The CDP68HC68T1 Real-Time Clock provides a time/calendar function, a 32 byte static RAM, and a 3 wire Serial Peripheral Interface (SPI Bus). The primary function of the clock is to divide down a frequency input that can be supplied by the on-board oscillator in conjunction with an external crystal or by an external clock source. The internal oscillator can operate with a 32kHz, 1MHz, 2MHz, or 4MHz crystal. An external clock source with a 32kHz, 1MHz, 2MHz, 4MHz, 50Hz or 60Hz frequency can be used to drive the CDP68HC68T1. The time registers hold seconds, minutes, and hours, while the calendar registers hold day-of-week, date, month, and year information. The data is stored in BCD format. In addition, 12 or 24 hour operation can be selected. In 12 hour mode, an AM/PM indicator is provided. The T1 has a programmable output which can provide one of seven outputs for use elsewhere in the system. Computer handshaking is controlled with a wired-OR interrupt output. The interrupt can be programmed to provide a signal as the result of:

1. An alarm programmed to occur at a predetermined combination of seconds, minutes, and hours.
2. One of 15 periodic interrupts ranging from sub-second to once per day frequency.
3. A power fail detect. The PSE output and the VSYS input are used for external power control. The CPUR output is available to reset the processor under power-down conditions. CPUR is enabled under software control and can also be activated via the CDP68HC68T1's watchdog. If enabled, the watchdog requires a periodic toggle of the CE pin without a serial transfer.