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SX power consumption — Parallax Forums

SX power consumption

NateNate Posts: 154
edited 2005-01-04 00:32 in General Discussion
I·am building a system that will be using the SX28 powered by a small battery source, so I was looking at the SX spec sheet to see exactly how small a battery·I could get away with.· This application will use the SX to respond to logic input signals to output logic signals, and will speed the majority of the time in sleep mode.· Thus, the current used by the SX while in sleep mode will be a large factor in deciding what battery to use.

The SX spec. sheet states that at 3.0V, with SX in sleep mode and WDT disabled, the SX should draw 1.0 uA Typ, 9.0 uA Max.·

I programmed the SX with a simple goto-sleep program and measured (with high quality bench meter) ~50 uA current draw by the SX in sleep mode at a supply voltage of 3.0V.· This seems quite high·compared to the spec. sheet.· Has anyone else measured this operating parameter and can confirm or dispute my measurement?

nate

Comments

  • allanlane5allanlane5 Posts: 3,815
    edited 2005-01-03 15:15
    Nope, I have not measured the 'sleep' current. I believe you may have to set all pins as Input for minimal current.

    50 uA at 3 volts is equivalent to a 60 KOhm resistor. Do you have pull-up resistors on your circuit?
  • BeanBean Posts: 8,129
    edited 2005-01-03 16:19
    I had a similar problem because I had the pins as inputs and had them open. This causes the gates to toggle and causes excessive current draw.

    Make sure all pins are outputs OR are tied high or low with a resistor.

    Bean.
  • NateNate Posts: 154
    edited 2005-01-03 18:36
    My mistake.· I did have one pin set to input with no active pull-up resitor.· With this rectified, the current draw of the SX28 does indeed drop below 1 uA in sleep mode @3.0 V (<1 uA @ 4.5V also).·

    Thanks for the help.

    nate
  • allanlane5allanlane5 Posts: 3,815
    edited 2005-01-03 19:29
    Yikes, I had it exactly backward! Thanks Bean.
  • David BDavid B Posts: 592
    edited 2005-01-03 23:14
    Are the Scenix internal pullup resistors suitable for safeguarding unused inputs against oscillation?
  • BeanBean Posts: 8,129
    edited 2005-01-03 23:24
    If I remember they are about 20K so I would think they are okay.
    Of course you have to take it into account if you must pull the pin low.
    At 3 volts it will draw 150uA to pull the pin low.
    Bean.
  • Tracy AllenTracy Allen Posts: 6,666
    edited 2005-01-03 23:59
    There are graphs of pullup current as a function of voltage in the data sheet. The resistance is nonlinear, so, while the resistance is about 20 kohms around the voltage level of the TTL threshold, the current with the input grounded is around 400 microamps on a 5 volt Vdd. 100 microamps with a 3 volt Vdd. What do you mean by "stop oscillation"? If the input is not connected to any external circuitry, that will certainly be sufficient to hold the input high and minimize power supply current. It might not be enough though if the pin is connected to an unterminated cable or something like that.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Tracy Allen
    www.emesystems.com
  • allanlane5allanlane5 Posts: 3,815
    edited 2005-01-04 00:32
    Exactly. An unterminated, high-impedance input is known as an "Antenna". It will pick up any stray RF (and there's LOTS of stray RF -- Flourescent lights put out 60 HZ really well.)

    So, if you enable the internal pull-ups, that SHOULD provide enough 'hard' signal to disable the "Antenna Effect".
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