19.0 WATCHDOG/Clock Monitor (Continued)
TABLE 35. WATCHDOG Service Window Select
Clock
Monitor
Bit 0
Service Window
for High Speed Mode
(Lower-Upper Limits)
2048-8k tC Cycles
Service Window
for Dual Clock & Low Speed Modes
(Lower-Upper Limits)
WDSVR
Bit 7
WDSVR
Bit 6
0
0
1
1
X
X
0
1
0
1
X
X
X
X
X
X
0
2048-8k Cycles of 32 kHz Clk
2048-16k Cycles of 32 kHz Clk
2048-32k Cycles of 32 kHz Clk
2048-64k Cycles of 32 kHz Clk
Clock Monitor Disabled
2048-16k tC Cycles
2048-32k tC Cycles
2048-64k tC Cycles
Clock Monitor Disabled
Clock Monitor Enabled
1
Clock Monitor Enabled
19.1 CLOCK MONITOR
When jumping to the boot ROM for ISP and virtual E2
operations, the hardware will disable the lower window error
and perform an immediate WATCHDOG service. The ISP
routines will service the WATCHDOG within the selected
upper window. The ISP routines will service the WATCH-
DOG immediately prior to returning execution back to the
user’s code in flash. Therefore, after returning to flash
memory, the user can service the WATCHDOG anytime
following the return from boot ROM, but must service it within
the selected upper window to avoid a WATCHDOG error.
The Clock Monitor aboard the device can be selected or
deselected under program control. The Clock Monitor is
guaranteed not to reject the clock if the instruction cycle
clock (1/tC) is greater or equal to 5 kHz. This equates to a
clock input rate on the selected oscillator of greater or equal
to 25 kHz.
19.2 WATCHDOG/CLOCK MONITOR OPERATION
The WATCHDOG is enabled by bit 2 of the Option register.
When this Option bit is 0, the WATCHDOG is enabled and
pin G1 becomes the WATCHDOG output with a weak pull-
up.
The WATCHDOG has an output pin associated with it. This
is the WDOUT pin, on pin 1 of the port G. WDOUT is active
low. The WDOUT pin has a weak pull-up in the inactive
state. Upon triggering the WATCHDOG, the logic will pull the
WDOUT (G1) pin low for an additional 16–32 cycles after the
signal level on WDOUT pin goes below the lower Schmitt
trigger threshold. After this delay, the device will stop forcing
the WDOUT output low. The WATCHDOG service window
will restart when the WDOUT pin goes high.
The WATCHDOG and Clock Monitor are disabled during
reset. The device comes out of reset with the WATCHDOG
armed, the WATCHDOG Window Select bits (bits 6, 7 of the
WDSVR Register) set, and the Clock Monitor bit (bit 0 of the
WDSVR Register) enabled. Thus, a Clock Monitor error will
occur after coming out of reset, if the instruction cycle clock
frequency has not reached a minimum specified value, in-
cluding the case where the oscillator fails to start.
A WATCHDOG service while the WDOUT signal is active will
be ignored. The state of the WDOUT pin is not guaranteed
on reset, but if it powers up low then the WATCHDOG will
time out and WDOUT will go high.
The WDSVR register can be written to only once after reset
and the key data (bits 5 through 1 of the WDSVR Register)
must match to be a valid write. This write to the WDSVR
register involves two irrevocable choices: (i) the selection of
the WATCHDOG service window (ii) enabling or disabling of
the Clock Monitor. Hence, the first write to WDSVR Register
involves selecting or deselecting the Clock Monitor, select
the WATCHDOG service window and match the WATCH-
DOG key data. Subsequent writes to the WDSVR register
will compare the value being written by the user to the
WATCHDOG service window value, the key data and the
Clock Monitor Enable (all bits) in the WDSVR Register. Table
36 shows the sequence of events that can occur.
The Clock Monitor forces the G1 pin low upon detecting a
clock frequency error. The Clock Monitor error will continue
until the clock frequency has reached the minimum specified
value, after which the G1 output will go high following 16–32
clock cycles. The Clock Monitor generates a continual Clock
Monitor error if the oscillator fails to start, or fails to reach the
minimum specified frequency. The specification for the Clock
Monitor is as follows:
>
1/tC 5 kHz—No clock rejection.
<
1/tC 10 Hz—Guaranteed clock rejection.
The user must service the WATCHDOG at least once before
the upper limit of the service window expires. The
WATCHDOG may not be serviced more than once in every
lower limit of the service window.
TABLE 36. WATCHDOG Service Actions
Key Data
Match
Window Data
Match
Clock Monitor
Match
Action
Valid Service: Restart Service Window
Error: Generate WATCHDOG Output
Error: Generate WATCHDOG Output
Error: Generate WATCHDOG Output
Don’t Care
Mismatch
Don’t Care
Mismatch
Don’t Care
Don’t Care
Don’t Care
Don’t Care
Mismatch
19.3 WATCHDOG AND CLOCK MONITOR SUMMARY
•
Both the WATCHDOG and CLOCK MONITOR detector
circuits are inhibited during RESET.
The following salient points regarding the WATCHDOG and
CLOCK MONITOR should be noted:
63
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