PIC18F2431-I/SO Microchip Technology, PIC18F2431-I/SO Datasheet - Page 277

• Manufacturer Part Number: PIC18F2431-I/SO
• Description: IC PIC MCU FLASH 8KX16 28SOIC
• Manufacturer: Microchip Technology
• Series: PIC® 18Fr

Specifications of PIC18F2431-I/SO

• Program Memory Type: FLASH
• Program Memory Size: 16KB (8K x 16)
• Package / Case: 28-SOIC (7.5mm Width)
• Core Processor: PIC
• Core Size: 8-Bit
• Speed: 40MHz
• Connectivity: I²C, SPI, UART/USART
• Peripherals: Brown-out Detect/Reset, LVD, Power Control PWM, QEI, POR, PWM, WDT
• Number Of I /o: 24
• Eeprom Size: 256 x 8
• Ram Size: 768 x 8
• Voltage - Supply (vcc/vdd): 4.2 V ~ 5.5 V
• Data Converters: A/D 5x10b
• Oscillator Type: Internal
• Operating Temperature: -40°C ~ 85°C
• Processor Series: PIC18F
• Core: PIC
• Data Bus Width: 8 bit
• Data Ram Size: 768 B
• Interface Type: EUSART/I2C/SPI/SSP
• Maximum Clock Frequency: 40 MHz
• Number Of Programmable I/os: 24
• Number Of Timers: 4
• Operating Supply Voltage: 2 V to 5.5 V
• Maximum Operating Temperature: + 85 C
• Mounting Style: SMD/SMT
• 3rd Party Development Tools: 52715-96, 52716-328, 52717-734, 52712-325, EWPIC18
• Development Tools By Supplier: PG164130, DV164035, DV244005, DV164005, PG164120, ICE2000, ICE4000, DM183021, DV164136
• Minimum Operating Temperature: - 40 C
• On-chip Adc: 5-ch x 10-bit
• Package: 28SOIC W
• Device Core: PIC
• Family Name: PIC18
• Maximum Speed: 40 MHz
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• For Use With: XLT28SO-1 - SOCKET TRANSITION 28SOIC 300MILI3-DB18F4431 - BOARD DAUGHTER ICEPIC3
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

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 system clock to the internal oscillator block. The
FSCM function is enabled by setting the Fail-Safe
Clock Monitor Enable bit, FCMEN (CONFIG1H<6>).
When FSCM is enabled, the INTRC oscillator runs at
all times to monitor clocks to peripherals and provide
an instant backup clock in the event of a clock failure.
Clock
accomplished by creating a sample clock signal, which
is the INTRC output divided by 64. This allows ample
time between FSCM sample clocks for a peripheral
clock edge to occur. The peripheral system 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 system 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
sample clock. If a sample clock falling edge occurs
while the CM is still set, a clock failure has been
detected
• the FSCM generates an oscillator fail interrupt by
• the system clock source is switched to the internal
• the WDT is reset.
Since the postscaler frequency from the internal
oscillator block may not be sufficiently stable, it may be
desirable to select another clock configuration and
enter an alternate power-managed mode (see
Section 23.3.1 “Special Considerations for Using
Two-Speed Start-up”
Sleep Commands”
done to attempt a partial recovery or execute a
controlled shutdown.
 2010 Microchip Technology Inc.
setting bit, OSCFIF (PIR2<7>);
oscillator block (OSCCON is not updated to show
the current clock source – this is the fail-safe
condition); and
Peripheral
Source
(32 s)
INTRC
Clock
monitoring
Fail-Safe Clock Monitor
(Figure
(2.048 ms)
488 Hz
23-4). This causes the following:
÷ 64
for more details). This can be
(shown
FSCM BLOCK DIAGRAM
and
(edge-triggered)
Clock Monitor
Section 4.1.4 “Multiple
Latch (CM)
C
S
in
Figure
Q
Q
Detected
23-3)
PIC18F2331/2431/4331/4431
Failure
Clock
is
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>, imme-
diately 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.
Adjustments to the internal oscillator block using the
OSCTUNE register also affect the period of the FSCM
by the same factor. This can usually be neglected, as
the clock frequency being monitored is generally much
higher than the sample clock frequency.
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
separate 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
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 Monitor
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 oscillator mode, such as
the OST or PLL timer). The INTOSC multiplexer provides
the system clock until the primary clock source becomes
ready (similar to a Two-Speed Start-up). The clock system
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.
The primary clock source may never become ready
during 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.
Entering a power-managed mode by loading the
OSCCON register and executing a SLEEP instruction
will clear the fail-safe condition. When the fail-safe
condition is cleared, the clock monitor will resume
monitoring the peripheral clock.
FSCM AND THE WATCHDOG TIMER
EXITING FAIL-SAFE OPERATION
DS39616D-page 277