Will this rtc module for raspberry pi on ebay be compatible with the rfm12b? So far its the only rtc module for raspberry Pi I've found that includes physical pins to allow stacking of additional modules. Is it fairly likely to work with an rfm12pi (v1 or 2)?
The ebay listing says it "includes a I2C logic level converter allowing you to connect other 5V I2C devices to your Raspberry Pi". I guess that means in protects itself from 5v i2c devices? The rfm12pi technical document seems to say that its 3.3v, but as far as I can tell its not actually an i2c device anyway?
Re: realtime clock module for raspberry pi
The RFM2Pi board doesn't touch the I2C pins of the Raspberry pi, so my assumption is that you can plug it on top of the RTC board and it will work.
Why do you need a RTC, isn't your Pi connected to the Internet via a WiFi dongle or LAN cable?
Re: realtime clock module for raspberry pi
Thanks very much for that.
Do I need an rtc? The use case is a system for my father, who lives in a rural area, prone to power brown outs and outages. We want to reliably record the state of the house power supply. He can get refunds from the power company with reasonable evidence of a supply issue. Graphs or tables of Vrms from an emon setup should do nicely. Its as much about making a point about their quality of service, as recovering money.
I don't know for sure that this needs an rtc. On a resumption of supply, his pi will ntp from his router, which will be doing its own ntp update, and perhaps has its own rtc anyway. I suppose there's scope for the pi to have some seconds of runtime with a poor time value. Having an onboard rtc would just make it certain that the pi knows its time, at power on.
Re: realtime clock module for raspberry pi
If there is a power failure, isn't the Pi going to shut down too? I think, it would make more sense to get the cheapest computer UPS, your Pi will run on it for quite a while. It just needs NTP for a short period upon booting to get its clock right.
Re: realtime clock module for raspberry pi
To follow up on this, I first bought the Wyolum Epoch rtc board from Seeed Studio, seems good, cheap delivery. It doesn't have the long pins to allow stacking, but given that the rtc board and the rfm12pi use different raspi io pins, I just soldered on some strips of header. Then added hot glue to support the header better. Put it together, followed various guides on enabling rtc, and it seems to be working nicely. Have a photo if anyone wants, it looks like this forum doesn't host photos directly.
I've also bought another rtc board from AB Electronics, which looks to be the same as my initial ebay link, but from the manufacturer, cheaper than ebay, with better postage. Still a bit more than the Wyolum board, but comes with the stackable header. Haven't tried it yet but I expect it will work perfectly. AB Electronics seem to have a few pretty nice raspi daughterboards, such as a 1-wire interface board with an rj12 socket, which seems pretty common for 1-wire sensors.
Thanks for the help!