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6.25 Mhz crystal — Parallax Forums

6.25 Mhz crystal

David BetzDavid Betz Posts: 14,530
in Propeller 1

I know that a 6.25 MHz crystal is often used with the P1 for circuits intended to generate video. I have a number of these crystals that I bought from Parallax ages ago along with P1 chips. I now have a need for a 6.25 MHz crystal for another project but the BOM for that project calls for a 6.25 MHz 30pF capacitor. The Parallax store doesn't list a capacitance for the crystals they sell. Does anyone know if a Parallax supplied crystal is 30 pF?

Comments

  • evanhevanh Posts: 16,965
    edited 2025-11-23 22:09

    The 6.25 MHz figure was just get a tidy 100 MHz. About as high as you'd want to push the Prop1. I went for 6 * 16 = 96 MHz in my Prop1 project. 6 MHz is common as mud and comes in cheap miniature oscillator module packages.

    PS: I took one look at existing sysclock setup code and decided to make my own.

    CON
' Custom PCB: 6 MHz SMD oscillator module
        CLKFRQ = 96_000_000
        OSCSEL = 0    ' 0 = XIN only,  1 = 4..16 MHz crystal,  2 = 8..32 MHz crystal,  3 = 20..60 MHz crystal
        CLKSEL = 7    ' 0=RCFAST  1=RCSLOW  2=XIN  3=PLL1X  4=PLL2X  5=PLL4X  6=PLL8X  7=PLL16X


PUB  main

    clkset( (CLKSEL>1)&32 | OSCSEL<<3 | (CLKSEL>2)&64, 13_000_000 )    ' config PLL but stay in RCFAST
    waitcnt(clkfreq>>6 + cnt)               'Wait 1/64 second for OSC/PLL stable
    clkset( (CLKSEL>1)&32 | OSCSEL<<3 | (CLKSEL>2)&64 | CLKSEL, CLKFRQ )    ' PLL switch over

...
  • evanhevanh Posts: 16,965
    edited 2025-11-23 23:14

    PPS: For a crystal, you don't want any capacitor added to the PCB. The OSCSEL = 1 is 36 pF loading internal to the Prop1 XI/XO pins. And if you think that's too much then use OSCSEL = 2 instead.

  • RaymanRayman Posts: 15,874

    The p1/p2 internal crystal caps seems to be a fairly unique thing. Nice to have seems…

  • David BetzDavid Betz Posts: 14,530

    Thanks for the comments. I was under the impression that the crystal itself had an internal capacitance of 30pF. The part number they actually call for is MP062-E. I'm just wondering if I can use one of my old Parallax 6.25 MHz crystals in place of the one the call for as they neglected to send that part in the kit I ordered. In case anyone is interested, I'm talking about this kit. It's a cool retro-like design that implements a simple processor using 7400 series chips.

    https://gigatron.io

  • evanhevanh Posts: 16,965
    edited 2025-11-24 00:47

    They have natural capacitance, yep. They'll all work stable, but maybe not to spec. The Prop1 applied capacitance shifts the resonant frequency. You should be able to measure the change when changing the clkset() mode bits.

    At least that's my limited understanding.

  • R BaggettR Baggett Posts: 233

    @"David Betz" said:
    Thanks for the comments. I was under the impression that the crystal itself had an internal capacitance of 30pF. The part number they actually call for is MP062-E. I'm just wondering if I can use one of my old Parallax 6.25 MHz crystals in place of the one the call for as they neglected to send that part in the kit I ordered. In case anyone is interested, I'm talking about this kit. It's a cool retro-like design that implements a simple processor using 7400 series chips.

    https://gigatron.io

    Interesting. The capacitance can usually be safely ignored if you don't need PPM accuracy and don't care about optimal start-up and stabilization time. I imagine the gigatron cares not about these things.
    That said, 30 pF is very common.

  • David BetzDavid Betz Posts: 14,530

    Thanks! I’ll try the Parallax crystal.

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