eZ80F92/eZ80F93
Product Specification
89
Real-Time Clock Alarm
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 seconds, 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. The alarm flag, ALARM, and corresponding interrupt to the CPU are
cleared by reading the RTC_CTRL register.
Alarm value registers and alarm control registers can be written at any time. Alarm condi-
tions are generated when the count value matches the alarm value. The comparison of
alarm and count values occurs whenever the RTC count increments (one time every sec-
ond). The RTC can also be forced to perform a comparison at any time by writing a 0 to
the RTC_UNLOCK bit (RTC_UNLOCK is not required to be changed to a 1 first).
Real-Time Clock Oscillator and Source Selection
The RTC count is driven by either an external 32 KHz on-chip oscillator or a 50/60 Hz
power-line frequency input connected to the 32 KHz RTC_X
OUT
pin.
An 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.
Real-Time Clock Battery Backup
The power supply pin (RTC_V
DD
) for the Real-Time Clock and associated low-power
32 KHz oscillator is isolated from the other power supply pins on the eZ80F92 device. To
ensure that the RTC continues to keep time in the event of loss of line power to the appli-
cation, a battery can be used to supply power to the RTC and the oscillator via the
RTC_V
DD
pin. All V
SS
(ground) pins should be connected together on the printed circuit
assembly.
Real-Time Clock Recommended Operation
Following a RESET from a powered-down condition, the counter values of the RTC are
undefined and all alarms are disabled. After a RESET from a powered-down condition, the
following procedure is recommended:
•
•
•
PS015309-1004
Write to RTC_CTRL to set RTC_UNLOCK and CLK_SEL
Write values to the RTC count registers to set the current time
Write values to the RTC alarm registers to set the appropriate alarm conditions
PRELIMINARY
Real-Time Clock
ZILOG [ ZILOG, INC. ]
