PIC16F946 Datasheet, PDF (82/274 Pages) Microchip Technology – 64-Pin Flash-Based, 8-Bit CMOS Microcontrollers with LCD Driver and nanoWatt Technology

PIC16F946
4.5.2
OSCILLATOR START-UP TIME-OUT
STATUS BIT
The Oscillator Start-up Time-out Status (OSTS) bit
(OSCCON<3>) indicates whether the system clock is
running from the external clock source, as defined by
the FOSC bits, or from the internal clock source. In
particular, OSTS indicates that the Oscillator Start-up
Timer (OST) has timed out for LP, XT or HS modes.
4.6 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
as the clock source and go back to Sleep without
waiting for the primary oscillator to become stable.
Note:
Executing a SLEEP instruction will abort
the oscillator start-up time and will cause
the OSTS bit (OSCCON<3>) to remain
clear.
When the PIC16F946 is configured for LP, XT or HS
modes, the Oscillator Start-up Timer (OST) is enabled
(see Section 4.3.1 “Oscillator Start-up Timer
(OST)”). The OST timer will suspend program
execution until 1024 oscillations are counted.
Two-Speed Start-up mode minimizes the delay in code
execution by operating from the internal oscillator as
the OST is counting. When the OST count reaches
1024 and the OSTS bit (OSCCON<3>) is set, program
execution switches to the external oscillator.
4.6.1
TWO-SPEED START-UP MODE
CONFIGURATION
Two-Speed Start-up mode is configured by the
following settings:
• IESO = 1 (CONFIG<10>) Internal/External
Switchover bit.
• SCS = 0.
• FOSC configured for LP, XT or HS mode.
Two-Speed Start-up mode is entered after:
• Power-on Reset (POR) and, if enabled, after
PWRT has expired, or
• Wake-up from Sleep.
If the external clock oscillator is configured to be anything
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.
4.6.2
TWO-SPEED START-UP
SEQUENCE
1. Wake-up from Power-on Reset or Sleep.
2. Instructions begin execution by the internal
oscillator at the frequency set in the IRCF bits
(OSCCON<6:4>).
3. OST enabled to count 1024 clock cycles.
4. OST timed out, wait for falling edge of the
internal oscillator.
5. OSTS is set.
6. System clock held low until the next falling edge
of new clock (LP, XT or HS mode).
7. System clock is switched to external clock
source.
4.6.3
CHECKING EXTERNAL/INTERNAL
CLOCK STATUS
Checking the state of the OSTS bit (OSCCON<3>) will
confirm if the PIC16F946 is running from the external
clock source as defined by the FOSC bits in the
Configuration Word (CONFIG) or the internal oscillator.
DS41265A-page 80
Preliminary
© 2005 Microchip Technology Inc.