Xycom XVME-113, Руководство пользователя

Страница: 50 / 75

Chapter 3 – Real Time Clock
3-14
3.5.4
RTC Alarm Function
The RTC alarm function can be used to determine when the RTC time has reached a specified time, without having to
read the entire set of RTC registers. The alarm time is specified by the user, and when the RTC time reaches the alarm
time, the AF is set. The alarm registers (64 Hz, Seconds, Minutes, Hours, Day of Week, and Day) each have an enable
(ENB) bit which controls whether that register is used in the comparison. Two or more alarm registers may be enabled
simultaneously.
The following flow chart and instructions demonstrate the procedure for setting up the RTC alarm function.
•
•
•••••••••••••••••••••••••••••
•
Clock Running ... •
•••••••••••••••••••••••••••••
•
•••••••••••••••••••••••••••••
•
Set the alarm time and • Enable only those alarm registers that are to be
• set appropriate ENB • used in the comparison.
• bits in alarm registers •
•••••••••••••••••••••••••••••
•
•••••••••••••••••••••••••••••
•
Clear AF • Clear the Alarm Flag in Control Register A.
•••••••••••••••••••••••••••••
•
•••••••••••••••••••••••••••••
•
Monitor the alarm time •
• (Check AF) •
•••••••••••••••••••••••••••••
Follow the procedure below to setup and monitor the alarm function:
1. Since interrupts are not used, there is no need to be concerned with handling an interrupt from a the
alarm function. (The AIE and CIE bits in Control Register A should always be written with '0').
2. Determine which of the alarm registers needs to be used in the comparison. For those registers, set
the desired alarm time and set the ENB bit to '1'.
3. Clear the AF bit in Control Register A to '0'. Note: To prevent the CF bit from being cleared as well,
set the CF bit to '1' for the write operation.
4. Monitor the AF bit in Control Register A until it becomes set to '1'. When this occurs, the alarm time
has been reached. The AF bit will remain set as long as the RTC time matches the alarm time. The
AF bit can be cleared by writing a '0' to it, but will be set again immediately if the alarm condition is
still met. To avoid this, either set the alarm time to a different time, or clear all of the ENB bits of the
alarm registers.