HD74LS191P中文资料瑞萨数据手册PDF规格书

Synchronous Up / Down 4-bit Binary Counter (single clock line)

Synchronous operation is provided by having all flip-flops clocked simultaneously so that the steering logic. This mode of operation eliminates the output counting spikes normally associated with asynchronous (ripple clock) counters. The outputs of the four master-slave flip-flops are triggered on a low-to-high-level transition of the clock input if the enable input is high. The direction of the count is determined by the level of the down / up input. When low, the counter counts up and when high, it counts down. Level changes at the down / up input should be made only when the clock input is high. This counter is fully programmable; that is, the outputs may be preset to either level by placing a low on the load input and entering the desired data at the data inputs. The output will change to agree with the data inputs independently of the level of the clock input. This feature allows the counters to be used as modulo-N dividers by simply modifying the count length with the preset inputs. The clock, down / up, and load inputs are buffered to lower the drive requirement which significantly reduces the number of clock drivers, etc., required for long parallel words.