PIC16F87-E Datasheet, PDF (47/228 Pages) Microchip Technology – 18/20/28-Pin Enhan lashMicrocontrollers with nanoWatt Technology

4.7.3
SEC_RUN/RC_RUN TO PRIMARY
CLOCK SOURCE
When switching from a SEC_RUN or RC_RUN mode
back to the primary system clock, following a change
of SCS<1:0> to ‘00’, the sequence of events that takes
place will depend upon the value of the FOSC bits in
the Configuration register. If the primary clock source
is configured as a crystal (HS, XT or LP), then the tran-
sition will take place after 1024 clock cycles. This is
necessary because the crystal oscillator has been
powered down until the time of the transition. In order
to provide the system with a reliable clock when the
changeover has occurred, the clock will not be
released to the changeover circuit until the 1024 count
has expired.
During the oscillator start-up time, the system clock
comes from the current system clock. Instruction
execution and/or peripheral operation continues using
the currently selected oscillator as the CPU clock
source, until the necessary clock count has expired, to
ensure that the primary system clock is stable.
To know when the OST has expired, the OSTS bit
should be monitored. OSTS = 1 indicates that the
Oscillator Start-up Timer has timed out and the system
clock comes from the primary clock source.
Following the oscillator start-up time, the internal Q
clocks are held in the Q1 state until eight falling edge
clocks are counted from the primary system clock. The
clock input to the Q clocks is then released and opera-
tion resumes with the primary system clock determined
by the FOSC bits (see Figure 4-10).
When in SEC_RUN mode, the act of clearing the
T1OSCEN bit in the T1CON register will cause
SCS<0> to be cleared, which causes the SCS<1:0>
bits to revert to ‘00’ or ‘10’ depending on what SCS<1>
is. Although the T1OSCEN bit was cleared, T1OSC will
be enabled and instruction execution will continue until
the OST time-out for the main system clock is com-
plete. At that time, the system clock will switch from the
T1OSC to the primary clock or the INTRC. Following
this, the T1 oscillator will be shut down.
Note:
If the primary system clock is either RC or
EC, an internal delay timer (5-10 µs) will
suspend operation after exiting Secondary
Clock mode to allow the CPU to become
ready for code execution.
PIC16F87/88
4.7.3.1
Returning to Primary Clock Source
Sequence
Changing back to the primary oscillator from
SEC_RUN or RC_RUN can be accomplished by either
changing SCS<1:0> to ‘00’, or clearing the T1OSCEN
bit in the T1CON register (if T1OSC was the secondary
clock).
The sequence of events that follows is the same for
both modes:
1. If the primary system clock is configured as EC,
RC or INTRC, then the OST time-out is skipped.
Skip to step 3.
2. If the primary system clock is configured as an
external oscillator (HS, XT, LP), then the OST
will be active, waiting for 1024 clocks of the
primary system clock.
3. On the following Q1, the device holds the
system clock in Q1.
4. The device stays in Q1 while eight falling edges
of the primary system clock are counted.
5. Once the eight counts transpire, the device
begins to run from the primary oscillator.
6. If the secondary clock was INTRC and the
primary is not INTRC, the INTRC will be shut
down to save current providing that the INTRC
is not being used for any other function, such as
WDT or Fail-Safe Clock monitoring.
7. If the secondary clock was T1OSC, the T1OSC
will continue to run if T1OSCEN is still set;
otherwise, the T1 oscillator will be shut down.
 2005 Microchip Technology Inc.
DS30487C-page 45