These synchronous, presettable counters feature
an internal carry look-ahead for application in
high-speed counting designs. The HC163
devices are 4-bit binary counters. Synchronous
operation is provided by having all flip-flops
clocked simultaneously so that the outputs
change coincident with each other when
instructed by the count-enable (ENP, ENT) inputs
and internal gating. This mode of operation
eliminates the output counting spikes normally
associated with synchronous (ripple-clock)
counters. A buffered clock (CLK) input triggers the
four flip-flops on the rising (positive-going) edge of
the clock waveform.
These counters are fully programmable; that is, they can be preset to any number between 0 and 9 or 15. As
presetting is synchronous, setting up a low level at the load input disables the counter and causes the outputs
to agree with the setup data after the next clock pulse, regardless of the levels of the enable inputs.
The clear function for the HC163 devices is synchronous. A low level at the clear (CLR\) input sets all four of
the flip-flop outputs low after the next low-to-high transition of CLK, regardless of the levels of the enable inputs.
This synchronous clear allows the count length to be modified easily by decoding the Q outputs for the maximum
count desired. The active-low output of the gate used for decoding is connected to CLR\ to synchronously clear
the counter to 0000 (LLLL).
The carry look-ahead circuitry provides for cascading counters for n-bit synchronous applications without
additional gating. ENP, ENT, and a ripple-carry output (RCO) are instrumental in accomplishing this function.
Both ENP and ENT must be high to count, and ENT is fed forward to enable RCO. Enabling RCO produces a
high-level pulse while the count is maximum (9 or 15 with QA high). This high-level overflow ripple-carry pulse
can be used to enable successive cascaded stages. Transitions at ENP or ENT are allowed, regardless of the
level of CLK.
These counters feature a fully independent clock circuit. Changes at control inputs (ENP, ENT, or LOAD\) that
modify the operating mode have no effect on the contents of the counter until clocking occurs. The function of
the counter (whether enabled, disabled, loading, or counting) is dictated solely by the conditions meeting the
stable setup and hold times.