Rasp Pi - MCP23017 and RTC DS3231 in harmony

Question

Hello there,

I've got three raspberry pis rev 2's (wheezy 12-2012) that have been purring away for over a year now performing simple logging tasks. Attached to each of them (SDA/SCL) is a chronodot DS3231 which has kept time accurately as glossy brochure assured.

Now I wish to expand the capabilities of each PI using the MCP23017 port expander. I've found plenty of tutorials and I can make LED's blink as per bash or python script. What I havent found easy to get my head around is attaching an RTC to this chip. The current RTC does a wonderful job of keeping time around both warm and cold restarts which is important to the logging I analyse. I am not using NTP btw.

This is closest I have found

http://www.puzsar.hu/raspberry_ext/

but the original page/site has gone.

How do I wire the clock to the MCP23017?, Is that two GPA's to the RTC SDA/SCL? How does the raspi OS 'see' through MCP23017 at restart for the local clock to be set?

Thank you

Anthony

shabaz · Accepted Answer

Hi Anthony,

The first byte of I2C transmission contains the address, and all devices have a 7-bit (or 8-bit depending on how you look at it) address. These are hard-coded in devices, but some device (like the MCP23017) have a few bits that are controlled by pins, for some address adjustment. Since the MCPxx has three pins (A0..2), this mean you could connect 2^3 (i.e. 8) of them on the same I2C bus, all with unique addresses. Figure 1-2 in the MCP datasheet shows the values of the remainder bits of the address which are hard-coded inside the device.

In a similar way, the DSxxx device you refer to has a hard-coded address too - it cannot be changed. The DS device does not have any A0,etc pins, so there is no flexibility to modify the address slightly. You could only connect one to the bus. The 0x68 mentioned refers to the adafruit tutorial, which is using a different DS1307 device. The datasheet for the DS1307 shows (in figure 4) the address to be 0x68 (1101000, i.e. binary 110 representing 6, and 1000 representing 8). The DS3231 datasheet will have a similar diagram, with a different value.

The bus doesn't actually use any termination, there will be a couple of resistors already on the Pi (I'm guessing), these create the bus default high state, and then any device on the bus is free to pull it low to begin toggling data on the wires. The pi is designated the master device, and the MCP and DS devices are slaves (they can only operate as slaves, they have no capability inside the chip to do anything else).