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pin voltage and current 90 mA 15 pins high => only 2.8 volts instead of 3.3 — Parallax Forums

pin voltage and current 90 mA 15 pins high => only 2.8 volts instead of 3.3

JkaneJkane Posts: 113
in Propeller 1
Hello

I have a device that high is 3.3 low 0 volts, if i use a dc power supply i can set the voltage of the device pins (15) to 3.3volts, but for this project i need to control the device by a micro controller, i am using 15 pins and need to set all 15 to 3.3 volts, but when i use the propeller and when i set all the pins high is get 2.8 volts on each pin, as far as current the combined current is about 90 milli amps. so any ideas how to get the propeller to set to 3.3 volts, I could use a relay to a power supply but that is messy.
by the way, this also happens if i only set one pin, not 15.

regards

Jeff

Comments

  • Peter JakackiPeter Jakacki Posts: 10,193
    A perfect switch will give you 3.3V at 90ma, but no switches are perfect as there is always some amount of resistance. Same goes for the MOSFETs that drive the pin, they have an ON resistance of around 45ohms or so. If you connect them to another CMOS input you will have 3.3V, if you try to drive a short circuit high you will have 0V. If you drive a heavy load the Voh will be less than 3.3V etc. The Voh of any device is specified at a certain current, as they say, RTM.

    It is normally a bad idea for a whole lot of reasons to try to drive a 90ma load from what I gather are paralled I/O. You should use a 5 cent transistor for this purpose which can not only switch at least 100's of milliamps, it can also handle higher load voltages. Bad design practices is what "messy" is.
  • Mark_TMark_T Posts: 1,981
    More information please about the "device" and what you are trying to achieve, or we'll be in xyproblem land...
  • wmosscropwmosscrop Posts: 409
    edited 2017-10-13 13:43
    Hi Jeff -

    Can you provide a datasheet for the device?

    You say you need to set all 15 pins to 3.3v, and when you do this the current is 90 milliamps and the voltage drops to 2.8.

    1) What is your power supply rated for? Is it batteries or a wall adapter? The current needed depends on the device but that sounds high.

    2) Are you using current-limiting resistors between the Propeller and the device inputs? I'd try 4K7 (4700 ohm) resistors. There will be a slight voltage drop but it shouldn't affect your device.

    3) Do you have a capacitor (0.1uf - "104") between V+ and GND on the device?

    Regarding setting only one pin: many devices need to have all input pins connected to either V+ or GND. In your case, I suspect the unconnected pins are "floating", causing random states in your device, and possibly causing the extra power demand.

    Walter
  • jmgjmg Posts: 15,173
    Jkane wrote: »
    by the way, this also happens if i only set one pin, not 15.
    That's harder to believe, as 15 pin in parallel, will drive higher than one.
    Maybe you are not actually driving all 15 ?


    (3.3-2.8)/90m = 5.555 Ohms, sounds lower than one pin, but higher than all 15 ?
    What is the Prop Supply In doing as you draw 90mA ?

    Usually, you would use a MOSFET (eg SOT23) to do this wort of switching.
    A 1 Ohm MOSFET will drop 90mV, for 3.21V out.
    A 0.1 Ohm MOSFET will give 3.291V etc

    NFET (drive low) have slightly better Rds values/$ than PFET (drive hi).



  • kwinnkwinn Posts: 8,697
    Also, keep in mind that Vdd and Vss supplied to the chip comes in on either 2 or 4 pairs of pins so there will be some voltage drop there as well since the total output current on the I/O pins plus that for the chip will be divided between those pins.
  • evanhevanh Posts: 15,916
    Trying to provide the power via signal lines is "messy" as Peter puts it.
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