I'm used to run PIC24 device family (e.g PIC24FJ256GB) at CPU freq 32MHz using the PLL.

Recently I got to use PIC24FJ256GA705 on my new project and when I read the datasheet, I found out that the PLL has mode "PLL8X" (Figure 9-2 page 108) I've 8MHz crystal at the primary clock input, so if I use this mode I should got CPU freq 8MHz x 8 = 64MHz right?

But last night when I try to blink the LED at 1 Hz using "__delay_ms(1000)", the LED won't blink.

The PLL was locked (OSCCONbits.LOCK) when checked.

So I'm just curious is anybody ever used this device at 64 MHz CPU freq???

Can this device really run at 64MHz?

  • \$\begingroup\$ The datasheet tells you this device will run up to 32MHz. The 8x PLL option is not intended to be used with an 8MHz crystal. \$\endgroup\$
    – brhans
    Commented Oct 16, 2018 at 14:38
  • \$\begingroup\$ Where did you get your datasheet from? I can't find PLL8X option in the microchip datasheet: ww1.microchip.com/downloads/en/DeviceDoc/… \$\endgroup\$ Commented Oct 16, 2018 at 14:48
  • \$\begingroup\$ I believe they got it from the 30010118b.pdf datasheet, page 333, PLLMODE "8x PLL is selected" but as brhans already said, it is not intended for use with 8MHz crystal \$\endgroup\$
    – pm101
    Commented Oct 16, 2018 at 15:01
  • \$\begingroup\$ @JohnGo-Soco Page 15 of your linked datasheet. \$\endgroup\$
    – awjlogan
    Commented Oct 16, 2018 at 15:16
  • \$\begingroup\$ How do you know "PLL was locked (OSCCONbits.LOCK)" executed? \$\endgroup\$
    – sstobbe
    Commented Oct 16, 2018 at 15:18

3 Answers 3


No, it will only work up to 32 MHz.


IESO: Two-Speed Oscillator Start-up Enable bit.
1 = Starts up the device with FRC, then automatically switches to the user-selected oscillator when ready.
0 = Starts up the device with the user-selected oscillator source.
PLLMODE<3:0>: Frequency Multiplier Select bits 1111 = No PLL is used (PLLEN bit is unavailable) 1110 = 8x PLL is selected.
1101 = 6x PLL is selected.
1100 = 4x PLL is selected

Choose the mode that it is rated for, if 32MHz max then , x4 or start slow with watch dog timer enabled then switch to fast. Include LED state codes for errors.


OK I got it, even on the first page of the datasheet written "Up to 16 MIPS Operation @ 32 MHz"


on the page 15 written "A Phase-Locked Loop (PLL) frequency multiplier, which allows processor speeds up to 32 MHz".

Thanx guys.


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