71M6521BE
Energy Meter IC
DATA SHEET
JANUARY 2008
VBAT
Battery
Current
BROWNOUT
MPU Mode
Xtal
MPU Clock
Source
14.5 CK32
cycles
WAKE
PLL_OK
Internal
RESETZ
1024 CK32
cycles
VBAT_OK
time
Figure 24: Power-Up Timing with VBAT only
Fault and Reset Behavior
Reset Mode: When the RESET pin is pulled high all digital activity stops. The oscillator module continues to run. Additionally,
all I/O RAM bits are set to their default states. As long as V1, the input voltage at the power fault block, is greater than VBIAS,
the internal 2.5V regulator will continue to provide power to the digital section.
Once initiated, the reset mode will persist until the reset timer times out, signified by WAKE rising. This will occur in 4100
cycles of the real time clock after RESET goes low, at which time the MPU will begin executing its preboot and boot sequences
from address 00. See the security section for more description of preboot and boot.
If system power is not present, the reset timer duration will be 2 cycles of the crystal clock, at which time the MPU will begin
executing in BROWNOUT mode, starting at address 00.
Power Fault Circuit: The 71M6521BE includes a comparator to monitor system power fault conditions. When the output of
the comparator falls (V1<VBIAS), the I/P RAM bits PLL_OK is zeroed and the part switches to BROWNOUT mode if a battery
is present. Once, system power returns, the MPU remains in reset and does not start Mission Mode until 4100 oscillator clocks
later, when PLL_OK rises. If a battery is not present, indicated by BAT_OK=0, WAKE will fall and the part will enter SLEEP
mode.
There are several conditions the part could be in as system power returns. If the part is in BROWNOUT mode, it will auto-
matically switch to mission mode when PLL_OK rises. It will receive an interrupt indicating this. No configuration bits will be
reset or reconfigured during this transition.
If the part is in LCD or SLEEP mode when system power returns, it will also switch to mission mode when PLL_OK rises. In
this case, all configuration bits will be in the reset state due to WAKE having been zero. The MPU RAM must be re-initialized.
The hardware watchdog timer will become active when the part enters MISSION mode.
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