Hi there

I've been slowly working on this now, but I seem to be having issues with my 74LS190. I've got it set up as in the attached image:

http://imageshack.us/photo/my-images/651/74ls109.png/

It works perfectly on Multisim - resetting the 190 every 4th count with the NAND gate and passing the single to the rest of the circuit (modeled in the image as an LED) on the 4th count. However in my physical circuit its acting very strange. Most of the time it passes the single through every 2nd count, but something its randomly passes through on the 3rd, 4th, 5th etc count. And sometimes it just stops working entirely. I've tried using new components but the same things happen. Can you see anything obviously incorrect in the way I've got it set up?

Regards

 

There's not a bypass capacitor in sight. U1 & 2 need .1 μF capacitors to ground right at pin 14. How clean is your Vcc?

 

Sorry, I never showed the caps in the image but I have them on every IC in my circuit. I landed up rewiring the whole thing, which sorted out some issues, but there still seems to be something wrong with the LS190. 95% of the time it just skips count 2, so its counting 1,3,4,reset,1,3,4,reset etc. Occasionally it counts 2 but barely ever. I would say maybe my input signal isn't clean but it's coming from a Schmitt Trigger and appears to be very clean. Any other possible reasons you can think of for this? Oh, and I've tried 3 separate 190s.

On another issue, if I want to prevent counting when another part of the circuit does x, how would I do this without losing the current count on the 190. I know i can put a HIGH on the enable input which would do this, but the datasheet specifically says to only do so when the clock input is high. Is there a way to do this (besides holding the load pin low which will clear the current count), where the clock doesn't need to be HIGH? I need to inhibit counting based on a signal which is in no way related to the counter's clock signal, so the clock can be at any stage when i receive the signal to inhibit counting.

Thanks

 

Hi

I'm trying to use a 74LS32 OR gate to send a LOW signal when both inputs are LOW, however it seems to be outputting it's 'LOW' at something around 1.2V, which is not small enough to be considered LOW. How can i drop this voltage to below 0.8V without affecting the HIGH level when it is HIGH? The HIGH level is around 4.5V if that helps!

 

If you are leaving inputs floating, the unused sections are oscillating and will eventually destroy the chip. Either tie them to ground, or use a 10K resistor to pull each one up.