Resetting Microcontroller

[Dubbie]

I am attempting to repair a blender. I thought it would be a simple repair but I am a bit stuck. The problem is, the microcontroller resets every 300ms. Up till it resets, it starts up and functions normally. The strange thing is, the power rail seems stable throughout. When I probe the crystal, I can see the 4mhz clock until it dies after 300ms then 10ms later starts back up. Repeat.

Anyone have any ideas where to go from here?

Ignore the wires tacked on, they are for scope probes.

[Ian.M]

I assume you've probed around and cant find any signal that could be resetting it with that periodicity.

Some MCUs disable the oscillator in RESET.  If it was being reset by its internal watchdog timer you might see that sort of behaviour on its OSC pins.

[Dubbie]

Yes I can’t see anything else with that period. Although it is difficult to discern between signals the uC is outputting and some input signal. I will have another look and also check what is going on with the uC reset pin.

[Armadillo]

I think something is/are taking longer to complete than necessary (yes I suspect too is a watchdog interrupt event). I think should be something external.

1. Firstly confirm oscillator frequency is OK
2  then check the any external interlock switches (jumper those temporary to see troubleshoot)
3. Check the sync and gate signal coming in are OK.

[Ian.M]

I'd also be looking for stuck buttons, sensors etc, as an input combination it isn't programmed to handle could well result in a watchdog reset.

[hermit]

I'd also be looking for stuck buttons, sensors etc, as an input combination it isn't programmed to handle could well result in a watchdog reset.

Good call.  I was in repair for a long time.  When something new would come out I'd see guys replace the part they least understood instead of checking all of the stuff that hadn't changed and they understood.  I see lots of easily checked components on that board.  Eliminate them first.

[Dubbie]

There is what the reset line looks like.

Gives me somewhere to look I think!



It runs up to R8, R9, R10 and Q1. Not 100% how they work.

I have checked all the switches and they seem fine.

I am not very experienced at repair, so I appreciate the help.

[Armadillo]

I don't think is a 3.3v line power supply....
How come the Reset voltage is so low? Have you checked the power supply?

Edit: Check electrolytic cap as well. The scope show drops period about 300ms!

[Dubbie]

The power supply is -5v and ground but it seems to drop to 3.3v when it enters the board. To be honest I don’t 100% understand it. It comes from another board and is just a simple diode and cap

[Dubbie]

https://www.dataman.com/media/datasheet/Holtek/HT48x30x-1.pdf

The data sheet says anywhere between 2.5v and 5.5v is ok.

[Dubbie]

I think it must be a power problem. I put a 1000uf cap on the power rail and now the reset line looks like this:



I am still slightly lost where to look for faulty components as there are so few on the board.

To be clear the power rail looks relatively flat with a every so slight ripple.

[Armadillo]

Can you hook up external 5v power supply to troubleshoot.

Edit: wire the laboratory power supply 5v directly solder to the microcontroller for troubleshooting.

Edit: for TTL circuit, the voltage cannot deep below ~ 3.5volts. So look for dried joints from wires or connectors etc... Did you board suffered any water damage lately e..g. forming conductive path now...? The voltage at the Reset pin must be SOLID ~ say 4.8V to 5.0V.

[Dubbie]

Sorry I missed your post armadillo.

I have checked the crystal frequency. It is good.
The mains sync comes in and goes through q2 and then into the processor. It looks perfect.
The gate drive going out works fine until the uC resets and it stops

[Dubbie]

Good idea. I just hooked up 5v supply and bingo. Everything works. You guys were right. Stop chasing complicated stuff and concentrate on the most likely culprits! So this board is fine. The other high voltage one has the fault. I should be good from here. Thanks to all who gave their 5c!

[Armadillo]

https://www.dataman.com/media/datasheet/Holtek/HT48x30x-1.pdf

The data sheet says anywhere between 2.5v and 5.5v is ok.

No, it's with respect to the VDD voltage. The datasheet says 0.9VDD minimum at the reset pin.