The TOD clock is a special purpose timer for real-time applications. TOD consists of a 24-hour (AM/PM) clock with 1/10th second resolution. It is organized into 4 registers: 10ths of seconds, Seconds, Minutes and Hours. The AM/PM flag is in the MSB of the Hours register for easy bit testing. Each register reads out in BCD format to simplify conversion for driving displays, etc. The clock requires an external 60 Hz or 50 Hz (programmable) TTL level input on the TOD pin for accurate time-keeping. In addition to time-keeping, a programmable ALARM is provided for generating an interrupt at a desired time. The ALARM registers or located at the same addresses as the corresponding TOD registers. Access to the ALARM is governed by a Control Register bit. The ALARM is write-only; any read of a TOD address will read time regardless of the state of the ALARM access bit.

A specific sequence of events must be followed for proper setting and reading of TOD. TOD is automatically stopped whenever a write to the Hours register occurs. The clock will not start again until after a write to the 10ths of seconds register. This assures TOD will always start at the desired time. Since a carry from one stage to the next can occur at any time with respect to a read operation, a latching function is included to keep all Time Of Day information constant during a read sequence. All four TOD registers latch on a read of Hours and remain latched until after a read of 10ths of seconds. The TOD clock continues to count when the output registers are latched. If only one register is to be read, there is no carry problem and the register can be read "on the fly," provided that any read of Hours is followed by a read of 10ths of seconds to disable the latching.