74HCT190

The 74HCT190 is high-speed Si-gate CMOS devices and are pin compatible with low power Schottky TTL (LSTTL). They are specified in compliance with JEDEC standard no. 7A.

The 74HCT190 is asynchronously presettable up/down BCD decade counters. They contain four master/slave flip-flops with internal gating and steering logic to provide asynchronous preset and synchronous count-up and count-down operation.

Asynchronous parallel load capability of the 74HCT190 permits the counter to be preset to any desired number. Information present on the parallel data inputs (D₀ to D₃) of the 74HCT190 is loaded into the counter and appears on the outputs when the parallel load (PL) input is LOW. As indicated in the function table, this operation overrides the counting function.

Counting is inhibited by a HIGH level on the count enable (CE) input. When CE is LOW internal state changes are initiated synchronously by the LOW-to-HIGH transition of the clock input. The up/down (U/D) input signal determines the direction of counting as indicated in the function table. The CE input may go LOW when the clock is in either state, however, the LOW-to-HIGH CE transition must occur only when the clock is HIGH. Also, the U/D input should be changed only when either CE or CP is HIGH.

Overflow/underflow indications of the 74HCT190 are provided by two types of outputs, the terminal count (TC) and ripple clock (RC). The TC output is normally LOW and goes HIGH when a circuit reaches zero in the count-down mode or reaches "9" in the count-up-mode. The TC output will remain HIGH until a state change occurs, either by counting or presetting, or until U/D is changed. Do not use the TC output as a clock signal because it is subject to decoding
spikes. The TC signal is used internally to enable the RC  output. When TC is HIGH and CE is LOW, the RC output follows the clock pulse (CP). This feature of the 74HCT190 simplifies the design of multistage counters as shown in Figs 5 and 6.

In Fig.5, each RC output of the 74HCT190 is used as the clock input to the next higher stage. It is only necessary to inhibit the first stage to prevent counting in all stages, since a HIGH on CE inhibits the RC output pulse as indicated in the function table. The timing skew between state changes in the first and last stages is represented by the cumulative delay of the clock as it ripples through the preceding stages. This can be a disadvantage of this configuration in some applications.

Fig.6 shows a method of causing state changes to occur simultaneously in all stages. The RC outputs propagate the carry/borrow signals in ripple fashion and all clock inputs are driven in parallel. In this configuration the duration of the clock LOW state must be long enough to allow the negative-going edge of the carry/borrow signal to ripple through to the last stage before the clock goes HIGH. Since the RC output of any package goes HIGH shortly after its CP input goes HIGH there is no such restriction on the HIGH-state duration of the clock.

In Fig.7, the configuration of the 74HCT190 shown avoids ripple delays and their associated restrictions. Combining the TC signals from all the preceding stages forms the CE input for a given stage. An enable must be included in each carry gate in order to inhibit counting. The TC output of a given stage it not affected by its own CE signal therefore the simple inhibit scheme of Figs 5 and 6 does not apply.