RMW Read modify write


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  1. #1
    Join Date
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    Default RMW Read modify write

    This is simple Led-blink code that does not work.

    LED5 flashes normally, and LED 4 stable HIGH.................

    Using a 12F1822 (RA4 is not open collector in this PIC).


    Code:
    ASM
     __CONFIG _CONFIG1, _FOSC_INTOSC & _WDTE_ON & _PWRTE_ON & _MCLRE_OFF & _CP_OFF & _CPD_OFF & _BOREN_ON & _CLKOUTEN_OFF & _IESO_OFF & _FCMEN_OFF
     __CONFIG _CONFIG2, _WRT_OFF & _PLLEN_ON & _STVREN_OFF & _BORV_HI & _LVP_OFF
    ENDASM
     
        DEFINE OSC 32
        LED5 VAR PORTA.5
        LED4 VAR PORTA.4
        
        OSCCON = %01110000               
        ANSELA = %00000011  ' AN0 = ANALOG PIN , C1IN0- = ANALOG PIN
        TRISA =  %00000000 
        
    LOOPA:
                                       
         LED4 = 1
         LED5 = 1
         PAUSE 100
         
         LED4 = 0
         LED5 = 0
         PAUSE 100
         
         GOTO LOOPA
    when LOOPA modified as above works fine and both LEDs blinking.

    Code:
    LOOPA:
                                       
         LED4 = 1
         LED5 = 1
         PAUSE 100
         
         LED4 = 0
         PAUSEUS 2  <---
         LED5 = 0
         PAUSE 100
         
         GOTO LOOPA
    EDIT : instead of the 2us pause , i tried another command like modifying a variable , XX = 1 and still works.
    So the conclusion is , two LEDx = 0 commands in the row cannot be processed normally (although LEDx = 1 works) .
    The UFO's coming ...
    Cheers!
    Last edited by LakisFM1; - 30th May 2014 at 17:08.

  2. #2
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    Default Re: Now , This is a bug ...

    Just my guess: RMW (read modify write) takes too long to execute before being asked to RMW again, thus the nops make it work.
    If you do not believe in MAGIC, Consider how currency has value simply by printing it, and is then traded for real assets.
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  3. #3
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    Default Re: Now , This is a bug ...

    Yes, classic symptoms of read-modify-write.
    So, now it's not a bug anymore - just sub optimal coding combined with capacitance on the output, high clock frequency and the hardware implementation of the PIC itself.

    If the PIC has a LAT register (which yours do have) then use it instead of the PORT register for things that may suffer from RMW issues.
    Code:
        LED5 VAR LATA.5
        LED4 VAR LATA.4

  4. #4
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    Default Re: Now , This is a bug ...

    I moved your thread because it is a clear example of RMW and can help others (like me).

    Robert

  5. #5
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    Default Re: Now , This is a bug ...

    Quote Originally Posted by HenrikOlsson View Post
    If the PIC has a LAT register (which yours do have) then use it instead of the PORT register for things that may suffer from RMW issues.
    Code:
        LED5 VAR LATA.5
        LED4 VAR LATA.4
    See Henrik, that's the part I left out due to lack of confidence, due to being a sub optimal coder I was thinking it however. Really my FIRST example where I could see that as the issue, thanks for the validation . . . (Backup?) So now I am reasonably sure I was correct. Thanks.
    If you do not believe in MAGIC, Consider how currency has value simply by printing it, and is then traded for real assets.
    .
    Gold is the money of kings, silver is the money of gentlemen, barter is the money of peasants - but debt is the money of slaves
    .
    There simply is no "Happy Spam" If you do it you will disappear from this forum.

  6. #6
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    Thumbs up Re: RMW Read modify write

    Thank you guys!

    Very clear response...

  7. #7
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    Default Re: RMW Read modify write

    when using the whole port as a data i/o bus is specifying LAT control possible or required for the port ?

  8. #8
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    Default Re: RMW Read modify write

    Hi Sheldon,
    Yes, it's possible. It's not required but generally recommended (I'd say).
    You say i/o but you can't use LAT to read the state of the pins when in input mode. If you read LAT you get the state of the output latch, not the state of the actual pins.

    /Henrik.

  9. #9
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    Default Re: RMW Read modify write

    Code:
    glcd_fs     var LATD.0     ' Font Select pin   High = 6x8 , Low = 8x8
        glcd_wr	 	var	LATD.1	   ' Write control pin ( Active Low)
        glcd_rd	 	var LATD.2	   ' Read control bit  ( Active Low)
        glcd_ce	 	var LATD.3	   ' Chip enable control bit  ( Active Low)
        glcd_cd	 	var LATD.4	   ' Command (high) and Data (low) toggle bit
        glcd_rst	var LATD.5	   ' Reset control bit (Active Low)
      
      
        glcd_tris 	var TRISE	   ' Data bus direction
        glcd_dat	var PORTE	   ' LCD 8 bit data bus 
        glcd_sta0	var PORTE.0	   ' Status Bit 0  - Status Read - Command excute capability      - 0 = not avaialble / 1 = Available 	
        glcd_sta1	var PORTE.1	   ' Status Bit 1  - Status Read - Data Read/Write capability     - 0 = not avaialble / 1 = Available
    yes the 8 bits of the port are used as both input / output

    i would think PBP uses the LAT registers in the above use ???




    i um PBP uses the LAT

  10. #10
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    Default Re: RMW Read modify write

    Not sure I understand the question, if there is any....

    In short:
    * To put data on the bus, write to the LATx register
    * To read data from the bus, read from PORTx register

    If you do
    Code:
    glcd_wr	 	var	LATD.1
    You can do glcd_wr = 1 but you can not do HIGH glcd_wr. HIGH/LOW etc only works on PORTx

    /Henrik.

  11. #11
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    Default Re: RMW Read modify write

    Code:
    glcd_dat	var PORTE	   ' LCD 8 bit data bus
    as this is 8bit data port with the TRIS set at $00 or $FF

    Code:
     glcd_tris 	var TRISE	   ' Data bus direction
    would PBP uses LAT port registers internally ?

  12. #12
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    Default Re: RMW Read modify write

    assuming you have lat registers on your pic chip , porte is not equal to late they are separate registers . check the memory map of your chip.
    pbp has no glcd native functions, whether your "glcd library" uses late is completely dependent on how the library was written .
    if you define your "data bus" as porte I would expect it will not use lat regs (that would probably require two defines , one for each direction)
    and lastly rmw has no impact at all when you are writing to the whole port

  13. #13
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    Default Re: RMW Read modify write

    thanks richard

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