PIC32MX440F256H-80I Datasheet, PDF (78/646 Pages) Microchip Technology – 64/100-Pin General Purpose and USB 32-Bit Flash Microcontrollers

PIC32MX3XX/4XX
EXAMPLE 4-1: ENABLING THE SOSC
SYSKEY = 0x12345678;
SYSKEY = 0xAA996655;
SYSKEY = 0x556699AA;
// ensure OSCCON is locked
// Write Key1 to SYSKEY
// Write Key2 to SYSKEY
// OSCCON is now unlocked
OSCCONSET = 2;
// make the desired change
// request clock switch
SYSKEY = 0x12345678;
// Relock the SYSKEY
// Write any value other than Key1 or Key2
// OSCCON is relocked
4.2.1.6 Internal Fast RC Oscillator (FRC)
The FRC oscillator is a fast (8 MHz nominal), user trim-
mable, internal RC oscillator with user selectable input
divider, PLL multiplier, and output divider.
4.2.1.6.1 FRC Postscaler Mode (FRCDIV)
Users are not limited to the nominal 8 MHz FRC output
if they wish to use the fast internal oscillator as a clock
source. An additional FRC mode, FRCDIV, implements
a selectable output divider that allows the choice of a
lower clock frequency from 7 different options, plus the
direct 8 MHz output. The output divider is configured
using the FRCDIV<2:0> bits (OSCCON<26:24>).
Assuming a nominal 8 MHz output, available lower fre-
quency options range from 4 MHz (divide-by-2) to
31 kHz (divide-by-256). The range of frequencies
allows users the ability to save power at any time in an
application by simply changing the FRCDIV bits. The
FRCDIV mode is selected whenever the COSC bits
(OSCCON<14:12>) are ‘111’.
4.2.1.6.2 FRC Oscillator with PLL Mode (FRCPLL)
The output of the FRC may also be combined with a
user selectable PLL multiplier and output divider to pro-
duce a SYSCLK across a wide range of frequencies.
The FRC PLL mode is selected whenever the COSC
bits (OSCCON<14:12>) are ‘001’.
Note:
In this mode, the PLL input divider is
forced to ‘2’ to provide a 4 MHz input to the
PLL. This parameter cannot be modified.
4.2.1.6.3 Oscillator Tune Register (OSCTUN)
The FRC Oscillator Tuning register OSCTUN allows
the user to fine tune the FRC oscillator over a range of
approximately ±12% (typical). Each bit increment or
decrement changes the factory calibrated frequency of
the FRC oscillator by a fixed amount.
4.2.1.7
Internal Low-Power RC Oscillator
(LPRC)
The LPRC oscillator is separate from the FRC. It oscil-
lates at a nominal frequency of 31.25 kHz. The LPRC
oscillator is the clock source for the Power-up Timer
(PWRT), Watchdog Timer (WDT), Fail Safe Clock Mon-
itor (FSCM) and PLL reference circuits. It may also be
used to provide a low-frequency clock source option for
the device in those applications where power
consumption is critical, and timing accuracy is not
required.
The desired PLL multiplier and output divider values
can be chosen to provide the desired device frequency
DS61143E-page 76
Preliminary
© 2008 Microchip Technology Inc.