DS89C420_05 Datasheet, PDF (38/47 Pages) Maxim Integrated Products – Ultra-High-Speed Microcontroller

DS89C420 Ultra-High-Speed Microcontroller
simplified diagram of the generation of the system clocks. Specifics of hardware restrictions associated with the
use of the 4X/2X CTM, CKRY, CD1, and CD0 bits are outlined in the SFR description.
Figure 15. System Clock Sources
Crystal
Oscillator
4X/2X
CTM
Clock
Multiplier
Ring
Enable
Divide 1024
Ring
Oscillator
CD0
CD1
MUX
Selector
System
Clock
BANDGAP-MONITORED INTERRUPT AND RESET GENERATION
The power monitor in the DS89C420 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 anytime 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 DS89C420, 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 DS89C420 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 a 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 reduced fashion, while in stop mode.
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