M48TMH1 STMICROELECTRONICS [STMicroelectronics], M48TMH1 Datasheet - Page 13

• Manufacturer Part Number: M48TMH1
• Description: 5V PC REAL TIME CLOCK
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

Table 10. Square Wave Frequency/Periodic Interrupt Rate
UPDATE CYCLE
The M48T86 executes an update cycle once per
second regardless of the SET bit (Register B; Bit
7). When the SET bit is asserted, the user copy of
the double buffered time, calendar, and alarm
bytes is frozen and will not update as the time in-
crements. However, the time countdown chain
continues to update the internal copy of the buffer.
This feature allows accurate time to be main-
tained, independent of reading and writing the
time, calendar, and alarm buffers. This also guar-
antees that the time and calendar information will
be consistent. 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 in all three positions.
There are three methods of accessing the real
time clock that will avoid any possibility of obtain-
ing inconsistent time and calendar data. The first
method uses the update-ended interrupt. If en-
abled, an interrupt occurs after every update cycle
which indicates that over 999ms are available to
RS3
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
RS2
Register A Bits
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
RS1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
RS0
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
Frequency
read valid time and date information. If this inter-
rupt is used, the IRQF bit (Register C; Bit 7) should
be cleared before leaving the interrupt routine.
A second method uses the Update-In-Progress
(UIP) bit (Register A; Bit 7) to determine if the up-
date cycle is in progress. The UIP bit will pulse
once per second. After the UIP bit goes high, the
update transfer occurs 244µs later. If a low is read
on the UIP bit, the user has at least 244µs before
the time/calendar data will be changed. Therefore,
the user should avoid interrupt service routines
that would cause the time needed to read valid
time/calendar data to exceed 244µs.
The third method uses a periodic interrupt to deter-
mine if an update cycle is in progress. The UIP bit
is set high between the setting of the PF bit (Reg-
ister C; Bit 6). Periodic interrupts that occur at a
rate greater than t
formation to be reached at each occurrence of the
periodic interrupt.The reads should be completed
within 1/(t
read during the update cycle.
8.192
4.096
2.048
1.024
None
256
128
512
256
128
64
32
16
8
4
2
Square Wave
PL/2
Units
+ t
kHz
kHz
kHz
kHz
Hz
Hz
Hz
Hz
Hz
Hz
Hz
Hz
Hz
Hz
Hz
BUC
BUC
) to ensure that data is not
allow valid time and date in-
976.5625
1.953125
3.90625
122.070
244.141
488.281
3.90625
Period
7.8125
7.8125
15.625
31.25
None
62.5
Periodic Interrupt
125
250
500
M48T86
Units
ms
ms
ms
ms
ms
ms
ms
ms
ms
ms
ms
us
us
us
us
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