PIC18F87K22 Datasheet, PDF (421/548 Pages) Microchip Technology – 64/80-Pin, High-Performance, 1-Mbit Enhanced Flash Microcontrollers with 12-Bit A/D and nanoWatt XLP Technology

PIC18F87K22 FAMILY
28.4 Two-Speed Start-up
The Two-Speed Start-up feature helps to minimize the
latency period from oscillator start-up to code execution
by allowing the microcontroller to use the INTOSC
(LF-INTOSC, MF-INTOSC, HF-INTOSC) 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 LP, XT or HS (Crystal-Based
modes). Other sources do not require an OST start-up
delay; for these, 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
internal 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.
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>,
immediately after Reset. For wake-ups from Sleep, the
INTOSC or postscaler clock sources can be selected
by setting the IRCF2:0> bits prior to entering Sleep
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.
28.4.1
SPECIAL CONSIDERATIONS FOR
USING TWO-SPEED START-UP
While using the INTOSC oscillator in Two-Speed Start-
up, the device still obeys the normal command
sequences for entering power-managed modes,
including multiple 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
application 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.
FIGURE 28-3:
TIMING TRANSITION FOR TWO-SPEED START-UP (INTOSC TO HSPLL)
INTOSC
Multiplexer
OSC1
PLL Clock
Output
CPU Clock
Q1
Q2
Q3
Q4
Q1
Q2 Q3 Q4 Q1 Q2 Q3
TOST⁽¹⁾
TPLL⁽¹⁾
1 2 n-1 n
Clock
Transition⁽²⁾
Peripheral
Clock
Program
Counter
PC
PC + 2
PC + 4
PC + 6
Wake from Interrupt Event
OSTS bit Set
Note 1: TOST = 1024 TOSC; TPLL = 2 ms (approx). These intervals are not shown to scale.
2: Clock transition typically occurs within 2-4 TOSC.
 2010 Microchip Technology Inc.
Preliminary
DS39960B-page 421