PIC18F44J50-I Datasheet, PDF (431/562 Pages) Microchip Technology – 28/44-Pin, Low-Power, High-Performance USB Microcontrollers

PIC18F46J50 FAMILY
27.4 Two-Speed Start-up
The Two-Speed Start-up feature helps to minimize the
latency period, from oscillator start-up to code execu-
tion, by allowing the microcontroller to use the INTRC
oscillator as a clock source until the primary clock
source is available. It is enabled by setting the IESO
Configuration bit.
Two-Speed Start-up should be enabled only if the
primary oscillator mode is HS or HSPLL
(Crystal-Based) modes. Since the EC and ECPLL
modes do not require an Oscillator Start-up Timer
(OST) delay, Two-Speed Start-up should be disabled.
When enabled, Resets and wake-ups from Sleep mode
cause the device to configure itself to run from the inter-
nal oscillator block as the clock source, following the
time-out of the Power-up Timer after a Power-on Reset
is enabled. This allows almost immediate code
execution while the primary oscillator starts and the
OST is running. Once the OST times out, the device
automatically switches to PRI_RUN mode.
In all other power-managed modes, Two-Speed
Start-up is not used. The device will be clocked by the
currently selected clock source until the primary clock
source becomes available. The setting of the IESO bit
is ignored.
FIGURE 27-3:
TIMING TRANSITION FOR TWO-SPEED START-UP (INTRC TO HSPLL)
INTRC
Q1
Q2
Q3
Q4
Q1
Q2 Q3 Q4 Q1 Q2 Q3
OSC1
PLL Clock
Output
CPU Clock
TOST⁽¹⁾
TPLL⁽¹⁾
1 2 n-1 n
Clock
Transition
Peripheral
Clock
Program
Counter
PC
PC + 2
PC + 4
Wake from Interrupt Event
OSTS bit Set
Note1: TOST = 1024 TOSC; TPLL = 2 ms (approx). These intervals are not shown to scale.
PC + 6
27.4.1
SPECIAL CONSIDERATIONS FOR
USING TWO-SPEED START-UP
While using the INTRC oscillator in Two-Speed
Start-up, the device still obeys the normal command
sequences for entering power-managed modes,
including serial SLEEP instructions (refer to
Section 4.1.4 “Multiple Sleep Commands”). In
practice, this means that user code can change the
SCS<1:0> bit settings or issue SLEEP instructions
before the OST times out. This would allow an applica-
tion to briefly wake-up, perform routine “housekeeping”
tasks and return to Sleep before the device starts to
operate from the primary oscillator.
User code can also check if the primary clock source is
currently providing the device clocking by checking the
status of the OSTS bit (OSCCON<3>). If the bit is set,
the primary oscillator is providing the clock. Otherwise,
the internal oscillator block is providing the clock during
wake-up from Reset or Sleep mode.
27.5 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.
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 27-4) 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.
The clock monitor is set on the falling edge of the
device clock source but cleared on the rising edge of
the sample clock.
 2011 Microchip Technology Inc.
DS39931D-page 431