Datasheet

71M6521DE/DH/FE Data Sheet
Page: 50 of 107 Rev 3
Battery Modes
Shortly after system power (V3P3SYS) is applied, the part will be in MISSION mode. MISSION mode means that the
part is operating with system power and that the internal PLL is stable. This mode is the normal operation mode
where the part is capable of measuring energy.
When system power is not available (i.e. when V1<VBIAS), the 71M6521DE/DH/FE can be in one of three battery
modes, i.e. BROWNOUT, LCD, or SLEEP mode. As soon as V1 falls below VBIAS or when the part wakes up under
battery power (with sufficient voltage margin), the part will automatically enter BROWNOUT mode (see Wake Up
Behavior section). From BROWNOUT mode, the MPU may enter either LCD mode or SLEEP mode by setting either
the LCD_ONLY or SLEEP I/O RAM bits (only one bit can be set at the same time in BROWNOUT mode, since setting
one bit will already force the part into SLEEP or LCD mode, disabling the MPU).
Figure 18 shows a state diagram of the various operation modes, with the possible transitions between modes. For
information on the timing of mode transitions refer to Figure 22 through Figure 24.
Meters that do not require functionality in the battery modes, e.g. meters that only use the SLEEP mode to
maintain the RTC, still need to contain code that brings the chip from BROWNOUT mode to SLEEP mode.
Otherwise, the chip remains in BROWNOUT mode, once the system power is missing, and consumes more
current than intended.
Similarly, meters equipped with batteries need to contain code that transitions the chip to SLEEP mode as
soon as the battery is attached in production. Otherwise, remaining in BROWNOUT mode would add
unnecessary drain to the battery.
The transition from MISSION mode to BROWNOUT mode is signaled by the IE_PLLFALL interrupt flag (in SFR
0xE8[7]). The transition in the other direction is signaled by the IE_PLLRISE interrupt flag (SFR 0xE8[6]), when the
PLL becomes stable.
Transitions from both LCD and SLEEP mode back to BROWNOUT mode are initiated by wake-up timer timeout
conditions or pushbutton events. When the PB pin is pulled high (pushbutton is pressed), the IE_PB interrupt flag
(SFR 0xE8[4]) is set, and when the wake-up timer times out, the IE_WAKE interrupt flag (SFR 0xE8[5]) is set.
In the absence of system power, if the voltage margin for the LDO regulator providing 2.5 V to the internal circuitry
becomes too low to be safe, the part automatically enters sleep mode (BAT_OK false). The battery voltage must stay
above 3V to ensure that BAT_OK remains true. Under this condition, the 71M6521DE/DH/FE stays in SLEEP mode,
even if the voltage margin for the LDO improves (BAT_OK true).
Table 60 shows the circuit functions available in each operating mode.
Note: Systems with batteries benefit from firmware code that supports safe power mode transitions. This code will
ensure safe operation under severe conditions such as repeated disruptions of the AC supply voltage of the meter.
This code is described in application note AN5268 (Firmware for Safe Power Mode Transitions) and is contained in
demo code revision 4.8 and later revisions.