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Power on reset / reset circuit help — Parallax Forums

Power on reset / reset circuit help

John AbshierJohn Abshier Posts: 1,116
edited 2011-02-10 14:00 in General Discussion
Chip is a MCP23008 I2C IO expander on a Propeller Professional Development Board. Power and reset (active low) are wired to the 3.3 volt supply. Problem: You can turn the power switch off, wait 5 minutes and the MCP23008 will not reset because power has not dropped enough for power on reset. Voltage for POR typical value is Vss. Min and Max not specified. One solution would be to put a resistor between 3.3 volts and ground. The down side to this solution is that it draws power all the time--not good for battery powered apps. I cannot think of a circuit that would pull the reset line low and then high on power up.

John Abshier

Comments

  • ctwardellctwardell Posts: 1,716
    edited 2011-02-10 13:35
    John,

    Tie /reset to 3.3V supply with a resistor, 10K or so, and then also tie /reset to ground with a 0.1uF capacitor.

    The capacitor pulls /reset low briefly on power up. Might need to tweak the values a bit, but those should be in the ballpark.

    C.W.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2011-02-10 13:40
    If the reset threshold is that low, you will probably have to use a supervisor chip to trigger the reset pin. Microchip makes those, too. (Use the MCPxxx types with the built-in delay.)

    -Phil
  • ctwardellctwardell Posts: 1,716
    edited 2011-02-10 13:45
    Phil,

    The description that was given sounds like /reset is tied directly to the 3.3V supply, so it is rising above the threshold before the chip is fully powered. The resistor/capacitor circuit should hold it low long enough.

    C.W.
  • LeonLeon Posts: 7,620
    edited 2011-02-10 13:47
    If a capacitor is holding charge, a reverse-connected diode could be used to discharge it. I've had to do that, on occasion.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2011-02-10 13:54
    ctwardell wrote:
    The description that was given sounds like /reset is tied directly to the 3.3V supply, ...
    It may be wired that way on the PPDB, but the chip itself has a separate /RESET pin. BTW, the input low threshold for /RESET is 0.2*Vdd, which doesn't help a lot when it's following Vdd. That's why a supervisor chip would be better, but it sounds like it would entail cutting a trace to install.

    I highly recommend supervisor chips. They're cheap and much more reliable than the usual cap/resistor/diode approach. They provide a fixed delay, regardless of how long the power has been off. No more "waiting for the bits to settle" to get a clean reset.

    -Phil
  • ctwardellctwardell Posts: 1,716
    edited 2011-02-10 14:00
    Phil,

    I don't see the MCP23008 on the PPDB schematic, I think it is being added by the poster. I agree about using the supervisor if there are going to be more chips that have resets in the overall solution,

    C.W.
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