PIC18F2420/2520/4420/4520
In all other power-managed modes, Two-Speed Start-
23.3 Two-Speed Start-up
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.
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
oscillator as a clock source until the primary clock
source is available. It is enabled by setting the IESO
Configuration bit.
23.3.1
SPECIAL CONSIDERATIONS FOR
USING TWO-SPEED START-UP
Two-Speed Start-up should be enabled only if the
primary oscillator mode is LP, XT, HS or HSPLL
(Crystal-Based modes). Other sources do not require
an OST start-up delay; for these, Two-Speed Start-up
should be disabled.
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 3.1.4 “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.
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.
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.
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 IRCF<2:0> bits prior to entering Sleep
mode.
FIGURE 23-2:
TIMING TRANSITION FOR TWO-SPEED START-UP (INTOSC TO HSPLL)
Q3
Q4
Q1
Q2 Q3 Q4 Q1 Q2 Q3
Q1
Q2
INTOSC
Multiplexer
OSC1
(1)
TOST
(1)
TPLL
1
2
n-1
n
PLL Clock
Output
Clock
Transition(2)
CPU Clock
Peripheral
Clock
Program
Counter
PC
PC + 2
PC + 6
PC + 4
OSTS bit Set
Wake from Interrupt Event
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.
DS39631E-page 260
© 2008 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
