PIC18F64J11T-I/PT Microchip Technology, PIC18F64J11T-I/PT Datasheet - Page 291

• Manufacturer Part Number: PIC18F64J11T-I/PT
• Description: 16KB, Flash, 1024bytes-RAM, 51I/O, 8-bit Family,nanoWatt 64 TQFP 10x10x1mm T/R
• Manufacturer: Microchip Technology
• Series: PIC® 18Fr

Specifications of PIC18F64J11T-I/PT

• Core Processor: PIC
• Core Size: 8-Bit
• Speed: 40MHz
• Connectivity: I²C, SPI, UART/USART
• Peripherals: Brown-out Detect/Reset, LVD, POR, PWM, WDT
• Number Of I /o: 51
• Program Memory Size: 16KB (8K x 16)
• Program Memory Type: FLASH
• Ram Size: 1K x 8
• Voltage - Supply (vcc/vdd): 2 V ~ 3.6 V
• Data Converters: A/D 12x10b
• Oscillator Type: Internal
• Operating Temperature: -40°C ~ 85°C
• Package / Case: 64-TFQFP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• For Use With: MA180018 - MODULE PLUG-IN 18F85J11AC162079 - HEADER MPLAB ICD2 18F85J90 64/80AC164327 - MODULE SKT FOR 64TQFP
• Eeprom Size: -
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: PIC18F64J11T-I/PTTR

23.5
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 provides
a backup clock in the event of a clock failure. Clock
monitoring (shown in Figure 23-4) 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 23-4:
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 23-5). 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.
 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
Fail-Safe Clock Monitor
(2.048 ms)
488 Hz
÷ 64
FSCM BLOCK DIAGRAM
(edge-triggered)
Clock Monitor
Latch (CM)
C
S
Q
Q
Detected
Failure
Clock
PIC18F85J11 FAMILY
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 attempt a partial recovery or execute a
controlled shutdown. See Section 4.1.4 “Multiple
Sleep Commands” and Section 23.4.1 “Special
Considerations for Using Two-Speed Start-up” for
more details.
The FSCM will detect failures of the primary or secondary
clock sources only. If the internal oscillator block fails, no
failure would be detected, nor would any action be
possible.
23.5.1
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
INTRC clock when a clock failure is detected. This may
mean a substantial change in the speed of code execu-
tion. 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 execu-
tion speed was changed and decreasing the likelihood
of an erroneous time-out.
If the interrupt is disabled, subsequent interrupts while
in Idle mode will cause the CPU to begin executing
instructions while being clocked by the INTRC source.
FSCM AND THE WATCHDOG TIMER
DS39774D-page 291