Rainbow Electronics 71M6542G - Manual (Page 85)

v1.1 © 2008–2011 Teridian Semiconductor Corporation 85
3.3 Fault and Reset Behavior
3.3.1 Events at Power-Down
Power fault detection is performed by internal comparators that monitor the voltage at the V3P3A pin and
also monitor the internally generated VDD pin voltage (2.5 VDC). The V3P3SYS and V3P3A pins must be
tied together at the PCB level, so that the comparators, which are internally connected only to the V3P3A
pin, are able to simultaneously monitor the common V3P3SYS and V3P3A pin voltage. The following
discussion assumes that the V3P3A and V3P3SYS pins are tied together at the PCB level.
During a power failure, as V3P3A falls, two thresholds are detected:
• The first threshold, at 3.0 VDC (VSTAT[2:0] = 001), warns the MPU that the analog modules are no
longer accurate. Other than warning the MPU, the hardware takes no action when this threshold is
crossed.
• The second threshold, at 2.8 VDC, causes the 71M654x to switch to battery power. This switching
happens while the FLASH and RAM systems are still able to read and write.
The power quality is reflected by the SFR VSTAT[2:0] field, as shown in Table 68 . The VSTAT[2:0] field is
located at SFR address 0xF9 and occupies bits [2:0], and it is read-only.
In addition to the state of the main power, the VSTAT[2:0] register provides information about the internal
VDD voltage under battery power. Note that if system power (V3P3A) is above 2.8 VDC, the
71M6541D/F and 71M6542F always switch from battery to system power.
Table 68: VSTAT[2:0] (SFR 0xF9[2:0])
VSTAT[2:0] Description
000 System Power OK. V3P3A > 3.0 VDC. Analog modules are functional and accurate.
001
System Power is low. 2.8 VDC < V3P3A < 3.0 VDC. Analog modules not accurate.
Switch over to battery power is imminent.
010
The IC is on battery power and VDD is OK. VDD > 2.25 VDC. The IC has full digital
functionality.
011
The IC is on battery power and 2.25 VDC > VDD > 2.0 VDC. Flash write operations are
inhibited.
101
The IC is on battery power and VDD < 2.0, which means that the MPU is nearly out of
voltage. A reset occurs in 4 cycles of the crystal clock CK32.
The response to a system power fault is almost entirely controlled by firmware. During a power failure,
system power slowly falls. This is monitored by internal comparators that cause the hardware to
automatically switch over to taking power from the VBAT input. An interrupt notifies the MPU that the part
is now battery powered. At this point, it is the MPU’s responsibility to reduce power by slowing the clock
rate, disabling the PLL, etc.
Precision analog components such as the bandgap reference, the bandgap buffer, and the ADC are
powered only by the V3P3A pin and become inaccurate and ultimately unavailable as the V3P3A pin
voltage continues to drop (i.e., circuits powered by the V3P3A pin are not backed by the VBAT pin).
When the V3P3A pin falls below 2.8 VDC, the ADC clocks are halted and the amplifiers are unbiased.
Meanwhile, control bits such as ADC_E bit (I/O RAM 0x2704[4]) are not affected, since their I/O RAM
storage is powered from the VDD pin (2.5 VDC). The VDD pin is supplied with power through an internal
2.5 VDC regulator that is connected to the V3P3D pin. In turn, the V3P3D pin is switched to receive
power from the VBAT pin when the V3P3SYS pin drops below 3.0 VDC. Note that the V3P3SYS and
V3P3A pins are typically tied together at the PCB level.