Issue 140 March 2002
Spy-Size Event Logger

And Now, Back To The Show

As a time-keeping reference, the DS1678 requires a standard 32-kHz crystal but no external crystal capacitors. Despite the fact that communications requires a Vcc of 5 VDC, Vbat allows the whole enchilada (minus communication) to continue operating as long as the attached battery remains above 2.6 VDC. Active current at 5 VDC is about 2 mA. When it’s running on battery power, the current drops to about 0.5 µA (about 4.5 mAh per year).

The 2000-compliant RTC is coded in BCD format down to seconds. The hours are available in either standard 12-hour (using a.m. and p.m.) or military format. Missing are the familiar I2C user-programmable address inputs A0–A2, which allows multiple devices to be set to different addresses and coexist on the same bus. Instead, to keep the pin count low, it is assumed there will be only one DS1678 (RTC) on the bus; as such, the device doesn’t need the multiple address capabilities of other devices (i.e., serial EEPROM). The ’1678 is a registered device, which means you must use the registered read and write formats to access all of the data registers within the device.

Figure 3 shows that the registers are divided into three main groups, the RTC, user, and event groups.