Designing Embedded Hardware, 2nd Edition

Adding a Real-Time Clock with I²C

We saw in the previous chapter how to interface a Real-Time Clock (RTC) to a microprocessor using a SPI interface. Now let's look at how we'd do the same using the I²C interface. For this example, we'll use the tiny Philips PCF8583. It also has 240 bytes of RAM, which, like the DS1305, may be used for parameter storage. Unlike the DS1305, it does not have an integrated battery-backup system. So, you would need to provide an external battery-backup circuit. There are many other I²C RTCs available, and some do incorporate battery-fail protection. I've chosen to look at this one because it makes for a very simple example of an I²C interface.

The PCF8583 has two pins (OSCI and OSCO) for connecting a 32.768 kHz watch crystal. This crystal pulses an internal circuit that performs the timekeeping functions. The address pin, A0, determines the address of the device on the I²C bus. Most I²C chips provide several address pins, allowing a range of possible addresses to be wired. The PCF8583 has only one, to reduce the pin count of the chip. Six of its address bits are hardwired internally. Only the least significant, A0, is available to the system designer. The address configuration of the PCF8583 is shown in Figure 8-8. (Note how the transfer direction [read or write] is incorporated into the address field.)

Figure 8-8. PCF8583 addresses

Connecting A0 directly ...