MiSTer FPGA Forum

Hello! I built an RTC board with an M41T00 RTC chip, and it doesn’t work as expected.

If I set the network time at for example 11:45, and then disconnect the network cable and power down the Mister, the time will be 11:00 the next time I power it on, and the clock will start from there, every subsequent reboot.

I’ve tried various load capacitors (none, 6,8, 10 and 12 pF), and nothing changes. I’ve also tried two different 32,768 kHz crystals.

The battery is brand new and measures fine.

It seems the M41T00 can not be used for this? It’s not actually in the schematic but I thought I’d try it as I had it on hand. Comparing data sheets, it seems really similar to the officially supported M41T81.

The M41T00 returns the data in a slightly different format than the M41T81

M41T00 does time to the nearest second and sends the entire clock data over in 9 bytes, but the M41T81 does to the nearest 100th of a second and sends the data in 10 bytes.

so setting it from the mister is setting it to something other than the current date and time due to the misalignment (so some things read back OK, but others are wildly different)