PIC18F2420/2520/4420/4520
attempt a partial recovery or execute a controlled shut-
down. See Section 3.1.4 “Multiple Sleep Commands”
23.4 Fail-Safe Clock Monitor
The Fail-Safe Clock Monitor (FSCM) allows the micro-
controller to continue operation in the event of an external
oscillator failure by automatically switching the device
clock to the internal oscillator block. The FSCM function
is enabled by setting the FCMEN Configuration bit.
and Section 23.3.1 “Special Considerations for
Using Two-Speed Start-up” for more details.
To use a higher clock speed on wake-up, the INTOSC or
postscaler clock sources can be selected to provide a
higher clock speed by setting bits, IRCF<2:0>, immedi-
ately after Reset. For wake-ups from Sleep, the INTOSC
or postscaler clock sources can be selected by setting the
IRCF<2:0> bits prior to entering Sleep mode.
When FSCM is enabled, the INTRC oscillator runs at
all times to monitor clocks to peripherals and provide a
backup clock in the event of a clock failure. Clock
monitoring (shown in Figure 23-3) is accomplished by
creating a sample clock signal, which is the INTRC out-
put divided by 64. This allows ample time between
FSCM sample clocks for a peripheral clock edge to
occur. The peripheral device clock and the sample
clock are presented as inputs to the Clock Monitor latch
(CM). The CM is set on the falling edge of the device
clock source, but cleared on the rising edge of the
sample clock.
The FSCM will detect failures of the primary or second-
ary clock sources only. If the internal oscillator block
fails, no failure would be detected, nor would any action
be possible.
23.4.1
FSCM AND THE WATCHDOG TIMER
Both the FSCM and the WDT are clocked by the
INTRC oscillator. Since the WDT operates with a
separate divider and counter, disabling the WDT has
no effect on the operation of the INTRC oscillator when
the FSCM is enabled.
FIGURE 23-3:
FSCM BLOCK DIAGRAM
Clock Monitor
Latch (CM)
(edge-triggered)
As already noted, the clock source is switched to the
INTOSC clock when a clock failure is detected.
Depending on the frequency selected by the
IRCF<2:0> bits, this may mean a substantial change in
the speed of code execution. If the WDT is enabled
with a small prescale value, a decrease in clock speed
allows a WDT time-out to occur and a subsequent
device Reset. For this reason, fail-safe clock events
also reset the WDT and postscaler, allowing it to start
timing from when execution speed was changed and
decreasing the likelihood of an erroneous time-out.
Peripheral
Clock
S
Q
Q
INTRC
Source
C
÷ 64
(32 μs)
488 Hz
(2.048 ms)
Clock
Failure
Detected
23.4.2
EXITING FAIL-SAFE OPERATION
The fail-safe condition is terminated by either a device
Reset or by entering a power-managed mode. On
Reset, the controller starts the primary clock source
specified in Configuration Register 1H (with any
required start-up delays that are required for the oscil-
lator mode, such as the OST or PLL timer). The
INTOSC multiplexer provides the device clock until the
primary clock source becomes ready (similar to a Two-
Speed Start-up). The clock source is then switched to
the primary clock (indicated by the OSTS bit in the
OSCCON register becoming set). The Fail-Safe Clock
Monitor then resumes monitoring the peripheral clock.
Clock failure is tested for on the falling edge of the
sample clock. If a sample clock falling edge occurs
while CM is still set, a clock failure has been detected
(Figure 23-4). This causes the following:
• the FSCM generates an oscillator fail interrupt by
setting bit, OSCFIF (PIR2<7>);
• the device clock source is switched to the internal
oscillator block (OSCCON is not updated to show
the current clock source – this is the fail-safe
condition) and
• the WDT is reset.
The primary clock source may never become ready dur-
ing start-up. In this case, operation is clocked by the
INTOSC multiplexer. The OSCCON register will remain
in its Reset state until a power-managed mode is
entered.
During switchover, the postscaler frequency from the
internal oscillator block may not be sufficiently stable for
timing sensitive applications. In these cases, it may be
desirable to select another clock configuration and enter
an alternate power-managed mode. This can be done to
© 2008 Microchip Technology Inc.
DS39631E-page 261
MICROCHIP [ MICROCHIP ]
