Ls373 Problem

[tanky321]

I'm designing a Nixie tube clock, using 74LS373 latches as well as Russian 74141 equivalents. I'm noticing a strange issue. If I remove my microcontroller, all of my Data buses sit at roughly 2V, when I insert the microcontroller they drop down to about 1.7V.

The outputs of my microcontroller are push-pull as are the outputs of the latch. Any insight as to why I'm having this issue?

Also, after doing a little more digging I believe I should have used a LS374 Flip Flop. The basic operation will be very similar to a multiplexed display, I will write a BCD to the bus and then strobe the appropriate Clock line to pass the data to the Nixie driver. I am also holding the output enable line LOW.


Any ideas?

Schematic

http://www.mediafire.com/?0v5liai6270bmt1

Thanks!

Andrew

[Sch3mat1c]

The buses will naturally pull up to a few volts (2V is reasonable) due to the nature of LS logic inputs. As long as the microcontroller has its outputs set to tristate (input, DDRx = 0), the voltages should remain constant, because the input pin leakage is small (fractional uA max, whereas TTL inputs are rated at a few uA input bias). With outputs enabled, the microcontroller should have no problem driving the LS inputs (they should easily pull up to >4V and down to <0.8V, easily satisfying the guaranteed TTL voltage levels).

I'm surprised you chose LS, 74HC is widely used these days. Then again, nixies aren't exactly latest technology either.

Tim

[tanky321]

Thanks,

Heres the problem, when i connect my micro and have outputs set, I get a small reduction in bus voltage, goes down to 1.7ish volts. Ive confirmed my micro is working on a break out board with LED's, but it does nothing on the PCB. Also I'm drawing about 200mA of current on the 5V supply with the 74141s disconnected. This seems to be way to high, the 373 was getting warm, about 150F.


Andrew

[millwood]

that sounds like a hardware problem. you probably want to figure out where the current draw is (the ls373?) and go after that.

[tanky321]

I believe I figured my problem out. Looks like I forgot to connect the AVCC pin...

That is the supply to the ADC, which is Port C, which is my data bus port... Oh well, nothing a little piece of wire wrap wire cant fix!