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Voltage to voltage — Parallax Forums

Voltage to voltage

RogerInHawaiiRogerInHawaii Posts: 87
edited 2009-06-22 01:38 in Propeller 1
I have a particular pin on a chip that has an output voltage of about 1.7V. That pin needs to connect to a pin on another chip that expects about 3V. how do I convert that 1.7V to 3V?

Comments

  • Peter JakackiPeter Jakacki Posts: 10,193
    edited 2009-06-22 00:07
    Digital or analog? If you simply want to detect the 1.7V as a logic high then the Prop will probably be happy with 1.7V (just) but it wouldn't hurt to buffer it with a simple NPN inverter
    (INPUT -->10K-->Base, GND-->Emitter, VCC-->10K-->Collector-->OUTPUT).

    Otherwise if you need an analog output you should just use an opamp configured as a voltage follower set to a gain of 1.76(total).

    *Peter*
  • localrogerlocalroger Posts: 3,452
    edited 2009-06-22 00:09
    Does the output pin pull low, high, or both?

    If it pulls high use a 2N2222 as an open collector inverter. 1K resistor from output to base of 2N2222, emitter to ground, collector through pullup to output voltage you want. When the output goes high current flows through the 2N2222 base-emitter circuit, turning the transistor on and crowbarring the collector to ground. It inverts the signal but works quite well.
  • RogerInHawaiiRogerInHawaii Posts: 87
    edited 2009-06-22 00:10
    It's digital, but it's not the Prop that needs to detect this. Its a different chip that does indeed expect 3V.
  • RogerInHawaiiRogerInHawaii Posts: 87
    edited 2009-06-22 00:20
    It goes from 0V (low) to +1.7V (high) and the target pin definitely expects a +3V to indicate high, so inverting the state is not an option.
  • RogerInHawaiiRogerInHawaii Posts: 87
    edited 2009-06-22 00:26
    Actually, looking at the documentation for this chip it seems that this pin SHOULD produce about +3V when high. I measured across the Vcc and GND pins and that shows +3V, but when I measure across this particular pin (when it's in its high state) and GND it shows only about 1.7V. Hmmm.
  • Clock LoopClock Loop Posts: 2,069
    edited 2009-06-22 00:35
    Couldn't you use a voltage divider but tie the GND resistor to 3v instead of gnd?

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  • localrogerlocalroger Posts: 3,452
    edited 2009-06-22 00:35
    RIH, that sounds suspiciously like the chip is putting out a 50% duty cycle wave instead of a high. Most digital chips only know two voltages, Vss and Vcc give or take a few resistive losses here and there.
  • RogerInHawaiiRogerInHawaii Posts: 87
    edited 2009-06-22 00:42
    The chip I'm using is the Linx Technologies HP Series RF Receiver, part number RXM-900-HP3-PPO:

    http://www.linxtechnologies.com/Products/RF-Modules/HP3-Series-Multiple-Channel-Radio-Frequency-Module/

    And it's the DATA line that shows only 0V (when low) and 1.7V when high. The documentation indicates it should be approximatley +3V when high. I have a question submitted to Linx about this (they don't have wonderful forums like this), but was hoping someone here might have a suggestion while I'm working on it today.
  • localrogerlocalroger Posts: 3,452
    edited 2009-06-22 00:47
    Well RIH I know what I'd do but it would be to hook the scope meter up and look at the signal, and I only have the scope meter because I'm *cough* borrowing it from another branch of the company I work for. I would bet serious money your chip is outputting a square wave but a simple meter can't tell you that.

    You could try using the prop to tell you whether the voltage is toggling or stable. Just repeat / text.hex ina[noparse][[/noparse]bit] might be rather informative.
  • Clock LoopClock Loop Posts: 2,069
    edited 2009-06-22 00:53
    localroger said...
    I would bet serious money your chip is outputting a square wave but a simple meter can't tell you that.

    If you have a meter that can measure frequency, that would be a dirty way to determine if that pin is square waving.
  • shanghai_foolshanghai_fool Posts: 149
    edited 2009-06-22 00:58
    RogerInHawaii said...
    The chip I'm using is the Linx Technologies HP Series RF Receiver, part number RXM-900-HP3-PPO:

    http://www.linxtechnologies.com/Products/RF-Modules/HP3-Series-Multiple-Channel-Radio-Frequency-Module/

    And it's the DATA line that shows only 0V (when low) and 1.7V when high. The documentation indicates it should be approximatley +3V when high. I have a question submitted to Linx about this (they don't have wonderful forums like this), but was hoping someone here might have a suggestion while I'm working on it today.
    The data sheet indicates output logic level should be Vcc - 0.3v. At what voltage are you operating this? Also, the output impedance is listed as 17k so your input impedance should be higher than that. If your termination impedance is 17k, you should get 1/2 (Vcc - 0.3v). If you operate this at 5 -6 Volts and use a 4.7-10k resistor between this and the propeller, it should work fine.
  • RogerInHawaiiRogerInHawaii Posts: 87
    edited 2009-06-22 01:01
    localroger said...
    Well RIH I know what I'd do but it would be to hook the scope meter up and look at the signal, and I only have the scope meter because I'm *cough* borrowing it from another branch of the company I work for. I would bet serious money your chip is outputting a square wave but a simple meter can't tell you that.

    You could try using the prop to tell you whether the voltage is toggling or stable. Just repeat / text.hex ina[noparse][[/noparse]bit] might be rather informative.

    This is a situation where I really only need to know whether it's high or low. It actually doesn't much matter whether it's a really nice square wave. I can indeed send a "bit" from the transmitter and have the DATA line on the receiver go high. And I can actually detect that DATA status via the Propeller, since the Propeller appears to be able to handle the 1.7V output. The problem is when I connect that DATA line to my OTHER chip that does indeed seem to require a good strong +3V that I'm having trouble, since all it's getting is that measly 1.7V.
  • RogerInHawaiiRogerInHawaii Posts: 87
    edited 2009-06-22 01:07
    shanghai_fool said...
    The data sheet indicates output logic level should be Vcc - 0.3v. At what voltage are you operating this? Also, the output impedance is listed as 17k so your input impedance should be higher than that. If your termination impedance is 17k, you should get 1/2 (Vcc - 0.3v). If you operate this at 5 -6 Volts and use a 4.7-10k resistor between this and the propeller, it should work fine.

    The voltage between Vcc and GND on this chip measures at +3V. So your comments about impedence resulting in 1/2 (Vcc - 0.3v) would seem to account for the 1.7V on the DATA pin.

    But, hey, I'm a software engineer. What do I know of impedance? [noparse]:)[/noparse] Nothing, that's what. So I have no clue how to measure it or what to do to correct it. I do have access to +5V on the propeller and MAYBE I can use that for this chip as well as the Prop. But, yikes, I'm afraid of frying something.

    Anyone care to hop on over to Waikiki and lend me a hand?
  • localrogerlocalroger Posts: 3,452
    edited 2009-06-22 01:08
    Well RIH it seems like you do have something electrically wonky then. Without seeing your hookup all I could suggest is making sure the grounds are all at 0V w/r/t each other (I've been deliberately violating that lately for a project so I'm very aware of it) and that Vcc on your TX chip is really the Vcc you think it is, and that the pin voltage is the same disconnected as it is connected to your receiver (not being loaded down).

    You have of course probably checked all this stuff already, but like my missing # in another post sometimes you just need a kick to look at it in the right other way.
  • shanghai_foolshanghai_fool Posts: 149
    edited 2009-06-22 01:38
    The problem may be the "other chip" you ae connecting it to. What chip is that? What voltage do you get with just the Propeller connected? Since the receive only draws ~ 20ma, you can certainly power it from the Propeller protoboard.

    Post Edited (shanghai_fool) : 6/22/2009 7:57:08 AM GMT
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