Re: Using 74xx574 D FF to expand PIC outputs

On Jul 16, 2:28 pm, "Peter S. May" <ps...@xxxxxxxxxxxx> wrote:
Greetings. I'm having a problem that reaches beyond my basic knowledge
of digital logic and electronics, and so I'm hoping an expert eye can
spot the mistake. (Also, please suggest a more appropriate group if I'm
in the wrong place.)

I'm working on a seemingly straightforward means of expanding the
limited outputs of a PIC 16F88 to 32 pins by way of four 74ABT574 octal
D-type flip-flops and a 2-to-4 decoder. The way the trivial design is
supposed to work is that each RBn pin of the PIC is attached to a
corresponding Dn pin on each of the four 574s. The program would assign
their data round-robin style, assigning one word to PORTB and then
clocking the destination DFFs using the demuxer. The outputs in this
experiment are LEDs run via 1K resistors (3 or 4mA).

I've had some problems, though, and they're somewhat spurious.
Sometimes, the outputs of the first 574 always echo PORTB; other times,
one or more of the 574s read correctly except for a split second of
flicker, which I'm guessing is a momentary read intended for another
output, but I have no idea why.

I've simplified the circuit to the minimum that should implement the
functionality, done away with the demuxer and three of the four 574s,
and the problem remains.

Here is the circuit:

16F88 74ABT574
---------- ---------------
| Vdd|--> +5V <-----|20 Vcc |
| Vss|--| GND |--+--|10 GND |
| | +--|1 /OE |

/OE should be held low to enable the outputs. Looks like you've left
it floating. If so, that is undoubtedly your problem. If you did
actually wire it low somehow, keep reading.

Also, do you have a decoupling capacitor (~0.1uF) between power and
ground physically close to each of the chips? Is your ground good and
solid between the PIC and the latches? Proper supply voltage for each
chip?

Sure would help if you did have a scope. :-\

.

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