71M6542F MAXIM [Maxim Integrated Products], 71M6542F Datasheet - Page 51

• Manufacturer Part Number: 71M6542F
• Description: 0.1% Accuracy Over 2000:1 Current Range Energy Meter ICs
• Manufacturer: MAXIM [Maxim Integrated Products]

2.5.4 Real-Time Clock (RTC)
2.5.4.1 RTC General Description
The RTC is driven directly by the crystal oscillator and is powered by either the V3P3SYS pin or the
VBAT_RTC pin, depending on the V3OK internal bit. The RTC consists of a counter chain and output
registers. The counter chain consists of registers for seconds, minutes, hours, day of week, day of
month, month, and year. The chain registers are supported by a shadow register that facilitates read
and write operations.
Table 42
2.5.4.2 Accessing the RTC
Two bits, RTC_RD (I/O RAM 0x2890[6]) and RTC_WR (I/O RAM 0x2890[7]), control the behavior of the
shadow register.
When RTC_RD is low, the shadow register is updated by the RTC after each two milliseconds. When
RTC_RD is high, this update is halted and the shadow register contents become stationary and are suitable
to be read by the MPU. Thus, when the MPU wishes to read the RTC, it freezes the shadow register by
setting the RTC_RD bit, reads the shadow register, and then lowers the RTC_RD bit to let updates to the
shadow register resume. Since the RTC clock is only 500Hz, there may be a delay of approximately 2 ms
from when the RTC_RD bit is lowered until the shadow register receives its first update. Reads to RTC_RD
continue to return a one until the first shadow update occurs.
When RTC_WR is high, the update of the shadow register is also inhibited. During this time, the MPU may
overwrite the contents of the shadow register. When RTC_WR is lowered, the shadow register is written into
the RTC counter on the next 500Hz RTC clock. A change bit is included for each word in the shadow
register to ensure that only programmed words are updated when the MPU writes a zero to RTC_WR.
Reads of RTC_WR returns one until the counter has actually been updated by the register.
The sub-second register of the RTC, RTC_SBSC (I/O RAM 0x2892), can be read by the MPU after the one
second interrupt and before reaching the next one second boundary. The RTC_SBSC register is expressed
as a count of 1/128 second periods remaining until the next one second boundary. Writing 0x00 to
RTC_SBSC resets the counter re-starting the count from 0 to 127. Reading and resetting the sub-second
counter can be used as part of an algorithm to accurately set the RTC.
The RTC is capable of processing leap years. Each counter has its own output register. The RTC chain
registers are not affected by the reset pin, watchdog timer resets, or by transitions between the battery
modes and mission mode.
Clock
OSC
MCK
CKCE
CKADC
CKMPU
CKICE
CKOPTMOD
CK32
shows the I/O RAM registers for accessing the RTC.
Crystal
Crystal/PLL
MCK
MCK
MCK
MCK
MCK
MCK
Derived
From
19.660800 MHz
4.9152 MHz …
Table 41: Clock System Summary
9.8304 MHz…
PLL_FAST=1
4.9152 MHz,
4.9152 MHz
2.4576 MHz
(600*CK32)
307.2 kHz
614.4 kHz
38.40 kHz
Fixed Frequency or Range
32.768 kHz
32.768 kHz
3.145728 MHz …
1.572864 MHz…
1.572864 MHz,
6.291456 MHz
0.786432 MHz
PLL_FAST=0
196.608 kHz
1.5728 MHz
(192*CK32)
98.304 kHz
38.6 kHz
71M6541D/F/G and 71M6542F/G Data Sheet
Controlled by
MPU_DIV[2:0]
MPU_DIV[2:0]
PLL_FAST
ADC_DIV
–
–
–
–
Crystal clock
Master clock
32 kHz clock
MPU clock
Modulation
ADC clock
Function
ICE clock
CE clock
Optical
UART