Sorgelig wrote:i found problem with M41T81S chip. After some time in power off it starts to show wrong date - several days behind the actual date.
I don't know if there is problem with particular chip or any M41T81S will work this way.
I have two other boards with M41T81 and MCP79410 - both keep time reliably.
The difference between M41T81 and M41T81S is very minor - the second one has flag of low battery voltage.
When I tried again today after the MiSTer had been powered off for 18 hours, the date and time was same as when I powered it off yesterday.
This tells me that the crystal isn't working. I used SER4080CT-ND from digikey.
Lucky for me I have some old fashion crystals (they looks like a tiny pencil with legs) previously used with another RTC chip. There's plenty of room for it on RTC and this time it's working. Powered off for 5 mins after setting the time and when I powered on, time had advance correct amount of time.