SparkFun Forums

[TabbyArtifact]

Hi everbody. I recently got my order from sparkfun and was super excited to start messing with everything it came with. One of the things I ordered was a shift register (specifically the Shift Register 8-Bit - 74HC595) Now I was just trying to mess around with it, by getting it light up 8 leds in a pattern. The problem is that I cant even seem to get it to work. I have Vcc and ground hooked up to power and ground respectively. I have the serial input pin hooked up to the avr as well as the clock pin and the latch pin. Now I thought that you made the serial input pin either high or low(to represent a 1 or a 0) and then you set the clock pin high. That shifts in the first bit. You would do that 8 times and then set the latch pin high and it would light up the leds, but i cant seem to get that to work.

[tecoist]

Well, the obvious suspects are:

a) OE# has to be low, or you won't see anything on Qn, ever, at all.
b) MR# has to be high, or the shift register will stay in reset (all zero).
c) Don't forget to set the clock (SH_CP) pin low again after you set it to 1 ("then you set the clock pin high"--better remember the low, too).
d) Similarly, don't forget to set the latch (ST_CP) pin low again after you set it to 1.
e) Don't forget to set all of these pins to be outputs on the AVR.

[stevech]

lot of pins going on with that chip
Vcc: 5V
input logic levels: per spec sheet.

*OE - low, to enable output pins when desired, or low all the time.
*MR - high, no reset
after shifting bits as need be, pulse ST_CP to copy shift register to output parallel bit register.
Be sure to leave the data line unchanged just before and after the rising edge of SH_CP; the setup time is about 50nSec, per one data sheet I saw. Don't clock until data has been stable for the spec'd hold time.

hope that helps.

[leon_heller]

The '595 is SPI-compatible, just connect it to the SPI pins on the AVR.

[TabbyArtifact]

Man you guys are super amazing! Or i think the truth is I am super stupid... I wasn't setting the latch pin back low so it would cycle the leds the first time really quick and then just stay there. Now I have it all figured out. Thanks so much guys.

[tecoist]

I wasn't setting the latch pin back low...

I had a feeling. Of course, I have NEVER made this mistake, ever at all. No. Not ever.

[TabbyArtifact]

Hahahaha. Ya I still cant believe I didn't think about that. I guess having other people to talk to really does help out.