Datasheet

dsPIC30F
DS70043F-page 20 © 2005 Microchip Technology Inc.
FIGURE 7-1: OSCILLATOR SYSTEM BLOCK DIAGRAM
7.4 Programmable Power-up Timer
(PWRT)
There are two internal timers that offer necessary
delays on power-up. One is the Power-up Timer
(PWRT), which provides a delay on power-up only. The
PWRT keeps the part in Reset while the power supply
stabilizes. The other is the Oscillator Start-up Timer
(OST), intended to keep the chip in Reset until the
crystal oscillator is stable. With these two timers on-
chip, most applications need no external Reset
circuitry.
7.5 Oscillator Start-up Timer/Stabilizer
(OST)
An Oscillator Start-up Timer (OST) is included to
ensure that a crystal oscillator (or ceramic resonator)
has started and stabilized. The OST is a simple 10-bit
counter that counts 1024 TOSC cycles before
releasing the oscillator clock to the rest of the system.
The time-out period is designated as TOST. The TOST
time is involved every time the oscillator has to restart
(i.e., on Power-On Reset (POR), Brown-Out Reset
(BOR) and wake-up from Sleep). The oscillator start-up
timer is applied to the LP oscillator, XT, XTL and HS
modes (upon wake-up from Sleep, POR and BOR) for
the primary oscillator.
OSC1
Secondary
Oscillator
32 kHz
PLL
x4, x8, x16
Clock
Switching
and
Control
Block
OSC2
SOSCO
SOSCI
FOSC
Primary Osc
PLL
Secondary Osc
To Timer1
Internal Fast
RC (FRC)
Oscillator
Internal Low
Power RC
(LPRC) Osc
Primary
Oscillator
Programmable
Clock
Divider
4 x FCY