These synchronous presettable counters feature an internal carry look ahead for application in high speed counting designs. The 74AS163 is a 4-bit binary counter. The 74AS163 clears synchronously. The carry output is decoded to prevent spikes during normal counting mode of operation. Synchronous operation is provided by having all flip-flops clocked simultaneously so that outputs change coincident with each other when so instructed by count enable inputs and internal gating. This mode of operation eliminates the output counting spikes which are normally associated with asynchronous (ripple clock) counters. A buffered clock input triggers the four flip-flops on the rising (positive-going) edge of the clock input waveform.
These counters are fully programmable, that is, the outputs may each be preset to either level. As presetting is synchronous, setting up a low level at the LOAD# input disables the counter and causes the outputs to agree with set up data after the next clock pulse regardless of the levels of enable input. LOW-to-HIGH transitions at the LOAD# input are perfectly acceptable regardless of the logic levels on the clock or enable inputs.
The 74AS163 clear function is synchronous; and a low level at the clear input sets all four of the flip-flop outputs LOW after the next clock pulse, regardless of the levels of enable inputs. This synchronous clear allows the count length to be modified easily, as decoding the maximum count desired can be accomplished with one external NAND gate. The gate output is connected to the clear input to synchronously clear the counter to all LOW outputs. LOW-to-HIGH transitions at the clear input of the 74AS163 is also permissible regardless of the levels of logic on the clock, enable or load inputs.
The carry look ahead circuitry provides for cascading counters for n bit synchronous application without additional gating. Instrumental in accomplishing this function are two count-enable inputs (P and T) and a ripple carry output. Both count-enable inputs must be HIGH to count. The T input is fed forward to enable the ripple carry output. The ripple carry output thus enabled will produce a high level output pulse with a duration approximately equal to the high level portion of QA output. This high level overflow ripple carry pulse can be used to enable successive cascaded stages. HIGH-to-LOW level transitions at the enable P or T inputs of the 74AS163 may occur regardless of the logic level on the clock.
The 74AS163 features a fully independent clock circuit. Changes made to control inputs (enable P or T, or load) that will modify the operating mode will have no effect until clocking occurs. The function of the counter (whether enabled, disabled, loading or counting) will be dictated solely by the conditions meeting the stable set-up and hold times.
Switching specifications at 50 pF
Switching specifications guaranteed over full temperature and VCC range
Advanced oxide-isolated, ion-implanted Schottky TTL process
Functionally and pin-for-pin compatible with Schottky and low power Schottky TTL counterpart
Improved AC performance over Schottky and low power Schottky counterparts
Internal look ahead for fast counting
Carry output for n-bit cascading
Load control line
16 pin DIP. Actual brand may vary from picture.