70592C Datasheet, PDF (227/314 Pages) Microchip Technology – High-Performance, 16-bit Microcontrollers

PIC24HJXXXGPX06A/X08A/X10A
21.4 Watchdog Timer (WDT)
For PIC24HJXXXGPX06A/X08A/X10A devices, the
WDT is driven by the LPRC oscillator. When the WDT
is enabled, the clock source is also enabled.
The nominal WDT clock source from LPRC is 32 kHz.
This feeds a prescaler than can be configured for either
5-bit (divide-by-32) or 7-bit (divide-by-128) operation.
The prescaler is set by the WDTPRE Configuration bit.
With a 32 kHz input, the prescaler yields a nominal
WDT time-out period (TWDT) of 1 ms in 5-bit mode, or
4 ms in 7-bit mode.
A variable postscaler divides down the WDT prescaler
output and allows for a wide range of time-out periods.
The postscaler is controlled by the WDTPOST<3:0>
Configuration bits (FWDT<3:0>) which allow the selec-
tion of a total of 16 settings, from 1:1 to 1:32,768. Using
the prescaler and postscaler, time-out periods ranging
from 1 ms to 131 seconds can be achieved.
The WDT, prescaler and postscaler are reset:
• On any device Reset
• On the completion of a clock switch, whether
invoked by software (i.e., setting the OSWEN bit
after changing the NOSC bits) or by hardware
(i.e., Fail-Safe Clock Monitor)
• When a PWRSAV instruction is executed
(i.e., Sleep or Idle mode is entered)
• When the device exits Sleep or Idle mode to
resume normal operation
• By a CLRWDT instruction during normal execution
FIGURE 21-2:
WDT BLOCK DIAGRAM
All Device Resets
Transition to New Clock Source
Exit Sleep or Idle Mode
PWRSAV Instruction
CLRWDT Instruction
SWDTEN
FWDTEN
LPRC Clock
WDTPRE
RS
Prescaler
(divide by N1)
If the WDT is enabled, it will continue to run during
Sleep or Idle modes. When the WDT time-out occurs,
the device will wake the device and code execution will
continue from where the PWRSAV instruction was exe-
cuted. The corresponding SLEEP or IDLE bits
(RCON<3,2>) will need to be cleared in software after
the device wakes up.
The WDT flag bit, WDTO (RCON<4>), is not automatically
cleared following a WDT time-out. To detect subsequent
WDT events, the flag must be cleared in software.
Note:
The CLRWDT and PWRSAV instructions
clear the prescaler and postscaler counts
when executed.
The WDT is enabled or disabled by the FWDTEN
Configuration bit in the FWDT Configuration register.
When the FWDTEN Configuration bit is set, the WDT is
always enabled.
The WDT can be optionally controlled in software when
the FWDTEN Configuration bit has been programmed
to ‘0’. The WDT is enabled in software by setting the
SWDTEN control bit (RCON<5>). The SWDTEN con-
trol bit is cleared on any device Reset. The software
WDT option allows the user to enable the WDT for crit-
ical code segments and disable the WDT during
non-critical segments for maximum power savings.
Note:
If the WINDIS bit (FWDT<6>) is cleared,
the CLRWDT instruction should be executed
by the application software only during the
last 1/4 of the WDT period. This CLRWDT
window can be determined by using a timer.
If a CLRWDT instruction is executed before
this window, a WDT Reset occurs.
Watchdog Timer
WDTPOST<3:0>
RS
Postscaler
(divide by N2)
Sleep/Idle
1
0
WDT
Wake-up
WDT
Reset
WINDIS
WDT Window Select
CLRWDT Instruction
© 2011 Microchip Technology Inc.
DS70592C-page 227