SN74HC161N binary counter

28 April 2018
When using LED chips — either segmented displays or dot matrices — there is typically one set of inputs that are powered in a fixed round-robin order, and in the ideal case all that is needed from the display controller is a signal indicating when to switch to the next power line in sequence. I did manage to find a chip that actually did this, but it was a low-volume legacy chip that sold for about €10 each, and I did not want to go down the road of programming a microcontroller to do the same task.

The Texas Instruments SN74HC161N (Farnell 2407119; datasheet) is a 4-bit binary counter, which when combined with a binary decoder such as the 74HC238 which expands binary out, provides a way for a single signal line to control the activation of parallel power bus. Had I known about these chips earlier, several of my past circuits would have been designed differently.

Pin-out

The counter data-sheet dates back to 1996, a time when presentation was somewhat different and it seems certainly less exhaustive, and the table below summarises what I worked out from reading it:

Formal name	V_cc	RCO	Q_A	Q_B	Q_C	Q_D	ENT	LOAD
Purpose	V_cc	Maximum	Binary output				Enable T	Load
Pin number	16	15	14	13	12	11	10	9
Pin number	1	2	3	4	5	6	7	8
Purpose	Clear	Clock	Load input				Enable P	Ground
Formal name	CLR	CLK	A	B	C	D	ENP	GND

Unusually the data-sheet for this chip does not list details for the pins, instead giving a combination of logic diagrams and verbose descriptions to piece information together from. Based on this, I managed to distil the following:

Maximum: Formally known as RCO (ripple carry output), this is asserted if the binary output is at is maximum value and Enable T is held high. Intended for cascading counters.
Enable T & Enable P: Clock toggling only has an effect if both Enable pins are high. Enable T (Pin 10) also enables the Maximum pin. For my purposes I simply tie both high and forget about them.
Load: This pin is active-low, and when activated the Load input is copied to the output pins on the next clock cycle.
Clock: Clocks the whole chip, but main practical purpose is to signal a counter increment. Incrementing happens on the rising edge.
Clear: This pin is active-low, and activating it immediately clears all the output pins.

In practice the pins of interest are Clear as it provides a way to force-sync the cycle to a known state, and Clock which does the actual incrementing.

How things might have been

Prior to the (first) 7-segment LCD display which made use of only four LED chips, I was in the process of designing a much larger display on 10x15cm stripboard, and had ordered in some breakout mini-PCBs to avoid the type of bus wiring that I later used in the 17-segment LCD display. Each LED chip would have its own counter, all of which would be driven by a common clock, and each chip would be activated when its counter was at a preset value. How this value would be defined was an unsolved problem, and building of the circuit was never started.

Concluding remarks

The SN74HC161N dates from the mid-1990s, which was a time when it seems electronic circuits were still mainly hard-wired, whereas these days the usual solution is to drop a microcontroller onto the board and instead do things in firmware. Back then it was discrete components such as NAND gates, whereas these days it is stringing together chips using I²C. This feels more like the electronics I remember from school-days. As an aside, the effect of a floating pin were a lot more random than more modern components.