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Expanding I/O Pins — Parallax Forums

Expanding I/O Pins

jkroylancejkroylance Posts: 12
edited 2006-12-03 17:39 in Propeller 1
I'm planning on using a propeller chip as the processor for a digital instrument panel I'm designing for my car and I need a way to control 392 LEDs with minimal I/O pins. I've seen 24-port I/O expanders on eBay but can't figure out what chip they're using. Any advice would be greatly appreciated.

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2006-12-03 01:28
    The Microchip MCP23017 is a 16 bit I2C I/O expander. You can have up to 8 devices on a pair of pins acting as an I2C bus (8 X 16 = 128). Each device can only handle 150ma, so you could have about 9ma per LED if all were on, otherwise you'd need a transistor driver.

    The 74HC595 is an 8 bit output expander that's cascadable, needs 4 I/O pins, but several can be shared among several strings of them. TI makes a similar chip that includes MOSFET drivers on chip.
  • LawsonLawson Posts: 870
    edited 2006-12-03 02:13
    I'd also look into driving the LED's with a 14x28 passive matrix. This would have only 28 LED's lit at any instant and require refressing of the whole matrix at a 60Hz plus rate. The advantage though is that it only requires 42 total pins. Thus it should be easier to fit on a PCB and cheaper. (note a 20x20 matrix would also work but would have a partially filled row. a 16x25 matrix would also work well and fit neatly into main memory)

    my 2 cents
  • OzStampOzStamp Posts: 377
    edited 2006-12-03 08:42
    Hi

    Why not use the I2C bus if high speed is not really an issue..
    You could use the 2 lines that connect to the eeprom (24LC256) ..

    Mike G suggested the MPC23017 that is a good one also consider looking at http://www.nxp.com/pip/PCA9635_2.html
    Or start at http://nxp.com and search thru the I2C devices.
    It is amazing what chips these people make for the I2C bus..

    Ronald Nollet
    Parallax Australia Distributor
  • jkroylancejkroylance Posts: 12
    edited 2006-12-03 17:39
    Speed is fairly important. The LEDs will be divided into banks. There are 2 banks of 80 LEDs for the tach and 2 banks of 80 LEDs for the speedometer, but I need individual control over each bank. Speed is not as much of an issue with the speedometer as it is with the tach, the tach will have to be updated much more than the speedometer will and I want to have as little lag as possible. The rest of the LEDs (72 of them) are for other gauges and speed isn't really an issue with them.
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