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Hot!High speed serial on PIC24EP128

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asm_uchip
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2021/01/11 02:40:22 (permalink)
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High speed serial on PIC24EP128

Hi all,
 
My team is developing a data logging board using the above PIC. Part of the requirements is to ensure that the board will be able to support high speed UART up to 16MBauds. The circuit is designed in-house, but the PCB layout is to be done externally by an external vendor. We've requested them to do the layout taking into account the 16Mbauds data rate and run some simulations to ensure signal integrity. They came back asking for input/output impedance of the PIC24, which I don't believe is available in the datasheet.
 
Being a complete beginner I am unsure about where to look for this information, any advice and help are greatly appreciated.
 
THanks! 
#1

39 Replies Related Threads

    JPortici
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    Re: High speed serial on PIC24EP128 2021/01/11 03:18:33 (permalink)
    3.5 (2)
    PIC24EP128 what?
    post the complete part number, please.
    In any case, the datasheet on the electric characteristics should give you important information.
    For example it could state that you may not be able to reach 16MBauds with the UART
    You will probably not find information about the input/output impedance of pins. What i would do is to use line drivers and buffers
    #2
    MBedder
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    Re: High speed serial on PIC24EP128 2021/01/11 04:48:28 (permalink)
    4.5 (2)
    asm_uchipThey came back asking for input/output impedance of the PIC24
    Tell'em just keep these 16 MHz traces as short as possible - no other hi-speed precautions are necessary.  All your problems will start outside the PCB.


    post edited by MBedder - 2021/01/11 04:49:37
    #3
    Antipodean
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    Re: High speed serial on PIC24EP128 2021/01/11 05:36:48 (permalink)
    5 (2)
    MBedder
    asm_uchipThey came back asking for input/output impedance of the PIC24
    Tell'em just keep these 16 MHz traces as short as possible - no other hi-speed precautions are necessary.  All your problems will start outside the PCB.

     
    They are probably looking to do defined impedance tracks to keep things from ringing at that clock rate.
    but I would be asking the question of the OP - why are you trying to use serial at that clock rate? There are better transmission formats for high speed data.

    Do not use my alias in your message body when replying, your message will disappear ...

    Alan
    #4
    asm_uchip
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    Re: High speed serial on PIC24EP128 2021/01/11 18:43:07 (permalink)
    0
    Jack_M
    PIC24EP128 what?
    post the complete part number, please.
    In any case, the datasheet on the electric characteristics should give you important information.
    For example it could state that you may not be able to reach 16MBauds with the UART
    You will probably not find information about the input/output impedance of pins. What i would do is to use line drivers and buffers


     
    The part number is PIC24EP128GP204-IPT
    Based on the datasheet it's stated that it's able to go 16Mbauds (max is 17.5), hence the reason for selecting it.
    #5
    asm_uchip
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    Re: High speed serial on PIC24EP128 2021/01/11 19:11:27 (permalink)
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    MBedder
    asm_uchipThey came back asking for input/output impedance of the PIC24
    Tell'em just keep these 16 MHz traces as short as possible - no other hi-speed precautions are necessary.  All your problems will start outside the PCB.




     
    OK, so keep the traces as short as possible all the way to the connector.
    Is it necessary to have the traces at the same length as well?
    post edited by asm_uchip - 2021/01/11 19:14:00
    #6
    ric
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    Re: High speed serial on PIC24EP128 2021/01/11 19:13:20 (permalink)
    5 (1)
    asm_uchip
    OK, so keep the traces as short as possible all the way to the connector.
    Is it necessary to have the traces at the same length as well?

    Does that mean you are taking the PIC signals off your PCB without any sort of line driver?
     
     

    I also post at: PicForum
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    #7
    asm_uchip
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    Re: High speed serial on PIC24EP128 2021/01/11 19:19:24 (permalink)
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    ric
    asm_uchip
    OK, so keep the traces as short as possible all the way to the connector.
    Is it necessary to have the traces at the same length as well?

    Does that mean you are taking the PIC signals off your PCB without any sort of line driver?
     
     




    Yes, that's correct. We offload the data directly to the system via the connector, no line driver in between
    #8
    ric
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    Re: High speed serial on PIC24EP128 2021/01/11 19:22:43 (permalink)
    4.5 (2)
    and what sort of cable?
    If it's not a coaxial cable with correct impedance matching, I predict massive problems trying to cram this signal down a cable!
    A differential line driver (e.g. RS485) would be the correct way to do it.
     
    post edited by ric - 2021/01/11 19:24:08

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    #9
    asm_uchip
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    Re: High speed serial on PIC24EP128 2021/01/11 21:19:39 (permalink)
    0
    ric
    and what sort of cable?
    If it's not a coaxial cable with correct impedance matching, I predict massive problems trying to cram this signal down a cable!
    A differential line driver (e.g. RS485) would be the correct way to do it.
     




    For the current config, the board is supposed to connect to another PCB which itself has a ribbon cable that connects to the main system. The cable is rated for 6 Mbauds, however we are designing our boards so that it could accommodate future upgrades of the system.
     
    At the moment, what's important that on our side we can achieve 16Mbauds, the other side will be dealt with later
    #10
    JPortici
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    Re: High speed serial on PIC24EP128 2021/01/12 01:53:27 (permalink)
    5 (1)
    asm_uchip
    Jack_M
    PIC24EP128 what?
    post the complete part number, please.
    In any case, the datasheet on the electric characteristics should give you important information.
    For example it could state that you may not be able to reach 16MBauds with the UART
    You will probably not find information about the input/output impedance of pins. What i would do is to use line drivers and buffers


     
    The part number is PIC24EP128GP204-IPT
    Based on the datasheet it's stated that it's able to go 16Mbauds (max is 17.5), hence the reason for selecting it.




    I looked at the datasheet (DS70000657J) and "TABLE 30-52: UARTx MODULE I/O TIMING REQUIREMENTS" states 15Mbps
    despite the fact that the front cover says 17.5
    did you actually test the UART module to see if the communication will actually run?
    post edited by JPortici - 2021/01/12 01:56:17
    #11
    ric
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    Re: High speed serial on PIC24EP128 2021/01/12 02:02:43 (permalink)
    5 (3)
    One figure comes from the "marketing" department, and one comes from "engineering".
     

    I also post at: PicForum
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    #12
    asm_uchip
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    Re: High speed serial on PIC24EP128 2021/01/12 02:11:44 (permalink)
    0
    Jack_M
    asm_uchip
    Jack_M
    PIC24EP128 what?
    post the complete part number, please.
    In any case, the datasheet on the electric characteristics should give you important information.
    For example it could state that you may not be able to reach 16MBauds with the UART
    You will probably not find information about the input/output impedance of pins. What i would do is to use line drivers and buffers


     
    The part number is PIC24EP128GP204-IPT
    Based on the datasheet it's stated that it's able to go 16Mbauds (max is 17.5), hence the reason for selecting it.




    I looked at the datasheet (DS70000657J) and "TABLE 30-52: UARTx MODULE I/O TIMING REQUIREMENTS" states 15Mbps
    despite the fact that the front cover says 17.5
    did you actually test the UART module to see if the communication will actually run?




    Check Section 20.0 of the datasheet:
    • Baud Rates Ranging from 4.375 Mbps to 67 bps at 16x mode at 70 MIPS
    • Baud Rates Ranging from 17.5 Mbps to 267 bps at 4x mode at 70 MIPS
    So the baud rate is possible for 70 MIPS, just need to make sure that the device is within -40 to +85C operating range 
    #13
    JPortici
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    Re: High speed serial on PIC24EP128 2021/01/12 03:03:50 (permalink)
    5 (1)
    While the baud rate may be possible in theory, the max frequency is actually limited by the IO pin driver and  the resulting signal integrity.
    IIRC the SPI on PPS pins is limited to 15MHz for the same reason, instead of up to 50MHz on dedicated pins...
     
    Since marketing and engineering disagree one must be wrong. I want to say marketing is wrong but i would at least test on a prototype before committing to a complete board.
    #14
    asm_uchip
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    Re: High speed serial on PIC24EP128 2021/01/12 03:32:46 (permalink)
    4 (1)
    Jack_M
    While the baud rate may be possible in theory, the max frequency is actually limited by the IO pin driver and  the resulting signal integrity.
    IIRC the SPI on PPS pins is limited to 15MHz for the same reason, instead of up to 50MHz on dedicated pins...
     
    Since marketing and engineering disagree one must be wrong. I want to say marketing is wrong but i would at least test on a prototype before committing to a complete board.




    This is a prototype board. We haven't commit to full production run.
     
    #15
    Chris A
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    Re: High speed serial on PIC24EP128 2021/01/12 10:24:18 (permalink)
    0
    I'll just say 70/4 = 17.5.  Guess what the next fastest baud will be!
    At these rates there are not many choices and those need to agree with the other end!
    #16
    Antipodean
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    Re: High speed serial on PIC24EP128 2021/01/12 13:18:52 (permalink)
    4.25 (4)
    asm_uchip
    ric
    and what sort of cable?
    If it's not a coaxial cable with correct impedance matching, I predict massive problems trying to cram this signal down a cable!
    A differential line driver (e.g. RS485) would be the correct way to do it.

     
    For the current config, the board is supposed to connect to another PCB which itself has a ribbon cable that connects to the main system. The cable is rated for 6 Mbauds, however we are designing our boards so that it could accommodate future upgrades of the system.
     
    At the moment, what's important that on our side we can achieve 16Mbauds, the other side will be dealt with later



    Well, that is going to produce some interesting debugging once you get the boards in your hands.
     

    Do not use my alias in your message body when replying, your message will disappear ...

    Alan
    #17
    ric
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    Re: High speed serial on PIC24EP128 2021/01/12 19:22:14 (permalink)
    3.67 (3)
    Don't quote Anti-podean's posts without inserting something between "anti" and "podean", or you will trigger the brain dead web server firewall, which clobbers your post.
     

    I also post at: PicForum
    Links to useful PIC information: http://picforum.ric323.co...opic.php?f=59&t=15
    NEW USERS: Posting images, links and code - workaround for restrictions.
    To get a useful answer, always state which PIC you are using!
    #18
    asm_uchip
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    Re: High speed serial on PIC24EP128 2021/01/12 20:06:42 (permalink)
    0
    ric
    Don't quote Anti-podean's posts without inserting something between "anti" and "podean", or you will trigger the brain dead web server firewall, which clobbers your post.
     




    Right...now I have problems viewing page 2 of the forums. Keeps on refreshing. Luckily it isn't Monday....
    #19
    ric
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    Re: High speed serial on PIC24EP128 2021/01/12 20:10:42 (permalink)
    4 (2)
    That is a side effect of the posts that have vanished. The page-count mechanism thinks they are still there.
    It will disappear in another post or two (when there really is a page#2).
    post edited by ric - 2021/01/12 20:11:54

    I also post at: PicForum
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    #20
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