71M6521DE Datasheet, PDF (57/107 Pages) Teridian Semiconductor Corporation – Energy Meter IC

71M6521DE/DH/FE Data Sheet
VBAT
Battery
Current
MPU Mode
BROWNOUT
MPU Clock
Source
WAKE
Xtal
14.5 CK32
cycles
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 and RTC module continue 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 2.5 V 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 the internal signal 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 71M6521DE/DH/FE 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 BROWN-
OUT 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
automatically 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 RTC clock will
not be disturbed, but the MPU RAM must be re-initialized. The hardware watchdog timer will become active when the
part enters MISSION mode.
If there is no battery when system power returns, the part will switch to mission mode when PLL_OK rises. All
configuration bits will be in reset state, and RTC and MPU RAM data will be unknown and must be initialized by the
MPU.
Wake Up Behavior
As described above, the part will always wake up in mission mode when system power is restored. Additionally, the
part will wake up in BROWNOUT mode when PB rises (push button pressed) or when a timeout of the wake-up timer
occurs.
Rev 2
Page: 57 of 107