Datasheet

PIC16(L)F1516/7/8/9
DS41452C-page 56  2010-2012 Microchip Technology Inc.
5.4 Two-Speed Clock Start-up Mode
Two-Speed Start-up mode provides additional power
savings by minimizing the latency between external
oscillator start-up and code execution. In applications
that make heavy use of the Sleep mode, Two-Speed
Start-up will remove the external oscillator start-up
time from the time spent awake and can reduce the
overall power consumption of the device. This mode
allows the application to wake-up from Sleep, perform
a few instructions using the INTOSC internal oscillator
block as the clock source and go back to Sleep without
waiting for the external oscillator to become stable.
Two-Speed Start-up provides benefits when the oscil-
lator module is configured for LP, XT or HS modes.
The Oscillator Start-up Timer (OST) is enabled for
these modes and must count 1024 oscillations before
the oscillator can be used as the system clock source.
If the oscillator module is configured for any mode
other than LP, XT, or HS mode, then Two-Speed
Start-up is disabled. This is because the external clock
oscillator does not require any stabilization time after
POR or an exit from Sleep.
If the OST count reaches 1024 before the device
enters Sleep mode, the OSTS bit of the OSCSTAT reg-
ister is set and program execution switches to the
external oscillator. However, the system may never
operate from the external oscillator if the time spent
awake is very short.
5.4.1 TWO-SPEED START-UP MODE
CONFIGURATION
Two-Speed Start-up mode is configured by the
following settings:
• IESO (of the Configuration Words) = 1;
Internal/External Switchover bit (Two-Speed
Start-up mode enabled).
• SCS (of the OSCCON register) = 00.
• FOSC<2:0> bits in the Configuration Words
configured for LP, XT or HS mode.
Two-Speed Start-up mode is entered after:
• Power-on Reset (POR) and, if enabled, after
Power-up Timer (PWRT) has expired, or
• Wake-up from Sleep.
TABLE 5-1: OSCILLATOR SWITCHING DELAYS
Note: Executing a SLEEP instruction will abort
the oscillator start-up time and will cause
the OSTS bit of the OSCSTAT register to
remain clear.
Switch From Switch To Frequency Oscillator Delay
Sleep/POR
LFINTOSC
HFINTOSC
31 kHz
31.25kHz-16MHz
Oscillator Warm-up Delay (T
WARM)
Sleep/POR EC, RC DC – 20 MHz 2 cycles
LFINTOSC EC, RC DC – 20 MHz 1 cycle of each
Sleep/POR
Secondary Oscillator,
LP, XT, HS
32 kHz-20 MHz 1024 Clock Cycles (OST)
Any clock source HFINTOSC 31.25 kHz-16 MHz 2 s (approx.)
Any clock source LFINTOSC 31 kHz 1 cycle of each
Any clock source Secondary Oscillator 32 kHz 1024 Clock Cycles (OST)