Microchip Technology MA330031-2 Data Sheet
2011-2013 Microchip Technology Inc.
DS70000657H-page 385
dsPIC33EPXXXGP50X, dsPIC33EPXXXMC20X/50X AND PIC24EPXXXGP/MC20X
27.5
Watchdog Timer (WDT)
For dsPIC33EPXXXGP50X, dsPIC33EPXXXMC20X/
50X and PIC24EPXXXGP/MC20X devices, the WDT is
driven by the LPRC oscillator. When the WDT is
enabled, the clock source is also enabled.
50X and PIC24EPXXXGP/MC20X devices, the WDT is
driven by the LPRC oscillator. When the WDT is
enabled, the clock source is also enabled.
27.5.1
PRESCALER/POSTSCALER
The nominal WDT clock source from LPRC is 32 kHz.
This feeds a prescaler that 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 WDT Time-
out period (T
This feeds a prescaler that 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 WDT Time-
out period (T
WDT
), as shown in Parameter
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
selection 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
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
selection 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 NOSCx bits) or by hardware
(i.e., Fail-Safe Clock Monitor)
after changing the NOSCx 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
27.5.2
SLEEP AND IDLE MODES
If the WDT is enabled, it continues to run during Sleep or
Idle modes. When the WDT time-out occurs, the device
wakes the device and code execution continues from
where the PWRSAV instruction was executed. The corre-
sponding SLEEP or IDLE bit (RCON<3,2>) needs to be
cleared in software after the device wakes up.
Idle modes. When the WDT time-out occurs, the device
wakes the device and code execution continues from
where the PWRSAV instruction was executed. The corre-
sponding SLEEP or IDLE bit (RCON<3,2>) needs to be
cleared in software after the device wakes up.
27.5.3
ENABLING WDT
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 control bit is cleared on any device Reset.
The software WDT option allows the user application
to enable the WDT for critical code segments and
disable the WDT during non-critical segments for
maximum power savings.
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.
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 control bit is cleared on any device Reset.
The software WDT option allows the user application
to enable the WDT for critical code segments and
disable the WDT during non-critical segments for
maximum power savings.
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.
27.5.4
WDT WINDOW
The Watchdog Timer has an optional Windowed mode,
enabled by programming the WINDIS bit in the WDT
Configuration register (FWDT<6>). In the Windowed
mode (WINDIS = 0), the WDT should be cleared based
on the settings in the programmable Watchdog Timer
Window select bits (WDTWIN<1:0>).
enabled by programming the WINDIS bit in the WDT
Configuration register (FWDT<6>). In the Windowed
mode (WINDIS = 0), the WDT should be cleared based
on the settings in the programmable Watchdog Timer
Window select bits (WDTWIN<1:0>).
FIGURE 27-2:
WDT BLOCK DIAGRAM
Note:
The CLRWDT and PWRSAV instructions
clear the prescaler and postscaler counts
when executed.
clear the prescaler and postscaler counts
when executed.
0
1
WDTPRE
WDTPOST<3:0>
Watchdog Timer
Prescaler
(Divide-by-N1)
Postscaler
(Divide-by-N2)
Sleep/Idle
WDT
WDT Window Select
WINDIS
WDT
CLRWDT
Instruction
SWDTEN
FWDTEN
FWDTEN
LPRC Clock
RS
RS
Wake-up
Reset
WDTWIN<1:0>
All Device Resets
Transition to New Clock Source
Exit Sleep or Idle Mode
PWRSAV
PWRSAV
Instruction
CLRWDT
Instruction