what I was planning was to scan across the blank-select pin (like a chip-select or latch) to make sure only 1 digit (display module) is eating led current at a time. I can't change the logic but I assume the current is mostly due to leds and that they were old, inefficient leds.
I remember when I first started (70's) that the usual led series resistor was 120 or 150 or maybe 180 ohms. now, I generally throw in a 1k (all for 5v) since modern leds are lots better than first or 2nd gen leds.
for the clock, I need to find some small pt src leds that look about the same size as these dots are, to make a center colon indicator. maybe some smd leds will do the trick but even then, I think those might be too large for the dot size I want.
I might made a board that takes 4 or 6 of these and gives a semi-simple i2c interface to them, so that would just be clk/data/5v/gnd. that would make interfacing with these much simpler.
another thought I had was to use an 8bit port expander per digit, make very small boards with the PE and the led display and let users stack them horizontally, as many as needed, to create chains of them.
you need 4 bits for the bcd digit, 1 bit that you pulse to latch the value, 1 bit for the display blanking (there is no 'space' char and 0-9,a-f are all the 16 combos), and then for completeness, you need 2 bits for the left and right decimal points. that's 8 bits, yeah?
it eats up bits like crazy this way but I see no other easy and clean way to do it.
in my test bed, I sent 8 wires over to the arduino. 4 for 'enable' lines (1 per char) and 4 bcd lines. I tied blanking to a constant (for now) and I'm only strobing the enable lines as I paint numbers across the display strip.
initial thoughts were to use a 16bit port expander to handle the 4 digit clock style display but I might as well just use an arduino and it will give me more digital lines, some pwm built in, and I could set the chip up as an i2c slave or even a serial slave, for flexibility. still just 1 28pin chip if I go skinnyDIP or the 'nano style' smd chip, taking even less space up on the display board.
once the board is done, it will be generic building block. I have used adafruit i2c display strips before and this is a similar idea.
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