Datasheet

 2002-2013 Microchip Technology Inc. DS30487D-page 47
PIC16F87/88
4.7.3.2 Returning to Primary Oscillator with
a Reset
A Reset will clear SCS<1:0> back to ‘00’. The
sequence for starting the primary oscillator following a
Reset is the same for all forms of Reset, including
POR. There is no transition sequence from the
alternate system clock to the primary system clock on
a Reset condition. Instead, the device will reset the
state of the OSCCON register and default to the
primary system clock. The sequence of events that
takes place after this will depend upon the value of the
FOSC bits in the Configuration register. If the external
oscillator is configured as a crystal (HS, XT or LP), the
CPU will be held in the Q1 state until 1024 clock cycles
have transpired on the primary clock. This is
necessary because the crystal oscillator has been
powered down until the time of the transition.
During the oscillator start-up time, instruction
execution and/or peripheral operation is suspended.
If the primary system clock is either RC, EC or INTRC,
the CPU will begin operating on the first Q1 cycle
following the wake-up event. This means that there is
no oscillator start-up time required because the
primary clock is already stable; however, there is a
delay between the wake-up event and the following
Q2. An internal delay timer of 5-10 s will suspend
operation after the Reset to allow the CPU to become
ready for code execution. The CPU and peripheral
clock will be held in the first Q1.
The sequence of events is as follows:
1. A device Reset is asserted from one of many
sources (WDT, BOR, MCLR
, etc.).
2. The device resets and the CPU start-up timer is
enabled if in Sleep mode. The device is held in
Reset until the CPU start-up time-out is
complete.
3. 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. While waiting for the OST,
the device will be held in Reset. The OST and
CPU start-up timers run in parallel.
4. After both the CPU start-up and OST timers
have timed out, the device will wait for one addi-
tional clock cycle and instruction execution will
begin.
FIGURE 4-10: PRIMARY SYSTEM CLOCK AFTER RESET (HS, XT, LP)
Note: If Two-Speed Clock Start-up mode is
enabled, the INTRC will act as the system
clock until the OST timer has timed out.
Q4 Q1
Q3 Q4 Q1 Q2
OSC1
Peripheral
Sleep
Program
PC 0000h
T1OSI
TOST
Q3
TT1P
(1)
Q4
OSC2
OSTS
System Clock
0001h
Q1 Q2 Q3 Q4
Q1 Q2
Clock
Counter
0004h 0005h
0003h
Q1 Q2 Q3 Q4
Reset
TCPU
(3)
Note 1: TT1P = 30.52 s.
2: T
OSC = 50 ns minimum.
3: T
CPU = 5-10 s (1 MHz system clock).
CPU Start-up
TOSC
(2)