cp3bt13 National Semiconductor Corporation, cp3bt13 Datasheet - Page 159

• Manufacturer Part Number: cp3bt13
• Description: Cp3bt13 Reprogrammable Connectivity Processor With Bluetooth-r And Can Interfaces
• Manufacturer: National Semiconductor Corporation

22.3
The Watchdog is an 8-bit down counter that operates on the
rising edge of a specified clock source. At reset, the Watch-
dog is disabled; it does not count and no Watchdog signal is
generated. A write to either the Watchdog Count (WDCNT)
register or the Watchdog Service Data Match (WDSDM)
register starts the counter. The Watchdog counter counts
down from the value programmed in the WDCNT register.
Once started, only a reset can stop the Watchdog from op-
erating.
The Watchdog can be programmed to use either T0OUT or
T0IN as its clock source (the output and input of Timer T0,
respectively). The TWCFG.WDCT0I bit controls this clock
selection.
Software must periodically “service” the Watchdog. There
are two ways to service the Watchdog, the choice depend-
ing on the programmed value of the WDSDME bit in the
Timer and Watchdog Configuration (TWCFG) register.
If the TWCFG.WDSDME bit is clear, the Watchdog is ser-
viced by writing a value to the WDCNT register. The value
written to the register is reloaded into the Watchdog counter.
The counter then continues counting down from that value.
If the TWCFG.WDSDME bit is set, the Watchdog is ser-
viced by writing the value 5Ch to the Watchdog Service
Data Match (WDSDM) register. This reloads the Watchdog
counter with the value previously programmed into the WD-
CNT register. The counter then continues counting down
from that value.
A Watchdog error signal is generated by any of the following
events:
T The Watchdog serviced too late.
T The Watchdog serviced too often.
T The WDSDM register is written with a value other than
A Watchdog error condition resets the device.
22.3.1
The Timer and Watchdog Configuration (TWCFG) register
is used to set the Watchdog configuration. It controls the
Watchdog clock source (T0IN or T0OUT), the type of
Watchdog servicing (using WDCNT or WDSDM), and the
locking state of the TWCFG, TWCPR, TIMER0, T0CSR,
and WDCNT registers. A register that is locked cannot be
read or written. A write operation is ignored and a read op-
eration returns unpredictable results.
If the TWCFG register is itself locked, it remains locked until
the device is reset. Any other locked registers also remain
locked until the device is reset. This feature prevents a run-
away program from tampering with the programmed Watch-
dog function.
5Ch
(TWCFG.WDSDME = 1).
WATCHDOG OPERATION
Register Locking
when
WDSDM
type
servicing
is
enabled
159
22.3.2
The Timer and Watchdog Module is active in both the Power
Save and Idle modes. The clocks and counters continue to
operate normally in these modes. The WDSDM register is
accessible in the Power Save and Idle modes, but the other
TWM registers are accessible only in the Active mode.
Therefore, Watchdog servicing must be carried out using
the WDSDM register in the Power Save or Idle mode.
In the Halt mode, the entire device is frozen, including the
Timer and Watchdog Module. On return to Active mode, op-
eration of the module resumes at the point at which it was
stopped.
Note: After a restart or Watchdog service through WDCNT,
do not enter Power Save mode for a period equivalent to 5
Slow Clock cycles.
22.4
The TWM registers controls the operation of the Timing and
Watchdog Module. There are six such registers:
The WDSDM register is accessible in both Active and Pow-
er Save mode. The other TWM registers are accessible only
in Active mode.
WDSDM
TWCFG
WDCNT
TWMT0
T0CSR
TWCP
Name
Power Save Mode Operation
TWM REGISTERS
Table 65 TWM Registers
FF FF2Ah
FF FF20h
FF FF22h
FF FF24h
FF FF26h
FF FF28h
Address
TWM Timer 0 Register
Configuration Register
Timer and Watchdog
Timer and Watchdog
TWMT0 Control and
Data Match Register
Watchdog Service
Watchdog Count
Clock Prescaler
Status Register
Description
Register
Register
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