Abstract:
An asynchronous ping-pong counter is disclosed. The asynchronous ping-pong counter comprises a first asynchronous counter, a second synchronous counter, and a controller, the asynchronous ping-pong counter operates between a first state and a second state. In the first state, the first asynchronous counter counts a first number of clock edges of a fast clock signal, and the second asynchronous counter holds a first counter output value. In the second state, the second asynchronous counter counts a second number of clock edges of the fast clock signal, and the first asynchronous counter holds a second counter output value. The controller determines a state transition based on a sampling of a slow clock signal by the fast clock signal.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/059,229, filed on Jun. 5, 2008 and entitled “ASYNCHRONOUS PING-PONG COUNTER”, the contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the design of a counter. In particular, it relates to the design of an asynchronous ping-pong counter. 
     2. Description of the Background Art 
     A counter is defined here as a building block that receives a fast clock and a slow clock, and generate an output value. A rising edge is defined as the transition of a digital signal from low to high. The output value represents the number of rising edges of the fast clock that exists between two neighboring rising edges of the slow clock. A synchronous counter clocked by the fast clock can be used to over-sample the slow clock to determine the number of rising edges by inspecting the sampled results. However, if the fast clock runs at a very high speed, e.g. 5 GHz, and the slow clock runs at an extremely slow speed, e.g. 10 MHz, it is almost impossible to meet both the setup and hold timing requirements of each flip-flop by using any synchronous counter in existing technologies. An asynchronous ping-pong counter is presented in this work to solve the aforementioned problems. 
     SUMMARY 
     In one embodiment, an asynchronous ping-pong counter comprises an edge detector, a dual asynchronous counter, and a finite state machine. The edge detector receives a first input clock and a second input clock, and generates a pulse signal to indicate the arrival of a rising edge of the second input clock. The rising edges of the first input clock are arranged into different time slots on the basis of the pulse signal generated by the edge detector. The dual asynchronous counter receives a binary select signal from the finite state machine and the first input clock, and generates a counter value that is the number of the rising edges in each time slot. The dual asynchronous counter includes a first asynchronous counter and a second asynchronous counter. When the first asynchronous counter is receiving the rising edges of the first input clock in the current time slot, the second asynchronous counter is calculating the number of the rising edges in the previous time slot and generating the counter value and vice versa. The choice of which asynchronous counter is selected depends on the binary value of the select signal. The finite state machine receives the pulse signal and the first input clock, and generate the select signal to indicate that the rising edges of the first input clock is coupled to either the first asynchronous counter or the second asynchronous counter. The select signal also indicates which asynchronous counter outputs the counter value in the previous time slot. 
     These and other features of the present invention will be readily apparent to persons of ordinary skill in the art upon reading the entirety of this disclosure, which includes the accompanying drawings and claims. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1(   a ) shows a block diagram of an asynchronous ping-pong counter. 
         FIG. 1(   b ) shows a timing diagram of the asynchronous ping-pong counter of  FIG. 1(   a ). 
         FIG. 2  schematically shows an asynchronous ping-pong counter in accordance with an embodiment of the present invention. 
         FIG. 3  shows a timing diagram of the asynchronous ping-pong counter of  FIG. 2 . 
         FIG. 4  schematically shows an asynchronous counter in accordance with an embodiment of the present invention. 
     
    
    
     The use of the same reference label in different drawings indicates the same or like components. 
     DETAILED DESCRIPTION 
     In the present disclosure, numerous specific details are provided, such as examples of electrical circuits, components, and methods, to provide a thorough understanding of embodiments of the invention. Persons of ordinary skill in the art will recognize, however, that the invention can be practiced without one or more of the specific details. In other instances, well-known details are not shown or described to avoid obscuring aspects of the invention. 
       FIG. 1(   a ) shows a proposed asynchronous ping-ping counter in accordance with an embodiment of the present invention. In the example of  FIG. 1(   a ), the asynchronous ping-pong counter receives a first input clock (CLKF in  FIG. 1(   a )), a second input clock (CLKS in  FIG. 1(   a )), and generates a digital output (CNT_VAL in  FIG. 1(   a )) that is the number of rising edges of the first input clock between two neighboring rising edges of the second input clock. Typical input and output waveforms are shown in the timing diagram of  FIG. 1(   b ), where the asynchronous ping-pong counter  100  calculates the number of the rising edges of the first input clock between two neighboring rising edges of the second input clock to generate the digital output CNT_VAL. The digital output CNT_VAL is a multi-bit digital value, with its bit width dependent on the maximally possible number of rising edges between two neighboring rising edges of the second input clock. 
     In one embodiment, an asynchronous ping-pong counter is configured for receiving a first input clock CLKF and a second input clock CLKS, and for generating a digital output CNT_VAL that represents the number of the rising edges of the first clock signal between two neighboring rising edges of the second clock signal.  FIG. 2  schematically shows an asynchronous ping-pong counter in accordance with an embodiment of the present invention. In the example of  FIG. 2 , the asynchronous ping-pong counter comprises a dual asynchronous counter  200 , an edge detector  210 , and a finite state machine  220 . 
     An edge detector  210  is first employed to detect a rising transition edge of the second clock and generates a corresponding pulse signal. In one embodiment, the edge detector is configured to receive the first input clock and the second input clock, and generate the pulse signal. In the example of  FIG. 2 , the edge detector  210  comprises a first flip-flop  211 , a second flip-flop  212 , and an AND gate  213 . The edge detector  210  applies the first clock signal to the clock pin of the first flip-flop  211  to sample the second clock signal. The output of the first flip-flop is connected to the data input pin D of the second flip-flop  212  that is also clocked by the first clock signal. The output of the first flip-flop and the negated output of the second flip-flop are ANDed together by the AND gate  213  to detect a rising transition edge of the second clock signal. Once a rising edge is detected, a corresponding pulse signal PULSE is generated. The time period between the rising edges of two neighboring pulses is called a time slot. 
     The asynchronous ping-pong counter comprises a dual asynchronous counter  200  to calculate the number of the rising edges of the first input clock in each time slot. In the example of  FIG. 2 , the dual asynchronous counter  200  comprises a first multiplexer  201 , a second multiplexer  202 , a third multiplexer  203 , a first asynchronous counter  204 , and a second asynchronous counter  205 . The dual asynchronous counter  200  operates in a ping-pong mode. When one asynchronous counter is used to receive the rising edges of the first input clock in the current time slot, the other asynchronous counter is used to calculate the number of the rising edges of the first input clock in the previous time slot and generate the digital output signal and vice versa. 
     The first and second asynchronous counters can be any asynchronous counter. In one embodiment, the asynchronous counter is configured to receive a series of pulses from an input signal CP, a reset signal RESET, and generate a digital output Q that represents the number of the rising edges of the input signal CP.  FIG. 4  schematically shows an asynchronous counter in accordance with an embodiment of the present invention. This embodiment is a ripple counter. The ripple counter comprises a series of connected flip-flops. The total number (i.e. N) of the required flip-flops depends on the maximally possible number of rising edges of the input signal CP. Each flip-flop has a clock input pin, a data input pin, an output pin, a negated output pin, and a reset pin. A rising edge at the clock input pin of a flip-flop samples a binary value at the data input pin into the output pin and its negative value into the negated output pin. A binary zero at the reset pin will reset the flip-flop such that the values at its output pin and negative output pin become a binary zero and a binary one, respectively. The clock input pin of the first flip-flop is driven by the input signal CP. The clock input pins of the other flip-flops are driven by the negated outputs of the proceeding flip-flops. Due to the nature of the asynchronous counter, the rising edges of the input signal CP are rippled through the counter. After the ripple stops, the data at the output pins of all the flip-flops represents the number of the rising edges of the input signal CP. When the reset signal RESET changes to a binary zero, all the flip-flops will be reset. 
     The choice of the asynchronous counter depends on the binary value of a signal SEL that is generated by the finite state machine  220 . When the signal SEL is a binary one, the first multiplexer  201  couples the first input clock CLKF to the input signal CP of the first asynchronous counter  204  whereas the input signal CP of the second asynchronous counter  205  is tied to a binary zero through the second multiplexer  202 . In the same time slot, the output signal Q of the second asynchronous counter  205  is coupled to the output signal CNT_VAL through the third multiplexer  203 . This configuration is for the first asynchronous counter to receive the input signal from the first input clock CLKF in the current time slot and for the second asynchronous counter to stop receiving the first clock and to generate the number of the rising edges of the first input clock in the previous time slot. 
     When the signal SEL is a binary zero, the second multiplexer  202  couples the first input clock CLKF to the input signal CP of the second asynchronous counter  205  whereas the input signal CP of the first asynchronous counter  204  is tied to a binary zero through the first multiplexer  201 . In the same time slot, the output signal Q of the first asynchronous counter  204  is coupled to the output signal CNT_VAL through the third multiplexer  203 . This configuration is for the second asynchronous counter to receive the input signal from the first input clock CLKF in the current time slot and for the first asynchronous counter to stop receiving the first input clock and to generate the number of the rising edges of the first input clock in the previous time slot. 
     The pulse signal PULSE generated by the edge detector  210  is connected to a finite state machine. The finite state machine  220  comprises a third flip-flop  221 , a fourth flip-flop  222 , a NAND gate  223 , and another NAND gate  224 . The pulse signal PULSE continuously toggles the third flip-flop  221  in the finite state machine  220 . The output of the third flip-flop  221  is connected to the data input of the fourth flip-flop  222  that is clocked by the falling edge of the first input clock CLKF. The binary data at the output pin of the fourth flip-flop  222  is the signal SEL. Because the fourth flip-flop  222  is clocked by the falling edge of the first input clock CLKF, the signal SEL always changes its value when the first input clock CLKF is a binary zero. In doing so, no glitches will be generated at the signal nets CP 1  and CP 0  inside the dual asynchronous counter  200  when the signal SEL switches its value from a binary one to a binary zero or from a binary zero to a binary one. 
     Before the signal SEL switches to a binary one, the NAND gate  223  is used to generate a binary zero to clear the old content of the first asynchronous counter  204 . When the value of the signal SEL becomes a binary one, the first input clock is coupled to the input of the first asynchronous counter  204  through the first multiplexer  201  and the first asynchronous counter  204  receives the rising edges of the first clock signal in the current time slot. Meanwhile, the second asynchronous counter  205  stop receiving any more rising edges of the first clock signal by tying its input to a binary zero through the second multiplexer  202  and its output is coupled to the output value CNT_VAL through the third multiplexer  203 . 
     Before the signal SEL switches to a binary zero, the NAND gate  224  generates a binary zero to clear the old content of the second asynchronous counter  205 . When the value of the signal SEL becomes a binary zero, the first input clock is coupled to the input of the second asynchronous counter  205  through the second multiplexer  202  and the second asynchronous counter  205  receives the rising edges of the first clock signal in the current time slot. Meanwhile, the first asynchronous counter  204  stop receiving any more rising edges of the first clock signal by tying its input to a binary zero through the first multiplexer  201  and its output is coupled to the output signal CNT_VAL through the third multiplexer  203 . The input, internal node, and output waveforms are shown in the timing diagram of  FIG. 3 . 
     An asynchronous ping-pong counter has been disclosed. While specific embodiments of the present invention have been provided, it is to be understood that these embodiments are for illustration purposes and not limiting. Many additional embodiments will be apparent to persons of ordinary skill in the art reading this disclosure.