© 2004 Microchip Technology Inc. DS70054C-page 7-19
Section 7. Oscillator
Oscillator
7
7.11 Low Power 32 kHz Crystal Oscillator
The LP or Secondary oscillator is designed specifically for low power operation with a 32 kHz
crystal. The LP oscillator is located on the SOSCO and SOSCI device pins and serves as a
secondary crystal clock source for low power operation. The LP oscillator can also drive Timer1
for a real-time clock application.
7.11.1 LP Oscillator Enable
The following control bits affect the operation of the LP oscillator:
1. The COSC<1:0> bits in the OSCCON register (OSCCON<13:12>).
2. The LPOSCEN bit in the OSCCON register (OSCCON<1>).
When the LP Oscillator is enabled, the SOSCO and SOSCI I/O pins are controlled by the
oscillator and cannot be used for other I/O functions.
7.11.1.1 LP Oscillator Continuous Operation
The LP oscillator will always be enabled if the LPOSCEN control bit (OSCCON<1>) is set. There
are two reasons to leave the LP oscillator running. First, keeping the LP oscillator ON at all times
allows a fast switch to the 32 kHz system clock for lower power operation. Returning to the faster
main oscillator will still require an oscillator start-up time if it is a crystal type source (see
Section 7.12 “Oscillator Start-up Timer (OST)”). Second, the oscillator should remain ON at
all times when using Timer1 as a real-time clock.
7.11.1.2 LP Oscillator Intermittent Operation
When the LPOSCEN control bit (OSCCON<1>) is cleared, the LP oscillator will only operate
when it is selected as the current device clock source (COSC<1:0> = 00). The LP oscillator will
be disabled if it is the current device clock source and the device enters Sleep mode.
7.11.2 LP Oscillator Operation with Timer1
The LP oscillator can be used as a clock source for Timer1 in a real-time clock application. Refer
to Section 12. “Timers” for further details.
7.12 Oscillator Start-up Timer (OST)
In order to ensure that a crystal oscillator (or ceramic resonator) has started and stabilized, an
Oscillator Start-up Timer is provided. It is a simple 10-bit counter that counts 1024 T
OSC cycles
before releasing the oscillator clock to the rest of the system. The time-out period is designated
as T
OST. The amplitude of the oscillator signal must reach the VIL and VIH thresholds for the
oscillator pins before the OST can begin to count cycles (see Figure 7-4).
The T
OST time is involved every time the oscillator has to restart (i.e., on POR, BOR and wake-up
from Sleep mode). The Oscillator Start-up Timer is applied to the LP oscillator and the XT, XTL
and HS modes for the Primary oscillator.
7.13 Internal Fast RC Oscillator (FRC)
The FRC oscillator is a fast (8 MHz nominal) internal RC oscillator. This oscillator is intended to
provide reasonable device operating speeds without the use of an external crystal, ceramic
resonator or RC network.
The dsPIC30F operates from the FRC oscillator whenever COSC<1:0> = 01.
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