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5v on the prop pins? — Parallax Forums

5v on the prop pins?

kutalinelucaskutalinelucas Posts: 80
edited 2011-08-14 08:51 in Propeller 1
Could anybody give me any ideas of the best way to interface a PIC18F and a propeller? I'm guessing the prop wouldn't be too happy with 5v on its pins!

I'm looking to communicate through a 2 way 4-bit data bus, so a voltage divider is out of the question. I have a 74LVC244AD line driver, but its surface mount and I'm on a breadboard, plus I haven't really looked into how to use it yet...

Cheers

Comments

  • RaymanRayman Posts: 14,876
    edited 2011-08-11 07:39
    I'd just try putting 10k resistors between PIC and Prop. 10k is enough to prevent damage to Prop.
  • Peter JakackiPeter Jakacki Posts: 10,193
    edited 2011-08-11 07:42
    The PIC18F has a sensible Vih of 2V or more even with a supply voltage of 5V (many have a Vih above 3.6V). This means that the Prop can talk to the PIC directly and all you need to do to keep the Prop happy when the PIC talks to it is to limit the current so the value of 10K that Ray mentioned is fine.
  • kutalinelucaskutalinelucas Posts: 80
    edited 2011-08-11 07:42
    That simple?...fantastic, thankyou very much
  • mindrobotsmindrobots Posts: 6,506
    edited 2011-08-11 07:45
    Chris Savage wrote a great article about all your options in the April Nuts & Volts. Here's a thread discussing it:

    http://forums.parallax.com/showthread.php?130733-Did-any-one-read-the-article-in-Nuts-and-Volts-by-Chris-Savage-about-Mixing-Voltage&highlight=chris+savage+nuts+volts

    There's also some interfacing the Prop to "foreign" voltages in the sticky thread at the top of the forum:

    http://forums.parallax.com/showthread.php?111166-Propeller-HYDRA-Key-Thread-Index

    Of course, when you've done it with success, please come back and share your solution with all of us!!
  • agimuhingagimuhing Posts: 39
    edited 2011-08-11 07:49
    put a resistor between the prop rx pin and PIC tx pin

    (note: tx stands for transmit, rx stands for receive)

    prop rx pin <
    /\/\/\
    <PIC tx pin
    10K resistor

    the prop has internal clamping diodes to keep the voltage below 3.3V, the resistor is there to limit current going through the diodes


    check the PIC's logic high voltage, make sure it's less than 3.3V, then you can directly connect the prop tx pin with the PIC rx pin. A better idea would be to put another resistor between the pins to prevent damage caused by setting the pin states incorrectly (ie: PIC rx pin set to output high instead of input, and prop output low)

    prop tx pin>
    /\/\/\
    >PIC rx
    [/INDENT][/INDENT]10K
    resistor is
    optional, but a good idea

    you could also read this http://www.sparkfun.com/tutorials/65
  • WossnameWossname Posts: 174
    edited 2011-08-11 09:23
    Great thread, very useful to know. :)
  • Toby SeckshundToby Seckshund Posts: 2,027
    edited 2011-08-11 11:44
    Would the PIC part of the circuit be happy at 3.3 Volts ?? If so the drop that down and just bolt the two parts together dirrectly.
  • kutalinelucaskutalinelucas Posts: 80
    edited 2011-08-14 08:51
    just putting 10k resistors worked like a charm...cheers guys
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