4 replies to this topic

[dino4net]

Hello,

 

I try to get a MAX6371 watchdog chip working for some time now.

 

When I power the MAX6371 from a 3V battery and connect it to the reset pin on a Netduino 2 or Plus 2 board, I get a nice reset every 105ms and a low on the reset pin for 170ms. It's working perfect ...

 

When I power it from the Netduino 5V or 3.3V and don't connect it to the reset pin, it's also perfect. The signal on the reset output from the watchdog is as it should be.

If I now connect it to the reset pin, a reset is every 211s and the low on the reset line maybe 1ms if lucky. Sometimes it resets, sometimes not.

 

I use a 100k resistor to pull the reset line high, also tried different values.

Tried to put a low pass filter to the input voltage ...

 

Is there someone with an idea???

 

Thanks, Guido

 

[dino4net]

Does anybody know if the Netduino 2 has a bidirectional reset pin that acts as both, input and output? 

[Paul Newton]

Hi Guido,

 

I have just looked at the Netduino plus V2 schematics.

 

The external reset appears to be an input only, I can see this because the /EXTERNAL_RESET signal is connected to the base of an NPN transistor. It can't act as an output, so it should not be affecting the reset chip.

 

Have you tried powering the reset chip from an external supply, whilst connected to the Netduino?

I am wondering whether the Netduino is affecting the 3.3V supply as it re-boots, causing the reset chip to give up.

 

On the schematic, the 3.3V on the header "+3V3_HEADER" appears to be connected to the 3.3V regulator via a transistor - it is controlled by a signal called "CTRL_OF_PWR_HEADERS" which is an output of the ARM chip.

 

The more I look, the more I think the power is being cut off when the ARM reboots. Also it looks quite hard to locate a place where you can access the unswitched 3.3V.

 

One place that looks like it might work, is the mini-jtag connector. This has +3.3V, Gnd, and /RESET pins. I guess that's perfect for a reset generator. If you were to modify the firmware, you might even be able to use one of the JTAG GPIO pins to send a don't reset signal to the reset chip using one of the other pads on the mini-jtag connector.

 

Hope this helps solve the problem - Paul

[dino4net]

Hi Paul,

 

That "CTRL_OF_PWR_HEADERS" was a good tip. I connected the watchdog to the 3.3V from the mini jtag and it works like a champ.

Not sure if I try to change the firmware or add a connector to my board.

 

Thanks, Guido

[Paul Newton]

Happy to help.

 

Would be great to see a picture.

Even better if you add a wiki page!