71M6541D Datasheet, PDF (81/166 Pages) Maxim Integrated Products – 0.1% Accuracy Over 2000:1 Current Range Energy Meter ICs

71M6541D/F/G and 71M6542F/G Data Sheet
3.2 Battery Modes
Shortly after system power (V3P3SYS) is applied, the part is in mission mode (MSN mode). MSN mode
means that the part is operating with system power and that the internal PLL is stable. This mode is the
normal operating mode where the part is capable of measuring energy.
When system power is not available, the 71M654x is in one of three battery modes:
• BRN mode (brownout mode)
• LCD mode (LCD-only mode)
• SLP mode (sleep mode).
An internal comparator monitors the voltage at the V3P3SYS pin (note that V3P3SYS and V3P3A are
typically connected together at the PCB level). When the V3P3SYS dc voltage drops below 3.0 VDC, the
comparator resets an internal power status bit called V3OK . As soon as system power is removed and
V3OK = 0, the 71M654x switches to battery power (VBAT pin), notifies the MPU by issuing an interrupt and
updates the VSTAT[2:0] register (SFR 0xF9[2:0], see Table 68). The MPU continues to execute code when
the system transitions from MSN to BRN mode. Refer to 3.2.1 BRN Mode for the settings that result in the
lowest possible power during BRN mode. Depending on the MPU code, the MPU can choose to stay in
BRN mode, or transition to LCD or to SLP mode (via the I/O RAM bits LCD_ONLY, I/O RAM 0x28B2[6] and
SLEEP, I/O RAM 0x28B2[7]). BRN mode is similar to MSN mode except that resources powered by V3P3A
power, such as the ADC are inaccurate. In BRN mode the CE continues to run and should be turned off
to conserve VBAT power. Also, the PLL continues to function at the same frequency as in MSN mode
and its frequency should be reduced to save power (CKGN = 0x24 (I/O RAM 0x2200).
When system power is restored, the 71M654x automatically transitions from any of the battery modes
(BRN, LCD, SLP) back to MSN mode, switches back to using system power (V3P3SYS, V3P3A), issues
an interrupt and updates VSTAT[1:0]. The MPU software should restore MSN mode operation by issuing
a soft reset to restore system settings to values appropriate for MSN mode.
Figure 29 shows a state diagram of the various operating modes, with the possible transitions between modes.
When the part wakes-up under battery power, the part automatically enters BRN mode (see 3.4 Wake Up
Behavior). From BRN mode, the part may enter either LCD mode or SLP mode, as controlled by the MPU.
MSN
RESET
V3P3SYS
rises
LCD_ONLY
VSTAT=00X
V3P3SYS
falls
VSTAT=001
V3P3SYS
rises
System Power
Battery Power
BRN
Wake Flags
V3P3SYS
rises
RESET &
VBAT
sufficient
LCD
Wake
event
VBAT
insufficient
RESET &
VBAT
insufficient
Wake
event
SLEEP or
VBAT
insufficient
VBAT
insufficient
SLP
Figure 29: Operation Modes State Diagram