Monday, October 20, 2014

Wiz820io power on problem.

Some months ago I have found that the wiz820io is not recovering correctly on power failure. There is needed to reset it after power on. Sadly I have connected the reset of atmega to wiz820io. There was two possible solutions I had on mind, first to use the Ethernet INT signal that is not used as separate reset, and second to add a watchdog chip to the board.

I went first with with the watchdog variant. I have found, and bought MCP130 which is a voltage supervisory device. It is designed to keep a micro controller in reset state until the system voltage has reached the proper level and it is stabilized. By looking at the schematic I have found that the R22 resistor, 10k reset pull-up, can be replaced by MPC130. The MPC130 is available in SOT-23 package and pins matches the footprint of R22 resistor except GND signal. I have replaced it and soldered extra wire form nearest ground to watchdog.

I happy to say that the watchdog keeps the reset long enough to satisfy the wiz820io and the main board is now recovering from complete power loss without any issue. Just for completeness the wiz820io has its own 10k pull-up.

If there will be some new revision of the main board, I will replace the R22 with  MCP130 and free the INT line to wiz820io for digital input. The price of MCP130 is 0.25 EUR for 1 piece in our local shop and it is well worth it :).

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