Chapter 3: Hardware
CAN-Engine
3-8
Figure 3.2 Location of watchdog timer enable jumper
Power-failure Warning
The supervisor supports power-failure warning and backup battery protection. When power failure is
sensed by the PFI=J1.11, pin 9 of the MAX691 (lower than 1.3 V), the PFO is low. The PFI pin 9 of 691 is
directly shorted to VCC by default. In order to use PFI externally, cut the trace and bring the PFI signal out.
You may design an NMI service routine to take protect actions before the +5V drops and processor dies.
The following circuit shows how you might use the power-failure detection logic within your application.
47K
2K
PFI, pin 9 of MAX691
(1.3 V min)
(8.35 V min)
9-14 V
C?
VCC = +5V
Using the supervisor chip for power failure detection
Battery Backup Protection
The backup battery protection protects data stored in the SRAM and RTC. The battery-switch-over circuit
compares VCC to VBAT (+3 V lithium battery positive pin), and connects whichever is higher to the
VRAM (power for SRAM and RTC). Thus, the SRAM and the real-time clock are backed up. In normal
J5 WatchDog