DS89C420-QCL Datasheet, PDF (81/139 Pages) Maxim Integrated Products – Ultra-High-Speed Flash Microcontroller User’s Guide

Ultra-High-Speed Flash
Microcontroller User’s Guide
VCC
VPFW
VRST
VSS
INTERRUPT
SERVICE ROUTINE
tCSU
XTAL1
tPOR
INTERNAL RESET
Figure 7-1. Power Cycle Operaton
Precision Voltage Monitor
A precision bandgap reference and other analog circuits monitor the state of the power supply during power-up and power-down tran-
sitions. This obviates the need for external circuits to perform these functions that other microcontroller systems would require. The
bandgap reference provides a precise voltage to compare with V_CC. When V_CC begins to drop, the power monitor compares it to its
reference. This enables the analog circuits to detect when V_CC passes through predetermined thresholds, V_PFW and V_RST. These are
specified in the product data sheet.
Early-Warning Power-Fail Interrupt
The precision voltage reference has the ability to generate a power-fail interrupt and/or reset in response to a low-supply voltage. When
V_CC reaches the V_PFW threshold, the microcontroller can generate a power-fail interrupt. This early warning of supply voltage failure
allows the system time to save critical parameters in nonvolatile memory and put external functions in a safe state.
The power-fail interrupt is optional and is enabled using the enable power-fail warning interrupt (EPFI) bit at WDCON.5. If enabled, V_CC
dropping below VPFW causes the device to vector to address 33h. The power-fail interrupt status bit, PFI (WDCON.4), is set anytime
VCC transitions below V_PFW. This flag is not cleared when V_CC is above V_PFW, and software should clear it immediately after reading
it. As long as the condition exists, PFI is immediately set again by hardware.
A typical application of the PFI is to place the device into a “safe mode” when a power loss appears imminent. When the interrupt
occurs, the code vectors to location 33h. At this time, software can disable the interrupt, save any critical data, clear PFI, and then con-
tinually poll the status of the power supply through the PFI flag. As long as PFI is set, power is still below VPFW. If power returns to the
proper level, PFI is not set once cleared by software. This indicates a safe operating condition. If power continues to fall, a power-fail
reset is invoked automatically.
Power-Fail Reset
The power-fail reset automatically invokes a reset when V_CC drops below V_RST. This halts device operation and places all outputs in
their reset state. This state continues to be held until V_CC drops below the voltage necessary to power the port pins. Because V_RST.
is lower than V_PFW, the microcontroller has the option to use the power-fail interrupt to place the device into a “safe” state before the
device halts operation with a power-fail reset. The power-fail reset function cannot be disabled.
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