Shop OBEX P1 Docs P2 Docs Learn Events
ready to cut the umbilical sx-key — Parallax Forums

ready to cut the umbilical sx-key

BebopALotBebopALot Posts: 79
edited 2006-09-06 15:39 in General Discussion
Hi folks. I am ready to disconnect my Sx-Key and let my SX ride the 5.125 MHz crystal all by itself in my circuit. Problem is, I don't know exactly how to do this. I have the crystal and inverter (if necessary) on my benchtop and am ready to go. Do I first make the oscillator with the 74AC04 (I don't need any help for this step...) or can I drive the SX with the crystal by itself? Anyhow, if someone could kindly give me a step-by-step I would appreciate it.

Thanks!

Comments

  • BeanBean Posts: 8,129
    edited 2006-08-07 00:35
    BebopALot,

    It depends how close to 5.125 MHz you need to be. The SX will probably work by just putting the xtal onto the OSC1 and OSC2 pins and using OSCXT1 or OSCXT2 on the device line.

    If not, you may need some capacitors on each side of the xtal to ground.

    That should be in the area of ±100ppM of 5.125 MHz.

    Bean.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Cheap 4-digit LED display with driver IC·www.hc4led.com

    Low power SD Data Logger www.sddatalogger.com

    "You're braver than you believe, stronger than you seem, and smarter than you think" Christopher Robin to Pooh
    ·
  • BebopALotBebopALot Posts: 79
    edited 2006-08-10 03:13
    Bean, I basically put a crystal lead in Pin 27 and one in Pin 26. It is not running at all...

    Post Edited (BebopALot) : 8/10/2006 3:43:10 AM GMT
  • BeanBean Posts: 8,129
    edited 2006-08-10 11:04
    Try putting caps on each side of the crystal to ground.

    Bean.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Cheap 4-digit LED display with driver IC·www.hc4led.com

    Low power SD Data Logger www.sddatalogger.com

    "You're braver than you believe, stronger than you seem, and smarter than you think" Christopher Robin to Pooh
    ·
  • BebopALotBebopALot Posts: 79
    edited 2006-09-01 14:04
    Bean you comment on how to choose capacitor specs that fall in the area of +-100ppm of 5.125 MHz - lets use .1uf cap. What are the calculations to determine?
  • BeanBean Posts: 8,129
    edited 2006-09-01 14:41
    0.1uF are WAY too big. Try 10 pF to 100 pF.

    Different value caps will offset the crystal frequency.

    You would need a calibrated frequency counter to be able to know how much the caps are pulling the frequency.
    To calculate it you need all kinds of technical data about the crystal (an the SX too).
    You can try finding the smallest and largest cap value that works, then pick a value in the center.

    Try different OSCxxx settings on the device line too.

    Bean.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Cheap used 4-digit LED display with driver IC·www.hc4led.com

    Low power SD Data Logger www.sddatalogger.com
    SX-Video Display Modules www.sxvm.com

    There are only two guaranteed ways to become weathy.
    Spend less than you make.
    Make more than you spend.
    ·
  • lawlopezlawlopez Posts: 6
    edited 2006-09-02 13:54
    .1 uf ???

    if you are putting .1 uf to ground either side of the crystal
    you are doomed.

    something like 20 pf would be a better choice.
  • BebopALotBebopALot Posts: 79
    edited 2006-09-06 15:39
    Thanks Bean and Mr. LawLopez. Can I use any 20-100 pf caps lying around, or do I need special ones ordered from digikey or mouser.com for example?
Sign In or Register to comment.