system upon return of power if the Alarm 2 time
has passed during loss of power. These bits
function as follows:
code is set in the year, month, date, day and
hours alarm byte. An alarm is generated every
minute with “don’t care” codes in the year,
month, date, day, hours and minutes alarm
bytes. The “don’t care” codes in all seven alarm
bytes creates an interrupt every second. As a
final example, an alarm is generated every one
of a certain day of the week, i.e., every Friday,
by specifying the “don’t care” code in the year,
month and date of month bytes.
If VTR is present: AL2_EN controls whether or
not alarm 2 is enabled as a wake-up function. If
AL2_EN is set and VTR=5V, the nPowerOn pin
will go active (low) when the date/time is equal
to the alarm 2 date/time and the power supply
will turn on the machine.
Update Cycle
If VTR is not present: AL2_REM_EN controls
whether or not alarm 2 will power-up the system
upon the return of VTR, regardless of the value
of AL2_EN. If AL2_REM_EN is set and VTR=0
at the date/time that alarm 2 is set for, the
nPowerOn pin will go active (low) as soon as
VTR comes back and the machine will power-
up.
An update cycle is executed once per second if
the SET bit in Register B is clear and the
DV0-DV2 divider is not clear. The SET bit in the
"1" state permits the program to initialize the
time and calendar bytes by stopping an existing
update and preventing
occurring.
a
new one from
The seven alarm 2 bytes may be used in two
ways. First, when the alarm time is written in
the appropriate year, month, date, day, hours,
minutes, and seconds alarm locations, the
alarm interrupt is initiated at the specified time
on the day of the week, on the date of the
month, in the year if the Alarm 2 Enable bit is
high. The second usage is to insert a “don’t
care” state into one or more of the alarm bytes.
The “don’t care” code is any hexadecimal byte
from C0 to FF inclusive. That is, the two most
significant bits of each byte, when set to “1”
create a “don’t care” situation. An alarm is
generated each year if the year byte is set to a
“don’t care” condition. Similarly, an alarm is
generated every month with “don’t care” codes
in the year and month bytes. An alarm is
generated on every day of every month of every
year with “don’t care” codes in the year, month,
date of month and day of week bytes. An alarm
is generated each hour, every day of the month,
every month, every year when the “don’t care”
The primary function of the update cycle is to
increment the seconds byte, check for overflow,
increment the minutes byte when appropriate
and so forth through to the year of the century
byte. The update cycle also compares each
alarm byte with the corresponding time byte and
issues an alarm if a match or if a "don't care"
code is present.
The length of an update cycle is shown in Table
67. During the update cycle, the time, calendar
and alarm bytes are not accessible by the
processor program. If the processor reads these
locations before the update cycle is complete,
the output will be undefined. The UIP (update in
progress) status bit is set during the interval.
When the UIP bit goes high, the update cycle
will begin 244 ms later. Therefore, if a low is read
on the UIP bit, the user has at least 244 ms
before time/calendar data will be changed.
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