ez80f91 ZiLOG Semiconductor, ez80f91 Datasheet - Page 168

• Manufacturer Part Number: ez80f91
• Description: Ez80 Acclaimplus!? Connectivity Assp Ez80f91 Assp
• Manufacturer: ZiLOG Semiconductor

PS027001-0707
Real-Time Clock Alarm
Real-Time Clock Oscillator and Source Selection
Real-Time Clock Battery Backup
Real-Time Clock Recommended Operation
The clock is 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 is 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 a specified hour on a 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 reading the
RTC_CTRL register.
Alarm value registers and alarm control registers are 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 increments (one time every second). The
RTC is also forced to perform a comparison 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 32 kHz RTC oscillator or an external
50/60 Hz CMOS-level clock signal (typically derived from the AC power line frequency).
The on-chip oscillator requires an external 32 kHz crystal connected to RTC_X
RTC_X
used, connect it to RTC_X
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
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 is used to supply power to the RTC and the oscillator via the RTC_V
V
Following a initial system reset from a power-down condition of V
counter values of the RTC are undefined and all alarms are disabled. The following proce-
dure is recommended to initialize the Real-Time Clock:
•
SS
Write to RTC_CTRL to set RTC_UNLOCK and disable the RTC counter; this action
also clears the clock divider
(ground) pins must be connected together on the printed circuit assembly.
OUT
as shown in
Figure 35
OUT.
DD
) for the RTC and associated low-power 32 kHz
on page 159. If an external 50/60 Hz clock signal is
Product Specification
DD
and V
eZ80F91 ASSP
Real-Time Clock
DD
DD
_RTC, the
IN
pin. All
and
160