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Microcontroller io pins pulse when turned on — Parallax Forums

Microcontroller io pins pulse when turned on

agimuhingagimuhing Posts: 39
edited 2012-07-08 00:07 in Propeller 1
When I turn on a microcontroller the io pins will output a brief pulse
This happens right when power is supplied to the chip

I noticed that this happens with just about all microcontrollers when they are turned on.

What causes this?

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2012-07-07 13:33
    As the power supply voltage is rising, there may be a very brief period when the output transistors have enough voltage applied to them for them to provide an output voltage, yet the rest of the chip still doesn't have enough voltage available nor enough time to initialize the internal circuitry to the point where it can inhibit the output circuitry. This sort of thing is more likely to happen when the power supply voltage is slow to rise.

    In the case of the Stamps and the Propeller, this could happen very briefly as the voltage is rising enough to measure a voltage at one of the I/O pins, but not yet enough to properly power the chip's internal circuitry. Both microcontrollers have "brown out" circuitry that will hold the chip in reset state for a time while and after the supply voltage is rising to a proper voltage level. There has to be enough voltage for the reset logic to actually work. The reset will keep the I/O pins in an input mode (high impedance) until the firmware is running and changes the I/O mode of some of the pins to outputs..
  • groggorygroggory Posts: 205
    edited 2012-07-08 00:07
    Mike Green wrote: »
    As the power supply voltage is rising, there may be a very brief period when the output transistors have enough voltage applied to them for them to provide an output voltage, yet the rest of the chip still doesn't have enough voltage available nor enough time to initialize the internal circuitry to the point where it can inhibit the output circuitry. This sort of thing is more likely to happen when the power supply voltage is slow to rise.

    In the case of the Stamps and the Propeller, this could happen very briefly as the voltage is rising enough to measure a voltage at one of the I/O pins, but not yet enough to properly power the chip's internal circuitry. Both microcontrollers have "brown out" circuitry that will hold the chip in reset state for a time while and after the supply voltage is rising to a proper voltage level. There has to be enough voltage for the reset logic to actually work. The reset will keep the I/O pins in an input mode (high impedance) until the firmware is running and changes the I/O mode of some of the pins to outputs..

    In other words, utilize the built in brown out circuitry by tie'ing it into the reset pin so the propeller is stable when it turns on.
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