EZ80F91MCU ZILOG [Zilog, Inc.], EZ80F91MCU Datasheet - Page 186

• Manufacturer Part Number: EZ80F91MCU
• Description: eZ80Acclaim-TM Flash Microcontrollers
• Manufacturer: ZILOG [Zilog, Inc.]

PS019209-0504
Real-Time Clock Alarm
Real-Time Clock Oscillator and Source Selection
Real-Time Clock Battery Backup
Real-Time Clock Recommended Operation
The clock can be programmed to generate an alarm condition when the current
count matches the alarm set-point registers. Alarm registers are available for sec-
onds, minutes, hours, and day-of-the-week. Each alarm can be independently
enabled. To generate an alarm condition, the current time must match all enabled
alarm values. For example, if the day-of-the-week and hour alarms are both
enabled, the alarm only occurs at the specified hour on the specified day. The
alarm triggers an interrupt if the interrupt enable bit, INT_EN, is set to 1. The
alarm flag, ALARM, and corresponding interrupt to the CPU are cleared by read-
ing the RTC_CTRL register.
Alarm value registers and alarm control registers can be written at any time. Alarm
conditions are generated when the count value matches the alarm value. The
comparison of alarm and count values occurs whenever the RTC count incre-
ments (one time every second). The RTC can also be forced to perform a compar-
ison at any time by writing a 0 to the RTC_UNLOCK bit (the RTC_UNLOCK bit is
not required to be changed to a 1 first).
The RTC count is driven by either the on-chip 32768 Hz crystal oscillator or a 50/
60 Hz power-line frequency input connected to the 32 KHz RTC_X
internal divider compensates for each of these options. The clock source and
power-line frequencies are selected in the RTC_CTRL register. Writing to the
RTC_CTRL register resets the clock divider.
The power supply pin (RTC_V
power 32 KHz oscillator is isolated from the other power supply pins on the
eZ80F91 device. To ensure that the RTC continues to keep time in the event of
loss of line power to the application, a battery can be used to supply power to the
RTC and the oscillator via the RTC_V
nected together on the printed circuit assembly.
Following a initial system reset from a powered-down condition of V
V
abled. The following procedure is recommended to initialize the Real-Time Clock:
•
•
DD
Write to RTC_CTRL to set RTC_UNLOCK and disable the RTC counter; this
action also clears the clock divider
Write values to the RTC count registers to set the current time
_RTC, the counter values of the RTC are undefined and all alarms are dis-
P R E L I M I N A R Y
DD
) for the Real-Time Clock and associated low-
DD
pin. All V
SS
(ground) pins should be con-
Product Specification
OUT
eZ80F91 MCU
DD
Real-Time Clock
pin. An
and
167