Rainbow Electronics DS89C450 - Manual (Page 40)

DS89C430/DS89C440/DS89C450 Ultra-High-Speed Flash Microcontrollers
40 of 48
Figure 14. System Clock Sources
4X/ 2X
CTM
CRYSTAL
OSCILLATOR
DIVIDE 1024
RING
OSCILLATOR
CLOCK
MULTIPLIER
CD0
CD1
SELECTOR
RING
ENABLE
MUX
SYSTEM
CLOCK
Bandgap-Monitored Interrupt and Reset Generation
The power monitor in the DS89C430 monitors the V
CC
pin in relation to the on-chip bandgap voltage reference.
Whenever V
CC
falls below V
PFW
, an interrupt is generated if the corresponding power-fail interrupt-enable bit EPFI
(WDCON.5) is set, causing the device to vector to address 33h. The power-fail interrupt status bit PFI (WDCON.4)
is set any time V
CC
transitions below V
PFW
, and can only be cleared by software once set. Similarly, as V
CC
falls
below V
RST
, a reset is issued internally to halt program execution. Following power-up, a power-on reset initiates a
power-on reset timeout before starting program execution. When V
CC
is first applied to the DS89C430, the
processor is held in reset until V
CC
> V
RST
and a delay of 65,536 oscillator cycles has elapsed, to ensure that power
is within tolerance and the clock source has had time to stabilize. Once the reset timeout period has elapsed, the
reset condition is removed automatically and software execution begins at the reset vector location of 0000h. The
power-on reset flag POR (WDCON.6) is set to logic 1 to indicate a power-on reset has occurred, and can only be
cleared by software.
When the DS89C430 enters stop mode, the bandgap, reset comparator, and power-fail interrupt comparator are
automatically disabled to conserve power if the BGS (EXIF.0) bit is set to logic 0. This is the lowest power mode. If
BGS is set to logic 1, the bandgap reference, reset comparator, and the power-fail comparator are powered up,
although in a mode that reduces their power consumption.