PIC16F87-E Datasheet, PDF (145/228 Pages) Microchip Technology – 18/20/28-Pin Enhan lashMicrocontrollers with nanoWatt Technology

PIC16F87/88
15.12.3 TWO-SPEED CLOCK START-UP
MODE
Two-Speed Start-up mode minimizes the latency
between oscillator start-up and code execution that
may be selected with the IESO (Internal/External
Switchover) bit in Configuration Word 2. This mode is
achieved by initially using the INTRC for code
execution until the primary oscillator is stable.
If this mode is enabled and any of the following condi-
tions exist, the system will begin execution with the
INTRC oscillator. This results in almost immediate
code execution with a minimum of delay.
• POR and after the Power-up Timer has expired (if
PWRTEN = 0);
• or following a wake-up from Sleep;
• or a Reset when running from T1OSC or INTRC
(after a Reset, SCS<1:0> are always set to ‘00’).
Note:
Following any Reset, the IRCF bits are
zeroed and the frequency selection is
forced to 31.25 kHz. The user can modify
the IRCF bits to select a higher internal
oscillator frequency.
If the primary oscillator is configured to be anything
other than XT, LP or HS, then Two-Speed Start-up
mode is disabled because the primary oscillator will not
require any time to become stable after POR, or an exit
from Sleep.
If the IRCF bits of the OSCCON register are configured
to a non-zero value prior to entering Sleep mode, the
system clock frequency will come from the output of
the INTOSC. The IOFS bit in the OSCCON register will
be clear until the INTOSC is stable. This will allow the
user to determine when the internal oscillator can be
used for time critical applications.
Checking the state of the OSTS bit will confirm
whether the primary clock configuration is engaged. If
not, the OSTS bit will remain clear.
When the device is auto-configured in INTRC mode
following a POR or wake-up from Sleep, the rules for
entering other oscillator modes still apply, meaning the
SCS<1:0> bits in OSCCON can be modified before the
OST time-out has occurred. This would allow the
application to wake-up from Sleep, perform a few
instructions using the INTRC 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 to remain clear.
15.12.3.1 Two-Speed Start-up Mode
Sequence
1. Wake-up from Sleep, Reset or POR.
2. OSCCON bits configured to run from INTRC
(31.25 kHz).
3. Instructions begin execution by INTRC
(31.25 kHz).
4. OST enabled to count 1024 clock cycles.
5. OST timed out, wait for falling edge of INTRC.
6. OSTS is set.
7. System clock held low for eight falling edges of
new clock (LP, XT or HS).
8. System clock is switched to primary source (LP,
XT or HS).
The software may read the OSTS bit to determine
when the switchover takes place so that any software
timing edges can be adjusted.
FIGURE 15-9:
TWO-SPEED START-UP MODE
CPU Start-up
Q1
Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4
INTRC
OSC1
OSC2
System Clock
Sleep
OSTS
Program
Counter
PC
TOST
0000h
0001h
0003h
0004h
0005h
 2005 Microchip Technology Inc.
DS30487C-page 143