Sequential Logic: Counters

74VHC393 four-bit counter

Examples: 74VHC393, 74VHC161

Counters count up or down sequentially on every clock (CK) pulse. A four-bit counter can have a modulus of up to 16; an eight-bit counter up to 256; and a 14-bit counter up to 16384. Some counters have the CLR input that is used to initialize the internal state to a known value.
Counters are incorporated in digital timers, electronic calculators, stopwatches, and many other devices.

Counters are broadly divided into asynchronous (ripple carry) and synchronous (parallel carry) counters. Let the propagation delay time of a single flip-flop be tpd. Then, an n-stage asynchronous counter incurs a large delay equal to n×tpd. It should also be noted that the asynchronous counter could produce hazards when the counter output is fed to a logic gate.

74VHC393 four-bit counter
Asynchronous: Ripple carry counter

Logic schematic of the 74VHC161 four-bit counter

Logic schematic of the 74VHC161 four-bit counter
Synchronous: Parallel carry counter

Timing diagram

Timing diagram: The following shows the timing diagram of a typical synchronous (parallel carry) counter, which counts upward sequentially on each edge of the clock (CK).

Chapter3 Basic CMOS Logic ICs

Basic CMOS Logic ICs
Combinational Logic: Inverters and Buffers
Combinational Logic: Bidirectional Bus Buffers
Combinational Logic: Schmitt-Trigger Devices
Combinational Logic: Decoders
Combinational Logic: Multiplexers
Combinational Logic: Analog Multiplexer/Demultiplexers
Combinational Logic: Analog Switches
Sequential Logic: Latches
Sequential Logic: Flip-Flops
Sequential Logic: Shift Registers


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