Hi All,

I'm trying to repair a PCB with a TN87C196KC processor on it, it also has a KAM regulator, this should pull the reset pin low for a period then allow it to go high.
At the moment reset never goes high, I'm assuming that there should be a weak pullup somewhere? Is this correct? I've spent quite a while trying and failing to find it but will persevere if I was sure it should be there? Google ai says there should be but i don't 100% trust it!!

Cheers Geoff

 

Do you have a datasheet for the processor? I remember an Intel part with a similar number; is this a 2nd source part?

Here is a link to the Intel part
87c196kc automotive.pdf

Here is an alternate link
Rochester Electronics TN87C196KC-20 Datasheet | View Rochester Electronics TN87C196KC-20 Microcontrollers Datasheets - Octopart

The way I read the datasheet it is a RESET input and open drain output. It needs an external pullup resistor. I'm guessing 20-40KΩ

 

Thanks, that's the way I understood it but it's quite an old part! It's on a tractor transmission control.

I've tried and tried to find a pull up with multi meter but can't do maybe a failed via? It played up intermittently until it stopped.......

 

I'm not familiar with these processors, is it likely the reset could go to any other ic on the board? And one of them be holding it down?

It has a AM28f512 on its data bus, is this likely to hold the program, or is the program likely on the processor memory? Or both?

There is also a MS6264l on the bus which I assume is ram?

I'm slightly concerned that if the processor has failed and some/all of the programme is on its memory I might not be able to repair it!

Cheers Geoff

 

A bit more prodding, I've found a weak (stronger than it ought be!!) pullup of 2k, the reset pin trace is also connected to a lm2901 comparator, to 1&2 output?

Better get on and trace the inputs but is this likely to be a voltage check to hold in reset until the voltage is stable?

 

Quick test with multi meter,
Out1& out2 are both connected to reset.

In2- and in1+ are connected.

In2- has a 10k pullup on.

This all seems a bit strange to me, I'd kind of expect the inputs tied if the outputs are? I'm minded to pull that one off the board and check again?

Or is this a common layout I just haven't come across before?

 

Just googled it!
So either comparator can pull reset low ?
It's not a pull up, it's a voltage divider to put 2.5v on negative input of one and positive of the other

 

Slightly annoyingly, it's now working, power up short delay and reset is pulled high.

When I started prodding reset was held low, in1- was .09v in2+ was 2.3v and the reference (in1+ & in2-) was 2.5v.

The second time I checked in2+ it rose to 4.8v and reset went high.

So I guess there is a dodgy joint/trace somewhere, I'll resolder that pin, I've just got a horrible feeling I'll put it all back together and will happen again.

Is this set up to protect against unstable power supply, the + and negative inputs have capacitors on the smd but they look reasonably big?

I'd like to understand what's going on.
Cheers Geoff

 

Hi All,

I'm trying to repair a PCB with a TN87C196KC processor on it, it also has a KAM regulator, this should pull the reset pin low for a period then allow it to go high.
At the moment reset never goes high, I'm assuming that there should be a weak pullup somewhere? Is this correct? I've spent quite a while trying and failing to find it but will persevere if I was sure it should be there? Google ai says there should be but i don't 100% trust it!!

Cheers Geoff

How did you establish that it never goes high?

 

How did you establish that it never goes high?

Pc based oscilloscope