Drive 74HC595 with only one pin?
CuriousOne
Posts: 931
Hello. I want to do a complex led scrolling marque using basic stamp, so a lot of 74HC595 -s will be involved (I know and I don't want to use MAX7219, as this is educational project). Is it possible somehow to drive 74HC595 with only one pin, without need for CLK/CS pins? Just one pulse come in - output 1 get enabled, next pulse come in - output 2 got enabled and so on, in loop.
Comments
CuriousOne,
The answer is no. You're wanting to use a single pin for both data and clock signals. Although some devices allow this ("one-wire protocol") the 74HC595 is not one of them.
The data bits are clocked in on the rising (low-to-high) transition of the shift register clock signal, rather than as different durations of signal high or low time (for example, a 20us pulse for a zero and a 40us pulse for a one).
Walter
And I don't meant to use different duration. I see it like this:
INPUT HIGH=OUTPUT 1 HIGH
INPUT LOW=OUTPUT 1 LOW
INPUT HIGH=OUTPUT 2 HIGH
INPUT LOW=OUTPUT 2 LOW
INPUT HIGH=OUTPUT 3 HIGH
and so on.
http://forums.parallax.com/showthread.php/134106-Updated-17seg-LED-driver-for-PPDB-now-available.?highlight=segment
As the Prop can do timings in a separate cog without any interruption it's not to hard, Not sure about the Basic stamp (can you turn off any interrupts?)
You simple use two RC filters and then you do 90%+ high state on clock pin most of the time, so the data will not be below 2.7V if you want a 1 and latch pins will not see an acceptable rising edge.
I got the idea from,
http://www.romanblack.com/shift1.htm
If you're willing to use a different IC then there are lots of options. A small microcontroller could be programmed to generate your sequence with a single input pin as a trigger. An inexpensive Arduino clone would be one of many possible inexpensive solutions.
If so then the HC238 is what you need (It's like a HC138 but active high output)
Though there is no state when all off, so you can only use 7bits unless you use a 4th Stamp pin for CS1
http://www.mouser.com/Search/Refine.aspx?Keyword=HC238&Ns=Pricing%7c0&FS=True
Could add a 4bit counter going it to HC238 if you just want to count single pin pulses.
You would have to connect HC238's CS1 to main clock pulse as your table shows that when low the output should be off.
2nd Options would be to use a single HC164
To make your own overbeck counter/ring counter as I can not find a IC that does that.
Stamp does one short pulse followed by 8 long clock pulses
wired up like this: http://m.eet.com/media/1128149/12116-figure_1.pdf
But HC164 does NOT have output enable, so off when clock is low you could do a low-side master Nmosfet (using same clk) on what I take is 8LEDs?