Vault 7: Projects

This publication series is about specific projects related to the Vault 7 main publication.

© 2009 Microchip Technology Inc. DS39700C-page 6-23
Section 6. Oscillator
Oscillator
6
6.8 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 or ceramic
resonator. The PIC24F operates from the FRC Oscillator whenever the COSC bits are 111’,
001’ or ‘000’.
6.8.1 Enabling the FRC Oscillator
Since it serves as the reference clock during device initialization, the FRC Oscillator is always
enabled at a POR. After the device is configured and PWRT expires, FRC remains active only if
it is selected as the device clock source.
6.8.2 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 post-
scaler that allows the choice of a lower clock frequency from 7 different options, plus the direct
8 MHz output. The postscaler is configured using the RCDIV<2:0> bits (CLKDIV<10:8>). Assum-
ing a nominal 8 MHz output, available lower frequency 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 RCDIV bits.
The FRCDIV mode is selected whenever the COSC bits are ‘111’.
6.8.3 FRC Oscillator with PLL Mode (FRCPLL)
The FRCPLL mode is selected whenever the COSC bits are 001’. In addition, this mode only
functions when the direct or divide-by-2 FRC postscaler options are selected (RCDIV<2:0> = 000
or 001).
For devices with the basic 4x PLL block, the output of the FRC postscaler block may also be
combined with the PLL to produce a nominal system clock of either 16 MHz or 32 MHz. Although
somewhat less precise in frequency than using the Primary Oscillator with a crystal or resonator, it
still allows high-speed operation of the device without the use of external oscillator components.
For devices with the 96 MHz PLL block, the output of the FRC postscaler block may also be com-
bined with the PLL to produce a nominal system clock of either 4 MHz, 8 MHz, 16 MHz or
32 MHz. It also produces a 48 MHz USB clock; however, this USB clock must be generated with
the FRC Oscillator meeting the frequency accuracy requirement of USB for proper operation.
Refer to the specific device data sheet for details on the FRC Oscillator electrical characteristics.
In cases where the frequency accuracy is not met for USB operation, the FRCPLL mode should
not be used when USB is active.
Note: Due to specified FRC accuracy, the FRC Oscillator modes may not meet the mini-
mum frequency accuracy requirements for serial communications (such as UART
and USB). Refer to the product family data sheet for more information about the
FRC accuracy.

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