Programmable Prescaler

This invention relates generally to prescaler circuits and, more particularly, to prescaler circuits which are programmable for providing various scaling factors.

A prescaler is a circuit which generates an output signal related to an input signal by a fractional scale factor. A typical example of a prescaler is a decade frequency divider which has an output frequency which is one tenth of the input frequency. Commonly known implementations of prescalers utilize static decode logic gates having inputs connected to binary control signals. The binary control signals are used to select a predetermined prescale rate, and the static decode logic is usually a one of 2.sup.n decoder where n is the number of control signal bits. Outputs of the static decode logic gates are coupled to another decoder for selecting and providing the correctly scaled output signal. Other implementations utilize a counter circuit consisting of serial flip flop circuits which are used to obtain a fractional output signal from a clock signal. Each of the flip flop circuits has a static decode logic circuit coupled to each flip flop for decoding binary control signals which control the switching of a clock signal. Known prescaler circuits utilize considerable decode circuitry associated with a counter and are therefore disadvantageous where circuit size is an important design factor. Prescalers which utilize both even and odd fractional scale factors also require complex state decoding logic to provide the odd fractional scale factors. Further, additional flip flop circuitry is generally required if the prescaler output signal is made synchronous with the input signal.

Accordingly, an object of the present invention is to provide an improved programmable prescaler.

Another object of the present invention is to provide a prescaler which is software programmable and which provides both odd and even fractional scale factors.

Yet another object of the present invention is to provide a prescaler which requires no predecoding of fractional scaling control signal bits.

In carrying out the above and other objects, there is provided, in one form, a prescaler circuit which provides an output signal which is synchronous with and proportional in frequency to a clock signal. Counter means having an input coupled to the clock signal count a predetermined number of cycles of the clock and provide a plurality of counter signals. Each counter signal is provided after a predetermined number of clock signal cycles. Decoder means are coupled to the counter signals of the counter means and connected directly to a binary control signal. The binary control signal selects the frequency of the output signal as a predetermined even or odd scaled fraction factor of the frequency of the clock signal. In response to both the counter means and the binary control signal, the decoder means couple a reference voltage node to a decoder output. Latch means hold the output of the decoder means and are coupled to delay means for providing the output signal after a predetermined amount of time delay.

The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings.

FIG. 1 illustrates in schematic form a programmable prescaler in accordance with a preferred embodiment of the present invention; and

FIG. 2 illustrates in graphical form waveforms associated with the decoder circuit of FIG. 1.

Shown in FIG. 1 is a programmable prescaler 10 constructed in accordance with a preferred embodiment and generally comprising a counter section 11, a decode section 12, a latch section 13 and a delay section 14. Although specific N-channel and P-channel MOS devices are shown, it should be clear that prescaler 10 could be implemented by completely reversing the processing techniques (e.g. P-channel to N-channel) or by using other types of transistors.

Counter section 11 comprises a NOR gate 16 having a first input coupled to a clock signal .phi..sub.2, a second input and an output. A flip flop 17 has a first input labeled "C" coupled to the output of NOR gate 16. An inverter 18 has an input coupled to the output of NOR gate 16 and an output coupled to a second input of flip flop 17 labeled "C". A first output of flip flop 17 labeled "Q.sub.1 " is coupled to a first input of a flip flop 19. A second output of flip flop 17 is coupled to a second input of flip flop 19. A first output of flip flop 19 labeled "Q.sub.2 " is coupled to a first input of a flip flop 20, and a second output of flip flop 19 is coupled to a second input of flip flop 20. A first output of flip flop 20 labeled "Q.sub.3 " is coupled to a first input of a flip flop 21, and a second output of flip flop 20 is coupled to a second input of flip flop 21 which has a first output labeled "Q.sub.4 " and a second output. The first inputs of flip flops 19, 20 and 21 are labeled "C" and the second inputs of flip flops 19, 20 and 21 are labeled "C". The second outputs of flip flops 19, 20 and 21 are each labeled "Q". Flip flops 17, 19, 20 and 21 each have a reset terminal labeled "R".

Decode section 12 comprises a decode transistor 25 having a first current electrode coupled to a reference voltage terminal 26 which is coupled to a reference voltage, say analog ground V.sub.AG. A control electrode of decode transistor 25 is coupled to the output of NOR gate 16, and a second current electrode of decode transistor 25 is coupled to a first current electrode of a decode transistor 27. A control electrode of decode transistor 27 is coupled to a predetermined bit of a binary control signal labeled "A". A second current electrode of decode transistor 27 is coupled to a first current electrode of a decode transistor 28. A control electrode of decode transistor 28 is coupled to a predetermined bit of the binary control signal labeled "B", and a second current electrode of decode transistor 28 is coupled to a decode output terminal 29. A decode transistor 31 has a first current electrode coupled to reference voltage terminal 26, a control electrode coupled to the output of NOR gate 16, and a second current electrode coupled to a first current electrode of a decode transistor 32. Decode transistor 32 has a control electrode coupled to the second output of flip flop 17 and a second current electrode coupled to a first current electrode of a decode transistor 33. Decode transistor 33 has a control electrode coupled to the first output of flip flop 19 and a second current electrode coupled to a first current electrode of a decode transistor 34. A control electrode of decode transistor 34 is coupled to the second output of flip flop 20 and a second current electrode of decode transistor 34 is coupled to a first current electrode of a decode transistor 35. A control electrode of decode transistor 35 is coupled to the second output of flip flop 21, and a second current electrode of decode transistor 35 is coupled to a first current electrode of a decode transistor 36. A control electrode of decode transistor 36 is coupled to a predetermined bit of the binary control signal labeled "A", and a second current electrode of decode transistor 36 is coupled to a first current electrode of a decode transistor 37. A control electrode of decode transistor 37 is coupled to bit B of the binary control signal, and a second current electrode of decode transistor 37 is coupled to decode output terminal 29. A decode transistor 40 has a first current electrode coupled to reference voltage terminal 26, a control electrode coupled to the output of NOR gate 16, and a second current electrode coupled to a first current electrode of a decode transistor 41. A control electrode of decode transistor 41 is coupled to the first output of flip flop 17, and a second current electrode of decode transistor 41 is coupled to a first current electrode of a decode transistor 42. A control electrode of decode transistor 42 is coupled to the first output of flip flop 19, and a second current electrode of decode transistor 42 is coupled to a first current electrode of a decode transistor 43. A control electrode of decode transistor 43 is coupled to the second output of flip flop 20, and a second current electrode of decode transistor 43 is coupled to a first current electrode of a decode transistor 44. Decode transistor 44 has a control electrode coupled to the second output of flip flop 21 and a second current electrode coupled to a first current electrode of a decode transistor 45. Decode transistor 45 has a control electrode coupled to bit A of the binary control signal and a second current electrode coupled to a first current electrode of a decode transistor 46. Decode transistor 46 has a control electrode coupled to a predetermined bit of the binary control signal labeled "B" and a second current electrode coupled to decode output terminal 29. A decode transistor 48 has a first current electrode coupled to reference voltage terminal 26, a control electrode coupled to the output of NOR gate 16 and a second current electrode coupled to a first current electrode of a decode transistor 48. Decode transistor 49 has a control electrode coupled to the second output of flip flop 17 and a second current electrode coupled to a first current electrode of a decode transistor 50. Decode transistor 50 has a control electrode coupled to the second output of flip flop 19 and a second current electrode coupled to a first current electrode of a decode transistor 51. A control electrode of decode transistor 51 is coupled to the first output of flip flop 20, and a second current electrode of decode transistor 51 is coupled to a first current electrode of a decode transistor 52. Decode transistor 52 has a control electrode coupled to the first output of flip flop 21 and a second current electrode coupled to a first current electrode of a decode transistor 53. A control electrode of decode transistor 53 is coupled to bit A of the binary control signal, and a second current electrode of decode transistor 53 is coupled to a first current electrode of a decode transistor 54. Decode transistor 54 has a control electrode coupled to bit B of the binary control signal and a second current electrode coupled to decode output terminal 29. In a preferred form, all decode transistors are N-channel transistors.

Latch section 13 comprises a latch preset transistor 60 having a first current electrode coupled to a supply voltage V.sub.DD, a control electrode coupled to the output of NOR gate 16 and a second current electrode coupled to decode output terminal 29. A latch transistor 61 has a first current electrode coupled to supply voltage V.sub.DD, a control electrode and a second current electrode. An inverter 62 has an input coupled to the second current electrode of latch transistor 61 and an output for providing a signal labeled "X" which is coupled to the control electrode of latch transistor 61. The input of inverter 62 is also coupled to the decode output terminal 29. In a preferred form, transistors 60 and 61 are P-channel transistors.

Delay means 14 comprise a switch 66 having a first current electrode coupled to the output of inverter 62 and a second current electrode coupled to both an input of an inverter 67 and a first current electrode of a switch 68. Clock signal .phi..sub.2 is coupled to an input of an inverter 69 which has an output coupled to a first control electrode of first conductivity type of switch 66. Clock signal .phi..sub.2 is coupled to a second control electrode of second conductivity type of switch 66 and is also coupled to a first control electrode of switch 68. A complementary clock signal, .phi.2, is coupled to a second control electrode of a second and opposite conductivity type of switch 68. An output of inverter 67 is coupled to both a first input of a NOR gate 70 and an input of an inverter 71. An output of inverter 71 is coupled to a second current electrode of switch 68. A second input of NOR gate 70 is coupled to the output of inverter 69. An output of NOR gate 70 is coupled to a prescaler output terminal 72, to a second input of NOR gate 16 and to the reset terminal of flip flops 17, 19, 20 and 21. In a preferred embodiment, switches 66 and 68 are CMOS transmission gates which are clocked in a conventional manner by clock signal .phi..sub.2. Switch 66 is constructed to be conductive when clock signal .phi..sub.2 is at a low state and nonconductive when clock signal .phi..sub.2 is at a high state. Conversely, switch 68 is constructed to be conductive when clock signal .phi..sub.2 is at a high state and nonconductive when clock signal .phi..sub.2 is at a low state.

In operation, a complementary clock signal .phi.2 is provided at the output of NOR gate 16 having a predetermined frequency as illustrated in FIG. 2 and is coupled to counter means 11. Assuming that the output signal is initially at a low state, the output of NOR gate 16 will change state every time clock signal .phi..sub.2 changes. Flip flops 17, 19, 20 and 21 function to count the number of changes in logic levels of clock signal .phi..sub.2. Every time clock signal .phi.2 changes logic level from a high level to a low level, a change in logic level of the Q.sub.1 output of flip flop 17 occurs. Every time clock signal Q.sub.1 changes its logic level from a high level to a low level, a change in logic level of the output signal of flip flop 19 occurs. The binary control signal is illustrated as having four bits, A A B B, where bit A is the most significant bit, and is coupled to decode section 12 for programmably selecting a predetermined divide rate for prescaler 11. In the illustrated embodiment, decode transistors 25, 27 and 28 are enabled to couple reference voltage V.sub.AG to decode output terminal 29 to provide a prescaler output which has a frequency which is equal to the frequency of clock signal .phi..sub.2. In other words, the divide factor is one. Decode transistors 31, 32, 33, 34, 35, 36 and 37 are enabled to couple reference voltage V.sub.AG to decode output terminal 29 to provide a prescaler output which has a frequency which is equal to one-third of the frequency of clock signal .phi..sub.2. Thus, the divide factor is three. Decode transistors 40, 41, 42, 43, 44, 45 and 46 are enabled to couple reference voltage V.sub.AG to decode output terminal 29 to provide a prescaler output which has a frequency which is equal to one-fourth of the frequency of clock signal .phi..sub.2. Decode transistors 48, 49, 50, 51, 52, 53 and 54 are enabled to couple reference voltage V.sub.AG to decode output terminal 29 to provide a prescaler output which has a frequency which is equal to one-thirteenth of the frequency of clock signal .phi..sub.2.

Shown in FIG. 2 is an illustration of waveforms associated with prescaler 10 when a binary input control signal of 1001 corresponding to A A B B is coupled to decode section 12. The example control signal makes decode transistors 36 and 37 conductive thereby selecting a divide by three prescale factor. Initially, assume that flip flops 17, 19, 20 and 21 have been reset so that the outputs Q.sub.1, Q.sub.2, Q.sub.3 and Q.sub.4 are at a low logic level. In a preferred form, as previously mentioned flip flops 17, 19, 20 and 21 are toggle flip flops which respond by providing a high logic level output at a Q output terminal in response to a negative going clock edge. Therefore, the Q.sub.1 output goes to a logic high level after clock signal .phi..sub.2 transitions to a low logic level. The Q.sub.1 output signal remains at a high logic level until clock signal .phi..sub.2 has another negative going clock edge. The second negative going clock edge of clock signal .phi..sub.2 results in a low logic level at the Q.sub.1 output and a high logic level at the Q.sub.2 output. At this time, decode transistors 32, 33, 34, 35, 36 and 37 are conductive. Upon the next rising or positive edge of clock signal .phi..sub.2, decode transistor 31 is also made conductive which results in reference node 26 being immediately coupled to decode output terminal 29. Simultaneously, clock signal .phi..sub.2 causes preset latch transistor 60 to be nonconductive. Preset transistor 60 presets decode output terminal 29 and resets the output of inverter 62. If reference voltage V.sub.AG is at a low logic level signal, then signal X at the output of inverter 62 is at a high logic level as shown in FIG. 2 thereby disabling latch transistor 61. Before reference node 26 is coupled to decode output terminal 29, transistor 61 latches a low logic level to the output of inverter 62.

Once signal X transitions to a high logic level, delay means 14 function to delay and latch signal X one-half cycle of clock signal .phi..sub.2 before the output signal transitions to a high logic level. The delay is accomplished by making switch 66 nonconductive when clock signal .phi..sub.2 is at a high logic level and by making switch 68 conductive. At the point in time when .phi..sub.2 transitions from a high logic level to a low logic level, signal X is latched internally thru the operation of inverters 67 and 71. Inverters 67 and 69 immediately couple a low logic level to both the first and second inputs of NOR gate 70. Therefore, when .phi..sub.2 transitions from a high logic level to a low logic level, the output signal at terminal 72 transitions from a low logic level to a high logic level. Since the output signal is coupled to the reset terminal of flip flops 17, 19, 20 and 21, all flip flops are immediately reset and the Q.sub.1 and Q.sub.2 outputs of flip flops 17 and 19, respectively, are reset to a low logic level. When clock signal .phi..sub.2 transitions back to a high logic level, the output signal returns to a low logic level. Further level changes in clock signal .phi..sub.2 do not change the output of NOR gate 70 until signal X returns to a high logic level again. The above described circuit operation then becomes repetitive. The Q.sub.3 and Q.sub.4 outputs of flip flops 20 and 21, respectively, never change logic level for a divide by three prescale operation. As a result of delay means 14, n-1 separate pulses of the .phi..sub.2 clock signal must occur before signal X transitions logic levels to provide a prescaled output signal which has a frequency which is 1/nth of the .phi..sub.2 clock signal, where n is an integer. Thus, counter section 11 functions to count a binary value associated with a clock signal input in response to an input control signal which determines the prescaled output signal rate. Decode section 12 functions to decode the numeric counted value. Prescaler 10 operates in an analogous manner for other input control signals to provide other programmed prescaled output signals. The control signals may be software programmed or programmed otherwise.

By now it should be apparent that a prescaler circuit which is simple and which requires no predecoding is provided. Static logic gates have been reduced and a one of n type decoder commonly associated with previous circuits has been eliminated. By utilizing the decode transistors of decode section 12, circuitry and space has been greatly reduced from previous predecoder circuits which predecode control signals. By utilizing the delay section 14, multiple odd divide rates may be provided in addition to providing even divide rates. This eliminates complex state decoding logic commonly associated with previous circuits which provided an analogous function. A plurality of divide rates of any rate may be provided by modifying decode section 12 to operate analogous to the circuit operation explained in detail. Further, to provide software programmability, a rate control register (not shown) may be used to enable software to manipulate the state of the bits of the encoded control signal.

While the invention has been described in the context of a preferred embodiment, it will be apparent to those skilled in the art that the present invention may be modified in numerous ways and may assume many embodiments other than that specifically set out and described above. Accordingly, it is intended by the appended claims to cover all modifications of the invention which fall within the true spirit and scope of the invention.