DM74ALS161B

*Switching specifications at 50 pF
*Switching specifications guaranteed over full tempera-ture and V_CC 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
*Synchronously programmable
*Internal look ahead for fast counting
*Carry output for n-bit cascading
*Synchronous counting
*Load control line
*ESD inputs

  These synchronous presettable counters feature an inter-nal carry look ahead for application in high speed counting designs. The DM74ALS162B is a four-bit decade counter,while the DM74ALS161B and DM74ALS163B are four-bit binary counters. The DM74ALS161B clears asynchro-nously, while the DM74ALS162B and DM74ALS163B clear synchronously. The carry output is decoded to prevent spikes during normal counting mode of operation. Synchro-nous 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 out-put 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 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 orenable inputs.

  The DM74ALS161B clear function is asynchronous. A low level at the clear input sets all four of the flip-flop outputs LOW regardless of the levels of clock, load or enable inputs. These two counters are provided with a clear on power-up feature. The DM74ALS162B andDM74ALS163B 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 out-put 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 DM74ALS162B and DM74ALS163B are 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 addi-tional 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 cas-caded stages. HIGH-to-LOW level transitions at the enable P or T inputs of the DM74ALS161B through DM74ALS163B may occur regardless of the logic level on the clock.

  The DM74ALS161B through DM74ALS163B feature 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.