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Um... Whoops. — Parallax Forums

Um... Whoops.

jeff-ojeff-o Posts: 181
edited 2010-11-14 16:15 in Propeller 1
Hey everyone,

A bit of an issue. I've designed a board that involves a propeller and an ADC. The ADC run on 5V, and communicates by SPI. Here comes the whoops: I forgot to implement level translation between the two! I checked over the datasheets and it looks like the ADC should be happy with the 3.3V data being sent from the Propeller, but no the other way around! The ADC will be sending at least 4.1V (and likely closer to 5) to the propeller.

Of course, the board has already been sent to the board house. So now, I either need assurance that the prop can handle 5V, or I need suggestions for a mod to the board - an inline diode, maybe?

Thanks!

Comments

  • Bill HenningBill Henning Posts: 6,445
    edited 2010-11-13 13:34
    An in-line resistor, 1k or so, between the output from the 5v device and the prop pin should do it :)
    jeff-o wrote: »
    Hey everyone,

    A bit of an issue. I've designed a board that involves a propeller and an ADC. The ADC run on 5V, and communicates by SPI. Here comes the whoops: I forgot to implement level translation between the two! I checked over the datasheets and it looks like the ADC should be happy with the 3.3V data being sent from the Propeller, but no the other way around! The ADC will be sending at least 4.1V (and likely closer to 5) to the propeller.

    Of course, the board has already been sent to the board house. So now, I either need assurance that the prop can handle 5V, or I need suggestions for a mod to the board - an inline diode, maybe?

    Thanks!
  • jeff-ojeff-o Posts: 181
    edited 2010-11-13 14:33
    Bill Henning wrote: »
    An in-line resistor, 1k or so, between the output from the 5v device and the prop pin should do it :)

    OK, so that means the prop has built-in clamping diodes? Otherwise, I'd need to add an external diode to 3.3V. I'm worried that this method will reduce the speed to the serial data transmission.
  • localrogerlocalroger Posts: 3,452
    edited 2010-11-13 14:43
    The Prop has clamping diodes, the main worry is making sure they don't dissipate too much current. It's quite possible the SPI transmitter in the ADC won't force enough current into the Prop pin to damage anything; before planning on board mods I'd make an effort to measure this current (use a small series resistor and an oscilloscope to determine the voltage drop across it) and if it's within the absolute max pin specs for the two chips, which it might be, you don't have to do anything.
  • Bill HenningBill Henning Posts: 6,445
    edited 2010-11-13 14:57
    Yes, like localroger said - the prop has built in clamping diodes.

    FYI, I've run 4mbps SPI through a 2k2 resistor without problems :)
    jeff-o wrote: »
    OK, so that means the prop has built-in clamping diodes? Otherwise, I'd need to add an external diode to 3.3V. I'm worried that this method will reduce the speed to the serial data transmission.
  • Dr_AculaDr_Acula Posts: 5,484
    edited 2010-11-13 15:26
    Bill's 1k resistor is the solution. I've used this on many boards. It seems to simple to be true, but it is the perfect bidirectional level translator.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2010-11-13 15:29
    Make that series resistor at least 2.2K to keep within the Prop's protection diode current limit (500 uA).

    -Phil
  • jeff-ojeff-o Posts: 181
    edited 2010-11-13 15:41
    Phil Pilgrim (PhiPi) wrote: »
    Make that series resistor at least 2.2K to keep within the Prop's protection diode current limit (500 uA).

    -Phil

    Excellent. 2.2k it is! Thanks, all. :)
  • Rick_HRick_H Posts: 116
    edited 2010-11-14 16:15
    What chip are you using, I had some ADC0838 laying around but when I made my proto board I swapped them out with TLV0838's. Thier Identical chips that run at 3.3V
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