 200 includes an input interface 210, simultaneous switching encoder circuit 220, databus 230, simultaneous switching decode control line 240, simultaneous switching decoder circuit 250, and output interface 260. Input interface 210 and output interface 260 may be used to connect bus 200 to components. Input interface 210 may include a plurality of inputs and output interface 260 may include a plurality of outputs. Input interface 210 may be coupled to simultaneous switching encoder circuit 220, and simultaneous switching decoder circuit 250 may be coupled to output interface 260. Databus 230 may be coupled to simultaneous switching encoder circuit 220 and simultaneous switching decoder circuit 250 and may transmit data values from simultaneous switching encoder circuit 220 to simultaneous switching decoder circuit 250.

[0018] In an embodiment, databus 230 is divided into parts. Databus 230 may include a first set of data lines 231 and a second set of data lines 232. In an embodiment, the first set of data lines 231 and second set of data lines 232 both have the same number of data lines. In FIG. 2, databus 230 comprises eight data lines, the first set of data lines 231 comprises four data lines, and second set of data lines 232 comprises four data lines. In other embodiments, databus 230, first set of data lines 231, and second set of data lines 232 may have any number of data lines, such as for example sixteen, twenty, sixty, etc.

[0019] Simultaneous switching encoder circuit 220 may include a counter circuit 221, a counter circuit 222, an inverter component 223, and a magnitude comparitor 228. In an embodiment, one half of the bits that are input to input interface 210 are coupled to counter circuit 221 as inputs, and the other half of the bits that are input to input interface 210 are coupled to counter circuit 222 as inputs. Counter circuit 221, counter circuit 222, and magnitude comparitor 228 may comprise a selector circuit that counts the number of signals in the first set of data lines and second set of data lines that have a certain value (e.g., a value of “one”) and selects a set of data lines to be inverted based on the count determined. In an embodiment, counter circuit 221 and counter 223 are both Wallace trees which are structures of Carry Save Adders (CSAs) that count the number of “ones” values input into the counter and output a binary sum. In other embodiments, counter circuit 221 and counter 223 may count the number of “zeros.” In an embodiment, magnitude comparitor 228 compares the magnitude of the sums generated by counter circuit 221 and counter circuit 222 and outputs a result that indicates which sum is greater. As shown in FIG. 2, the output of magnitude comparitor 228 is coupled to simultaneous switching decode control line 240 and is input to inverter component 223. In an embodiment, each of the bits that are input to input interface 210 are coupled to inverter component 223 as inputs.

[0020] As shown in FIG. 2, simultaneous switching decoder circuit 250 contains an inverter component 255 coupled to inputs of inverter 255. In this embodiment, simultaneous switching decode control line 240 is also coupled to an input of inverter component 255. The outputs of inverter component 255 are coupled to the outputs of output interface 260.

[0021] The operation of bus 200 according to an embodiment of the invention will now be described. A set of bit values may be received at input interface 210 during each cycle. Simultaneous switching encoder circuit 220 determines a value to be transmitted on the simultaneous switching decode control line 240 and encodes (i.e., inverts) a value to be transmitted on one set of data lines. In an embodiment, if the first half has more “ones” simultaneous switching encoder circuit 220 will select the first half of the input bits to be inverted and will send a value on simultaneous switching decode control line 240 indicating that the first half has been inverted. In this embodiment, if the second half has more “ones” simultaneous switching encoder circuit 220 will select the second half of the input bits to be inverted and will send a value on simultaneous switching decode control line 240 indicating that the second half has been inverted. In an embodiment, the first half of the inputs will be inverted if both halves have the same number of “ones.” The inverted set of data lines and the non-inverted set of data lines are then transmitted during the bus cycle over databus 230 to simultaneous switching decoder circuit 250. In addition, the value determined for simultaneous switching decode control line 240 is transmitted over simultaneous switching decode control line 240 to simultaneous switching decoder circuit 250. Simultaneous switching decoder circuit 250 uses the value received on simultaneous switching decode control line 240 to determine which set of data lines needs to be re-inverted, re-inverts the values received on these data lines, and transmits the values for all of the data lines to output interface 260 as the bus output. Thus, the data lines that were inverted by simultaneous switching encoder circuit 220 are inverted back to their original state and output by simultaneous switching decoder circuit 250, and the data lines that were never inverted by simultaneous switching encoder circuit 220 are output by simultaneous switching decoder circuit 250 in their original state (i.e., they are not inverted by either simultaneous switching encoder circuit 220 or simultaneous switching decoder circuit 250). This process is repeated for the next group of inputs during the next bus cycle. In an embodiment, the selector circuit (e.g., counter circuit 221, counter circuit 222, and magnitude comparitor 228) and inverter component ensure that one-half the signals in databus 230 are inverted.

[0022] In a further embodiment, the simultaneous switching decode control line 240 is included in the parity generation. In this embodiment, the inputs at input interface 210 are coupled to inputs of a parity generator 281. The output of magnitude comparitor 228 is coupled to an input of parity generator 281 and to an input of inverter component 282. Inverter component 282 may be an inverter coupled to a multiplexer. The output of parity generator 281 is also coupled to an input of inverter component 282. In this embodiment, an output of inverter component 282 is coupled to a parity line 283 which is coupled to an input of parity check circuit 284. In this embodiment, a predictive method must be used to avoid serializing simultaneous switching decode control line 240 and parity generation. An arbitrary but consistent assumption is made that the simultaneous switching decode control line 240 value will be “one.” A value of “one” for simultaneous switching decode control line 240 is fed into the parity generation (not shown) while the actual simultaneous switching decode control line 240 value is computed in parallel. If the computed simultaneous switching decode control line 240 value is “zero,” then the parity bit is inverted by inverter component 282. That is, the simultaneous switching encoder circuit will invert the value for the parity line based on the determination of the value transmitted on the simultaneous switching decode control line. If the computer simultaneous switching decode control line 240 value is really “one” (i.e., the prediction was correct), inverter component 282 does not invert the value generated by parity generator 281. Inverter component 282 transmits the parity bit value over parity line 283 to parity check circuit 284. According to this embodiment, simultaneous switching decode control line 240 generation should be completed by the time parity generation completes. This embodiment may be used where the two halves of the data bus have even numbers of signals. If the databus has an odd half and that side gets inverted by the decode control line 240, then parity does not get changed.

[0023] FIG. 3 is a table 300 that contains eleven examples of sample bus states according to an embodiment of the present invention. Table 300 includes an example number column 301, a bus input data column 302, a column that shows the number of “ones” on the left part of the input data 303, a column that shows the number of “ones” on right part of the input data 304, a column that shows the total number of “ones” in input data 305, a decode control line value column 306, a data sent from encoder column 307, and a column that shows the number of “ones” sent from the encoder 308. The examples in this table are for a thirty-two bit bus. The first example bus state will now be described. The example number 301 for this example is 1 to indicate that this is the first example. The bus input data 302 in this example is FFFF 0000. As would be appreciated by a person of skill in the art, the number of “ones” on the left part of the input data 303 is 16and the number of “ones” on right part of the input data 304 is 0. Thus, the total number of “ones” in input data 305 is 16. Because the left side has more “ones,” in this embodiment the left side is inverted and the decode control line value 306 is “one” to indicate that the left side is inverted. With the left side inverted, the data sent from the encoder 307 is 0000 0000. Thus, the total number of “ones” transmitted from the encoder 308 in this example is 0. Of course, a person of skill in the art would understand examples two to eleven to operate similarly. A comparison of the examples (e.g., a comparison of example 1 with example 2, or a comparison of example 2 with example 3) shows that even when all of the values for the bus input data 302 may change from one state to another state in next cycle, the data sent from the encoder 307 across the databus will never change from all “ones” during one cycle to all “zeros” in the next cycle (or vise-versa). A person of skill in the art would appreciate that the bus illustrated by table 300 has 2³²×2 possible states but, when encoded according to an embodiment of this invention, the bus only uses one-half of the states. Thus, the encoder circuit has eliminated certain state changes and, therefore, minimized the impact of simultaneous switching.

[0024] FIG. 4 is a flow diagram of a method of encoding a bus to minimize simultaneous switching according to an embodiment of the present invention. This method may be used with the apparatus described above. A multi-bit value is received that is to be sent over a bus, and this value comprises two sets of input values (401). The first set of input values may be viewed as the values for the right side of a bus (the “right set”), and the second set of input values may be viewed as the values for the left side of the bus (the “left set”). According to this embodiment, the number of bits in each set that has a value of “one” is counted and compared (402). If the number of “ones” in the left set of input values is equal to or less than the number of “ones” in the right set, then the right set of input values is inverted (403), a value is transmitted over the decode control line indicating that the right set of values was inverted (404), and the inverted right set of values and non-inverted left set of values are transmitted over the databus (405). If the number of “ones” in the left set of input values is greater than the number of “ones” in the right set, then the left set of input values is inverted (406), a value is transmitted over the decode control line indicating that the left set of values was inverted (407), and the inverted left set of values and non-inverted right set of values are transmitted over the databus (408).

[0025] The receiving agent receives both sets of data values and receives the decode control line value. The decode control line value is examined to determine which set of data values was inverted (409). If the right set of data values was inverted, then the right set of values are re-inverted (410). If the left set of data values was inverted, then the left set of values are re-inverted (411). The un-inverted and re-inverted data values are then output from the bus (412). This process is repeated for each bus cycle.

[0026] The invention disclosed in this application provides a method and apparatus for encoding a bus to minimize the effect of simultaneous switching. The SSO effect is known to cause large di/dt resulting in noise and local supply and ground degradation. The invention disclosed in this application may be employed to manage the input/output (I/O) design and performance by eliminating extreme conditions for the I/O and system designer.

[0027] The apparatus and method according to the present invention have been described with respect to several exemplary embodiments. It can be understood, however, that there are many other variations of the above described embodiments which will be apparent to those skilled in the art. It is understood that these modifications are within the teaching of the present invention, which is to be limited only by the claims appended hereto. For example, the number of “zeros” may be counted instead of the number of “ones.” As another example, the method disclosed with reference to FIG. 4 may be performed in a different order.