As I’m building my own IQ house control system, I need to have RTC in my system. So started playing with MCP7940N and i2c interface. I’m using Raspberry Pi for my experiments with i2c/SPI.

Construction is pretty simple, just use MCP7840N Datasheet.

 

Then you can start checking with Pi. In my case, I’m using RevB, so my bus has number 1 and RTC got 0x6f address.

what next? Check address 0x00, if the onboard oscilator is running

0x00 means NO, it’s NOT running. So, turn it on

and check few times address 0x00

Increments! Great. That means – our oscilator is working = our clock is working too. Next step is to set current time and date. But wait. How are values stored and read/written? RTC uses BCD encoding for values, that means, eg. number 94 is stored as 9 and 4, both in 4 bits = 9 is stored as 1001 and 4 as 0100, in hexadecimal 0x94. Easy? Eg. day is using 6 bits, upper two for 0123 values, bottom bits for 0-9, so 31th will be stored as 0x31. For more details please read the PDF – RTCC Memory map. So back to setting the date & time:

Fine. But if we’re setting ’00’ as seconds – address 0x00 – why value 0x80? 7th bit = ST – onboard oscilator enabled.

I’ve written simple Python code to read from RTC and print out the value:

Just run by

Voila! :) RTC is up and running. I’d like to check tomorrow (resp. today) morning, if everything is still working correctly, and can create next module for my IQ house control system :)

S PCA9555D