PIC18F4520-I/P Microchip Technology Inc., PIC18F4520-I/P Datasheet - Page 263

• Manufacturer Part Number: PIC18F4520-I/P
• Description: 40 Pin, 32 KB Flash, 1536 RAM, 36 I/O
• Manufacturer: Microchip Technology Inc.

Specifications of PIC18F4520-I/P

• A/d Inputs: 13-Channel, 10-Bit
• Comparators: 2
• Cpu Speed: 10 MIPS
• Eeprom Memory: 256 Bytes
• Input Output: 36
• Interface: I2C/SPI/USART
• Memory Type: Flash
• Number Of Bits: 8
• Package Type: 40-pin PDIP
• Programmable Memory: 32K Bytes
• Ram Size: 1.5K Bytes
• Speed: 40 MHz
• Timers: 1-8-bit, 3-16-bit
• Voltage, Range: 2-5.5 V
• Lead Free Status / Rohs Status: RoHS Compliant part Electrostatic Device

23.4
The Fail-Safe Clock Monitor (FSCM) allows the
microcontroller 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.
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.
FIGURE 23-3:
Clock failure is tested for on the falling edge of the sam-
ple 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
• the device clock source is switched to the internal
• the WDT is reset.
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
 2004 Microchip Technology Inc.
Peripheral
setting bit, OSCFIF (PIR2<7>);
oscillator block (OSCCON is not updated to show
the current clock source – this is the fail-safe
condition) and
Source
(32 s)
INTRC
Clock
Fail-Safe Clock Monitor
(2.048 ms)
488 Hz
÷ 64
FSCM BLOCK DIAGRAM
(edge-triggered)
Clock Monitor
Latch (CM)
C
S
Q
Q
PIC18F2420/2520/4420/4520
Detected
Failure
Clock
Preliminary
an alternate power managed mode. This can be done to
attempt a partial recovery or execute a controlled shut-
down. See Section 3.1.4 “Multiple Sleep Commands”
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, IRCF2:IRCF0, immedi-
ately after Reset. For wake-ups from Sleep, the INTOSC
or postscaler clock sources can be selected by setting the
IRCF2:IRCF0 bits prior to entering Sleep mode.
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
Both the FSCM and the WDT are clocked by the
INTRC oscillator. Since the WDT operates with a sep-
arate divider and counter, disabling the WDT has no
effect on the operation of the INTRC oscillator when the
FSCM is enabled.
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
IRCF2:IRCF0 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.
23.4.2
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 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 pri-
mary clock (indicated by the OSTS bit in the OSCCON
register becoming set). The Fail-Safe Clock Monitor
then resumes monitoring the peripheral clock.
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.
FSCM AND THE WATCHDOG TIMER
EXITING FAIL-SAFE OPERATION
DS39631A-page 261