Microchip Technology MA330027 Data Sheet
dsPIC33EPXXX(GP/MC/MU)806/810/814 and PIC24EPXXX(GP/GU)810/814
DS70616G-page 482
2009-2012 Microchip Technology Inc.
29.4
Watchdog Timer (WDT)
For dsPIC33EPXXX(GP/MC/MU)806/810/814 and
PIC24EPXXX(GP/GU)810/814 devices, the WDT is
driven by the LPRC Oscillator. When the WDT is
enabled, the clock source is also enabled.
PIC24EPXXX(GP/GU)810/814 devices, the WDT is
driven by the LPRC Oscillator. When the WDT is
enabled, the clock source is also enabled.
29.4.1
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 nominal
WDT Time-out (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 nominal
WDT Time-out (T
WDT
) period 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 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
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 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)
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
29.4.2
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 bits (RCON<3,2>) need 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 bits (RCON<3,2>) need to be
cleared in software after the device wakes up.
29.4.3
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.
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.
cleared following a WDT time-out. To detect subsequent
WDT events, the flag must be cleared in software.
FIGURE 29-2:
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.
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
instruction window can be determined by
using a timer. If a CLRWDT instruction is
executed before this window, a WDT Reset
occurs.
the CLRWDT instruction should be executed
by the application software only during the
last 1/4 of the WDT period. This CLRWDT
instruction window can be determined by
using a timer. If a CLRWDT instruction is
executed before this window, a WDT Reset
occurs.
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
All Device Resets
Transition to New Clock Source
Exit Sleep or Idle Mode
PWRSAV Instruction
CLRWDT Instruction
Transition to New Clock Source
Exit Sleep or Idle Mode
PWRSAV Instruction
CLRWDT Instruction