Need to modify reset on Wiz820io


I have retrofit a Wiz820io on a board that used a Wiz812MJ. I don’t have a dedicated reset line on the mother/main board. I am unable to reset the Wiz820io and it does not function. I can manually reset it using a jumper on the board but that is not possible when in actual use.

I now see the problem is that the Wiz820io has a 10k pullup on the RESETn line, and the Wiz812MJ does not. My design relies on my controller to pull the RESETn line low through a 5k resistor, so clearly the W5200 RESETn pin never sees a low level input. I have two ideas/questions:

Q1: Can you post the location of the pullup resistor on the RESETn line? I can’t trace it visually since it jumps to the other side of the board under the Magjack. I should be able to remove this and resolve the problem. NOTE: I am using Wiz820io Rev 1.0.

Q2: Is there a way to force the W5200 into a known state without using the RESETn line? I tried to send a software reset by setting bit7 of MODE register but that does not work. Can I assert/release CSn several times (or some other sequence) so that the W5200 will recognize me writing the reset bit to the MODE register?

Thank you.


The Pull-up resistor of RESETn is located on bottom of PCB(WIZ820io).
I have attached file as below. Please, refer to it.


Hi, ags

Q2) Did you mean that software reset doesn’t work or operation like H/W reset doesn’t work??
If you reply, I will answer.

Best regards.


Hi hjjeon0608,

Q2: Imagine that you have no connection to the RESETn pin of the Wiz820io module. It is floating, which means the 10k pullup on the 820io module will keep the W5200 RESETn pin tied to VDD. You power up the module, but what state is it in without performing a hardware reset (using the RESETn pin)?

I have tried to force a software reset by writing ‘1’ to bit7 of common MODE register, but this does not reset the Wiz820io. Is there some sequence of steps that I can use to perform a software reset that will work - if the Wiz820io is in a state after power-up without reseting using the RESETn pin?

Is this question clear? Thank you for your answer.


Hi, ags

I understood.

W5200 has power of reset inside. So if there is pull up resistor to reset pin, w5200 can be reset after power up.

But if not reset, I recommend to connect power of reset IC outside w5200 additionally. Then you don’t need to handle reset.

And power of reset must be satisfied with our chip reset timing.

And 812MJ is used by w5100, w5100 has pull up and down resistor inside chip. So 812MJ don’t have pull up and down resistors.

Best regards.