SDAS210C - DECEMBER 1982 - REVISED JULY 1996
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The 'ALS191A are synchronous 4-bit reversible up/down binarycounters. Synchronous counting operation is provided by having allflip-flops clocked simultaneously so that the outputs changecoincidentally with each other when instructed by the steering logic.This mode of operation eliminates the output counting spikes normallyassociated with asynchronous (ripple-clock) counters.
The outputs of the four flip-flops are triggered on alow-to-high-level transition of the clock (CLK) input if the countenable (
) input is low.A high at inhibitscounting. The direction of the count is determined by the level ofthe down/up (D/U\) input. When D/U\ is low, the counter counts up,and when D/U\ is high, the counter counts down.
These counters feature a fully independent clock circuit. Changesat the control inputs ( andD/U\) that modify the operating mode have no effect on the contentsof the counter until clocking occurs. The function of the counter isdictated solely by the conditions meeting the stable setup and holdtimes.
These counters are fully programmable. Each output can be presetto either level by placing a low on the 
input and entering the desired data at the data inputs.The output changes to agree with the data inputs independently of thelevel of the clock input. This feature allows the counters to be usedas modulo-N dividers by simply modifying the count length with thepreset inputs.
CLK, D/U\, and are bufferedto lower the drive requirement, which significantly reduces theloading on (current required by) clock drivers, for long parallelwords.
Two outputs are available to perform the cascading function:ripple clock and maximum/minimum count. The latter output produces ahigh-level output pulse with a duration approximately equal to onecomplete cycle of the clock while the count is minimum (0) countingdown or maximum (15) counting up. The ripple-clock output (
The SN54ALS191A is characterized for operation over the fullmilitary temperature range of -55°C to 125°C. TheSN74ALS191A is characterized for operation from 0°C to70°C.
View more information about generic part numbers:SN54ALS191A, SN74ALS191A
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