Best way to use USART/HSERIN

[RossWaddell]

Coming back to this thread after recovering from some encoder/code porting issues from last week, I'm still trying to figure out the best way to send/receive data from 1 Master chip to 3-4 slaves via USART. I've read over many threads here regarding HSEROUT/HSERIN and the permutations & combinations leave me confused.

As of last night, I've got a PIC12F1840 sending the following on the Tx pin based on a rotary encoder turn (there's an IOC ISR to determine the direction of turn which sets RotEncDir):

DEFINE OSC 16                ' Set oscillator 16Mhz

DEFINE HSER_TXSTA 20h        ; Set transmit status and control register
DEFINE HSER_BAUD  2400       ; Set baud rate
DEFINE HSER_RCSTA 90h
DEFINE HSER_CLROERR 1        ; Clear overrun error upon
                             ; execution of every HSERIN
                             ; command

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Main:
    TOGGLE LED_0

    If ValueDirty = 1 THEN
        ' Send RotEncDir to receiving chip
        '    Chip Id,Itm Id,     <modifier>, "~"
        HSEROUT ["2",   "3", SDEC RotEncDir, TxEndChar]  ' use 'SDEC' if RotEncDir can be -1 for CCW
            
        ValueDirty = 0               ' Clear the flag

        #IFDEF USE_LCD_FOR_DEBUG
            HSEROUT [LCD_INST, LCD_CLR]
            pause 5
            '    Chip Id,Itm Id,     <modifier>,  "~"
            HSEROUT ["2",   "3", SDEC RotEncDir, TxEndChar]  ' use 'SDEC' if RotEncDir can be -1 for CCW
        #ENDIF
    ENDIF
 
    pause 500
 
    GOTO Main

The output on my LCD shows this for a CW turn:

231~

and this for CCW (RotEncDir actually gets set to 0 but I change it to -1 as on the slave I want to just add this value:

23-1~

The first number represents the slave chip which is the intended target of the serial data (I'll have 2-3 slaves in this project). The second number represents what to change on that chip with the rotary encoder info. The next part is either '1' for CW or '-1' for CCW (I could go back to using '0' for CCW if that makes the overall serial data easier to interpret on the slaves).

• Should I have a sync character at the start of the serial data so that on the slave I can determine if new serial data is sent before processing the first message (or there's an error)?
• Should I convert the HSEROUT line to multiple lines?

For the slaves, I've decided I need to use a Rx interrupt as I have other code running in the Main loop.

DEFINE OSC 16                ' Set oscillator 16Mhz

DEFINE HSER_TXSTA 20h        ; Set transmit status and control register
DEFINE HSER_BAUD  2400       ; Set baud rate
DEFINE HSER_RCSTA 90h
DEFINE HSER_CLROERR 1        ; Clear overrun error upon
                             ; execution of every HSERIN
                             ; command
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' ***************************************************************
' ASM Interrupt Definitions
' ***************************************************************

ASM
INT_LIST  macro    ; IntSource,           Label,  Type, ResetFlag?
        INT_Handler     RX_INT,        _UsartRx,   PBP,  yes
    endm
    INT_CREATE     ; Creates the interrupt processor
ENDASM

' Enable interrupt
@ INT_ENABLE   RX_INT      ; USART Rx interrupt

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• Do I need to make the RX_INT a low priority interrupt, or is that only important if there are other time-sensitive interrupts (I am using SPWM_INT.bas for one slave)?
• As Henrik suggests above, should I make the serial data entirely consistent in terms of number of bytes to make the parsing here easier? If that's true, then I probably need to revert to using '0' for CCW rotary encoder changes.
• The code below is from DT on another thread but I can't see how the whole serial data (e.g. "231~" for slave chip #2 to increment item #3 by 1) will be pieced together.
• Does the ISR get 1 byte every time? i.e. the first time it goes into the above ISR, 'SerialData' will be "2" and then the next iteration will have "3"?
• Should the ISR have the ResetFlag set or not? I've seen both.

'*********** 'ISR for RX_INT interrupt *************************************
Getbytes:       
    HSERIN [Serialdata]                    ' Get the serial data
    IF Serialdata = SyncByte THEN          ' if it's a Sync byte,
        IF Sync THEN ERROR = 1            '  last command was corrupted
        Sync = 1                          '  indicate Sync was rcvd
        ForMe = 0                          '  ID has not been rcvd
    ELSE
        IF Sync THEN
            IF !ForME THEN                ' if we haven't rcvd the ID
                IF Serialdata = ID THEN    '  and this byte matches the ID
                    ForMe = 1              '    indicate ID rcvd
                ELSE
                    Sync = 0              ' Not my packet
                    ForMe = 0              '  reset receive state   
                ENDIF                      '  and wait for next Sync
            ELSE
                CmdBuf = serialdata        ' store the command
                IF CmdRcvd THEN LATE = 1  '  last command not finished
                CmdRcvd = 1                '  indicate a command was rcvd
                Sync = 0                  '  reset receive state
                ForMe = 0                 
            ENDIF
        ENDIF
    ENDIF
@ INT_RETURN

All slaves Rx pin will be connected to the master's Tx pin so each slave needs to determine if the serial data is for them or not (which the above code does).