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  1. #1
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    Default Active low transmitter question

    Name:  Transmitter Active Low Switched from PIC.JPG
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    Can someone kindly help please?

    I have a RF Transmitter that transmits active low but to avoid 'floating voltages' (that could trigger a not wanted transmission) I was thinking the above cct might prevent this happening.

    My thinking is the 10k resistor would hold the Transmitter input and pin3 on the Pic high until my code would make pin3 go low for a few milliseconds and make the Transmitter transmit, am I in the right ballpark with my thinking?

    As ever all help very much appreciated.

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    Default Re: Active low transmitter question

    I suppose it boils down to how much "oomph" the transmitter packs. If it's a weak signal, it may not be able to overcome the 10k pull-up. If it has sufficient amp capability, then you're golden. The best answer will be given from your test. It either works or it doesn't. As posted, not enough information to go on.

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    Default Re: Active low transmitter question

    Thanks for the reply mpgmike

    Wouldn't making pin3 on the pic go low for say 50ms make the transmitter which transmits when 'active low' transmit as the Tx pin on the transmitter is on the same line as the pic pin3 set Output LOW 50ms in code?
    Last edited by LEDave; - 26th June 2021 at 03:47.

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    Default Re: Active low transmitter question

    WHAT??? Your transmitter is "Talking" and your PIC should be "Listening"; or receiving. In order for your PIC to receive/listen, your pin3 must be configured as a digital (ANSEL Register reads 0 for that pin) input (TRIS Register set to 1 for that pin). For you to have your PIC make that transmission line low, you have to clear the TRIS bit to make it an output, then use either the PORT or LAT Register to make the pin3 low.

    WHAT DOES ANY OF THIS HAVE TO DO WITH RECEIVING A TRANSMISSION??!!?!?!??? What do you hope to accomplish??

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    Default Re: Active low transmitter question

    "WHAT DOES ANY OF THIS HAVE TO DO WITH RECEIVING A TRANSMISSION??!!?!?!??? What do you hope to accomplish??"

    mpgmike - I've never mentioned or talked about 'RECEIVING A TRANSMISSION??!!?!?!???' Exactly the opposite. I'm asking if pin3 on the pic is pulled low in code, there will be a wire from pin3 on the pic to the Tx pin on the RF transmitter, the transmitter operates when the Tx pin goes low so I'm asking if I pull pin3 LOW for say 50ms that should make the transmitter transmit.

    All I'm trying to do is replicate and replace a push button from the transmitter Tx pin to ground by using a pic to send the Tx line LOW. The 10k resistor being added to keep the line High to stop any floating voltage causing a 'false' transmit.

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    Default Re: Active low transmitter question

    If your question is, "Can the PIC safely overcome the 10k pull-up resistor?", the answer is absolutely yes. Again, will it do what you want, you'll just have to try it and see.

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    Default Re: Active low transmitter question

    Thanks for your help mpgmike, anyone else have any thoughts on this?

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    Default Re: Active low transmitter question

    anyone else have any thoughts on this?
    using a resistor to drive a pin to a default logic level is not an uncommon practice.
    so common that i would not be surprised that your [type not disclosed ] transmitter has such a pullup
    resistor already installed. the only reason i could see for an additional pullup would be if the pic was routinely disconnected
    from a device that did not have a built in pullup or that an open drain/collector pic pin was in use.
    summing up with the detail[almost non existent] you have provided it seems to be a pointless waste of a component
    that could easily be eliminated with good programming practice.
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    Default Re: Active low transmitter question

    I think it would be best to have the Data sheet of the that transmitter.

    Ioannis

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    Default Re: Active low transmitter question

    Hi Ioannis - Looking back at the thread I've done a pretty miserable job of it so far certainly information wise and I apologise to everyone.

    *I think it would be best to have the Data sheet of the that transmitter.* For sure, I'll add a screen shot then a full data sheet if needed.

    I intend to use maybe a 12F683 to pull a Transmitter Input pin LOW for a few m/s to make it transmit then hold the Tx pin high to stop false triggering via floating voltages if this is safe to do this without damaging the transmitter or pic. I'll add some code as soon as possible, here's the screenshot:

    Name:  Tx.JPG
Views: 185
Size:  125.3 KB

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    Default Re: Active low transmitter question

    I intend to use maybe a 12F683 to pull a Transmitter Input pin LOW for a few m/s to make it transmit then hold the Tx pin high to stop false triggering via floating voltages if this is safe to do this without damaging the transmitter or pic
    what is your doubt ?

    the data so far indicates no need for pullups , the schematic shows none.

    a pic pin when configured as output will never "float" its either high or low when set
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    Default Re: Active low transmitter question

    "what is your doubt ?

    the data so far indicates no need for pullups , the schematic shows none.

    a pic pin when configured as output will never "float" its either high or low when set"


    Hi Richard / Everyone - Thanks for your input. What you write makes total sense, I completely got the wrong end of the stick,
    lack of knowledge on my part, so no pullups needed yet (bear with me, I think I will need one on the Rx though pulldown)

    Tx first though:

    Here is my simple Tx test code given no pullups needed, Tx driven directly by OUTPUT pin state.


    '12F683'

    ANSEL = %00000000 'Disable analog select so ports work as digital i/o
    CMCON0 = %00000111 'Disable analog comparators
    OPTION_REG.7=1 'Disable internal pull-ups

    TRISIO = %00000000 'Set to OUTPUT
    GPIO = %11111111 'Set all outputs HIGH to stop a false Tx

    TestTx VAR BYTE

    FOR TestTx = 1 TO 3 'Test transmit loop

    LOW GPIO.4 'Pull Tx line LOW on transmitter to cause a Tx
    PAUSE 50 'Wait for Tx
    HIGH GPIO.4 'Stop Tx pull line HIGH
    PAUSE 2500 'Wait 2.5 seconds

    NEXT TestTx
    END

    So how does this look for basic a Tx test loop?

    I'll post the Rx Data sheet and the reason given what Richard said above why I think I need a pulldown resistor later tonight.

    Cheers David

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    Default Re: Active low transmitter question

    Name:  Rx.JPG
Views: 175
Size:  102.2 KB

    Here's the Rx schematic. When an Rx is received it outputs 5v onto the corresponding pin, which will then be read by the pic.

    My question now is (and this should have been the question all along):

    On the 12F683 pic I need to set a Tris pin to INPUT and read the pin for state change from LOW to High as the Rx is received.

    TRISIO = %00000001 'Set to Input

    TestRx 'LABEL
    Do SomeThing 'LABEL

    TestRx
    IF
    GPIO.0 = 0 THEN TestRx 'Nothing has happened continue to loop and check for state change

    IF

    GPIO.0 = 1 then DoSomeThing

    DoSomeThing

    Set alarm off in this case........

    So, here's the $64k question from me: How do I hold GPIO.0 LOW until a 5v Rx is received? Is it a pulldown resistor?

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    Default Re: Active low transmitter question

    posting pictures of text is really annoying , post the actual data sheet or a link.

    the annoying picture that cannot be seen when replying says that the output pins are set to reflect the incoming pkt data
    why would the pins change between pkts ?
    how can the pic even know when new data has arrived ?
    how do u know when data is valid ?
    there is no mention of pullups or pull downs in schematic
    a pull down here would be nothing but detrimental in my view
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    Default Re: Active low transmitter question

    Data sheet is needed for sure.

    The module says 5V tolerant, so I guess it works on 3.3V maybe? In that case you better lower the 5V of the PIC to 3.3V just to be sure that no problems may come up.

    Also the module will transmit when a change on input pins occurs. This means either when the switch is closed (grounding the input) or the switch is opened, leaving the input pin on the air.

    So you will have two transmissions and this is how the receiver will reflect transmitters status.

    Ioannis

    P.S. By the way, do you get email notifications when there is a reply? I do not once again...
    Last edited by Ioannis; - 1st July 2021 at 13:21.

  16. #16
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    Default Re: Active low transmitter question

    So how does this look for basic a Tx test loop?
    wont work like you think , it will tx twice once as pin goes low then again as it goes high


    Telemetry mode - Transmit (TX)
    GAMMA module configured as a transmitter
    Operation: When configured as a transmitter the GAMMA module will automatically default to
    low power sleep mode. A change on any input(s) will cause the GAMMA to wake, read all inputs
    and initiate RF transmission, (multiple input changes may take place simultaneously).
    data sheet
    Last edited by richard; - 1st July 2021 at 13:02.
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    Default Re: Active low transmitter question

    http://www.rfsolutions.co.uk/downloa...S-GAMMA-12.pdf

    Ioannis / Richard - Hopefully I've uploaded a link to the RF Module (PDF was too large to upload directly).

    It looks like there are two versions [Supply voltage Standard version 1.8 - 3.6 *V] & [Supply voltage high Voltage version “R” 3.6 - 16 *V]

    Will post again this evening.

    Ioannis - No email notifications received.

  18. #18
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    Default Re: Active low transmitter question

    Ok - So the transmitter will always Tx twice, on a button press to ground, then open contacts (if using a button). Or a transition from HIGH to LOW to High if using a pic GPIO pin to cause a Tx, correct?

    From the data sheet:

    'When configured as transmitter: Pins 3, 4, 7, 8, 9, 11, 12, 13 Inputs are internally pulled high, active low'

    So no pullup needed. How about if I set a TRIS bit to INPUT, corresponding GPIO pin left open but GPIO pin connected to Tx pin 3 say which is internally pulled HIGH, active low. Then when I want to TX, switch the TRIS bit to OUTPUT and pull the corresponding GPIO pin LOW then switch the TRIS bit to INPUT again mimicking a button press.

    Barking mad or genius?

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    Default Re: Active low transmitter question

    Barking mad or genius?
    not genius yet

    Inputs are internally pulled high, active low'
    not that i can see, there is no active low, a tx occurs on state change
    if tx telemetry mode engaged

    you need to explain what mode you are employing and what you are trying to achieve
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    Default Re: Active low transmitter question

    Hi Richard

    I got the idea of a Tx being initiated from the Data sheet 'When configured as transmitter: Pins 3, 4, 7, 8, 9, 11, 12, 13 Inputs are internally pulled high, active low'

    But as you and Ioannis both say a Tx occurs on a pin state change. I assumed that if a pin changed from HIGH to LOW and stayed in the LOW state the transmitter would continue to transmit being 'active low' but my knowledge is very limited I'll be the first to admit but always keen to learn.

    'you need to explain what mode you are employing and what you are trying to achieve'

    In the short term all I'm aiming to do is to have an input into a pic from a sensor trigger a Tx, and the Rx via another pic to trigger an alarm.

    Longer term, I'm fascinated by the idea of data transmission and would love to be able to configure a pic to transmit data - Set the Baud rate, SEROUT on Tx then SERIN and a lcd to read what's sent but that's for another day.

    Right now just a simple sensor (beam gets broken) set off a Tx, Rx to trigger an alarm on receipt via another pic.

  21. #21
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    Default Re: Active low transmitter question

    So to make the transmitter Tx once only, the way to do it is just have one pin state change - High to Low or Low to High?

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    Default Re: Active low transmitter question

    beam gets broken set off a Tx , Rx to another pic a beam broken state.
    beam gets restored set off a Tx , Rx to another pic a beam restored state.
    the remote end always reflects the "beam" state
    where is the problem ?
    why the tx once only obsession ?

    you might want to send a heartbeat to indicate tx link is active and working
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    Default Re: Active low transmitter question

    'you might want to send a heartbeat to indicate tx link is active and working'

    Good idea.

    'why the tx once only obsession ?'

    Not an obsession, I'm just trying to understand the mechanism here.

    Another Question:

    If an input pin is internally held HIGH and you put an external HIGH onto that pin, does the input pin 'see' the external HIGH as a state change? Or does it ignore it because it's internally held in a HIGH state already.

  24. #24
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    Default Re: Active low transmitter question

    logic pins by definition have two states high or low.
    a state change will be caused by any action that causes the pin voltage to reach
    the level of the logically opposite threshold
    so a low state is maintained until vih is reached
    a high state is maintained until vil is reached
    see data sheet for vih/vil threshold values
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    Default Re: Active low transmitter question

    'a high state is maintained until vil is reached'

    Ok, cheers, so:

    If an input pin on the transmitter is internally held HIGH and you put an external HIGH onto that pin (from the pic) no Tx would occur as no state change has taken place, is that correct? (vih would would always equal vih).

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    Default Re: Active low transmitter question

    no state change has taken place, is that correct? (vih would would always equal vih).
    more like ( ̶v̶i̶h̶ Vin would would always ̶e̶q̶u̶a̶l̶ exceed vih)

    the state will remain high till Vin is less than or equal vil where its guaranteed to go low
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    Default Re: Active low transmitter question

    'the state will remain high till Vin is less than or equal vil where its guaranteed to go low'

    So switching Vin LOW (from pic) causes a Tx?

    And

    Transmitter will continue to Tx until Vin goes High?

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    Default Re: Active low transmitter question

    No. Only when there is a state change. We said that. Transmission happens on change once and only.
    But you shouldn’t care at all. You should care only for your logic state. And maybe your voltage levels not to fry your module.

    Ioannis

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    Default Re: Active low transmitter question

    'No. Only when there is a state change. We said that.'

    Yes you did Ioannis, I'm just trying to understand the mechanism here.

    'You should care only for your logic state. And maybe your voltage levels not to fry your module.'

    The data sheet does say [High impedance inputs, LVCMOS/LVTTL compatible, 5V tolerant] would you say I could leave 5v on the Input pin to transmitter and switch to 0v/ground to cause a Tx safely? There are two module versions below, should I go for number 2 to be safe? Or is there a low voltage 12f683 that you know of, or is 5v fine?

    1/ Part Numbering 2.4 -3.6V Version

    2/Part Numbering 3.6 -15V Version

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    Default Re: Active low transmitter question

    Transmitter will continue to Tx until Vin goes High?
    what you have is a lora long distance two way packet radio system that transmits the entire device state each time any input pin changes
    to paired receiver constrained by the tx transmission time, that time is between 55 and 1002 mS depending on mode.
    its not some cheap OOK thing

    5V Tolerant is and means 5v tolerant , it would the least of you problem here
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    Default Re: Active low transmitter question

    Also you can feed just 3.3 power supply to most pics. They work just fine.

    Ioannis

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    Default Re: Active low transmitter question

    Quote Originally Posted by Ioannis View Post
    Also you can feed just 3.3 power supply to most pics. They work just fine.

    Ioannis
    Thanks for that Ioannis - I think 3.3v is the best option here.

    I would love given time to learn how to send data with the LoRa module and pic but that's for the future and a lot more learning my end (end of Summer/Autumn/Winter project).

    What I am after here, is to use the Tx/Rx as a switch at about 350m range so question:

    Am I right in assuming that (once paired) a state change on say Input pin1 on the Tx would be reflected as Output on pin1 on the receiver (acting as a simple switch in effect) or is that far to simple?

    what you have is a lora long distance two way packet radio system that transmits the entire device state each time any input pin changes
    to paired receiver constrained by the tx transmission time, that time is between 55 and 1002 mS depending on mode.
    its not some cheap OOK thing
    Hi Richard - I appreciate that, its a pretty suffisticated and versatile piece of kit, I have a lot of learning to do before I can do it justice but as I say above, right now to get it to operate at 350m as a mirrored switch is all I'm after.

  33. #33
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    Default Re: Active low transmitter question

    It does exactly that. Richard already told you so.

    Rx reflects what Tx sees at the inputs.-

    Ioannis

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    Default Re: Active low transmitter question

    Hi Ioannis - I came across this short instructional / demo video for anyone else who's interested in this topic, simple video but shows Tx/Rx in action. It's not the exact same model as the ones I've now ordered but very similar, you may have to scroll down the page a tad to see the video.


    http://www.rfsolutions.co.uk/radio-m...ule-1000m-p378

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    Default Re: Active low transmitter question

    Sorry guys another question from me.

    The demo video shows a lady pressing a push button to ground on Tx Input pin1, which on the Rx latches an led on, so:

    How do I mimic a 'single' button press to ground on Tx Input pin1 from a pic? do I:

    Just set GPIO.0 = 0 'Pulls Tx pin1 LOW causes a Tx
    then set GPIO.0 = 1 'pulls Tx pin1 High, from data sheet Tx Inputs pins are internally pulled high anyway
    Or would the above cause two Tx's

    Or do I:

    Set GPIO.0 = 0 'Pulls Tx pin1 LOW causes a Tx
    Then Change TRISIO = %00000001 'Set bit.0 to input and mimic a floating open switch Tx Inputs pins are internally pulled high

  36. #36
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    Default Re: Active low transmitter question

    The demo video shows a lady pressing a push button to ground on Tx Input pin1, which on the Rx latches an led on, so:
    yes on a different device to what was previously discussed and not to mention the woefully inadequate description of the hardware
    is the switch momentary or latching ?

    if the switch is momentary then the operational description in the data sheet completely fails to explain the toggle action
    occurring

    consult the salesman
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    Default Re: Active low transmitter question

    Ok, ignoring the video and the different device and back onto this one.


    How do I mimic a 'single momentary' button press to ground on Transmitter Input pin1 using a pic instead of a button? do I:

    Set TRISIO = %11111110 'set bit 0 to output
    Then set GPIO.0 = 0 'Pulls Tx pin1 LOW causes a Tx
    Pause 50ms 'Alow time for Tx
    Then set GPIO.0 = 1 'pulls Tx pin1 High, from data sheet Tx Input pins are internally pulled high anyway
    Or would the above cause two Tx's (two state changes).

    Or do I:

    Set TRISIO = %11111110 'set bit 0 to output
    Then set GPIO.0 = 0 'Pulls Tx pin1 LOW causes a Tx
    pause 50ms 'Alow time for Tx
    Then Change TRISIO = %00000001 'Set bit.0 to input and mimic a floating open switch, Tx Input pins are internally pulled high (data sheet)
    And then when I want to Tx again set the TRIS bit 0 then GPIO.0 LOW etc.

  38. #38
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    Default Re: Active low transmitter question

    if an input pin on the module distinguishes between a high impedance high and a low impedance high then
    its not a normal LVCMOS/LVTTL logic input , the datasheet has not described such a capability.


    Pause 50ms 'Alow time for Tx
    the datasheet does not specify this need ,the min tx time is 55ms anyway isn't it ?
    the datasheet does not specify a min hold/reaction time at all that i can see

    no point guessing ---- consult the salesman
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    Default Re: Active low transmitter question

    if an input pin on the module distinguishes between a high impedance high and a low impedance high then
    its not a normal LVCMOS/LVTTL logic input , the data sheet has not described such a capability.
    no point guessing ---- consult the salesman ---- Will do later today if possible and get back.
    Just to help me - Data sheet says:

    Inputs are internally pulled high, active low
    Ok - This is my understanding / misunderstanding of how these things work - Please tell me which of these statements is correct / incorrect so that I can learn.

    A/ INPUTS are high impedance 'Z' state (floating)
    B/ 'Z' state could be anywhere from High to Low unless pulled into a High/Low state via a resistor(10k)
    C/ OUTPUTS are just High / Low

    Data sheet for Transmitter says 'Input internally held high' (So an internal pullup resistor to +v) and then pulled low via push button or in my case a Pic, this will cause a Tx, Lora Mode 5 is the Tx default (720ms) not sure if I hold Tx Input pin low for this length of time, another question for the sales guy I guess.

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    Default Re: Active low transmitter question

    A/ INPUTS are high impedance 'Z' state (floating)
    no, INPUTS are always high impedance , there is no "high Z" state for digital inputs and they should be pulled into a High/Low state via a resistor so as not to float if left unconnected and they are active


    B/ 'Z' state could be anywhere from High to Low unless pulled into a High/Low state via a resistor(10k)
    hmmm. high z is high z its not an output

    C/ OUTPUTS are just High / Low
    no outputs are what you could tristate (high z) to stop that pin from loading a shared bus inappropriately
    otherwise they are just High / Low if loaded correctly.



    Data sheet for Transmitter says 'Input internally held high' (So an internal pullup resistor to +v) and then pulled low via push button or in my case a Pic, this will cause a Tx, Lora Mode 5 is the Tx default (720ms) not sure if I hold Tx Input pin low for this length of time, another question for the sales guy I guess.
    pure speculation , i believe the demo totally misleads by not specifying if the sw is push on / push off or momentary.
    there is no mention of debouncing , rearming time or several other things

    \
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