Ring buffer hardware Usart

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  1. #1

    Default Ring buffer hardware Usart

    Thanks to excellent examples on here I now have a ring buffer working on my 16F88 accepting a stream
    of 9600 baud 12 byte data packets and squirting the bytes back out of the TX as quickly as they arrive.

    That's great but i now need to manipulate the bytes in the packets before they are tramsmitted
    and before they are overwritten by fresh incomming data.

    It's general ideas I am after rather than example code.

    Should I have a seperate RX & TX buffer and then move the bytes from one to the other
    when i'm ready to transmit something.

    Or should i just wait until first packet recd and whilst second packet is coming in manipulate the first packet and transmit it?

    I have to drive a serial lcd in the spare time so need the interrupt driven capability.

    So for example 12 byte packet arrives in 24 byte RX buffer. When first 12 byte packet has arrived and whilst second twelve bytes are arriving the first 12 bytes are manipulated as reqd and then transmitted.

    There is some spare time between packets about 12ms in fact. I don't think the incomming data will cathc
    up with the tx data.

    Another issue is the packets have specific header bytes $87 or $AA, how can i make the ring buffer
    RX routine wait until they appear before starting to recieve the following 11 bytes? I need to have th bytes in a consistent places in the buffer to manipulate them so for instance buffer position 0 always contains $87 and position 12 always contains $AA?

    Can you also have an interrupt driven TX routine?

    Thanks Peter

  2. #2
    Join Date
    Oct 2005


    Hi Peter,

    If it where me and I had the RAM to spare I would probably go with a dual buffer, one for transmitting and one for receiving, and maintain "pointers" into both indicating where to get and put the next byte.

    You can have a 'start of packet flag' which you set in your receive routine if the received byte is either $87 or $AA. Each time you enter the receive routine you check if the flag is set or not, if it is set you write the received byte to the buffer, if it's not set you check the byte for being $87 or $AA.

    Another idea is to always write the received byte to the buffer and have a 'start of packet pointer' - a byte variable containing the packets 'start adress' in the buffer array. You then use that 'adress' as the starting point for your manipulating.

    You can have a interrupt driven transmit routine. When enabled the TX interrupt fires as soon as there's room in the USARTS TX register. In the interrupt rounting you take your character from your ring buffer and stuff it into the TX-reg, increment your 'pointer' so you know which character is next, decide if you've sent everything or not and either disable the interrupt or leave it enabled. Then you exit the interrupt service routine. As soon as the USART is finished sending the character the interrupt fires again and the process repeats.

    Mostly 'general' advice and not much specifics but hopefully you'll get some ideas from it - as always there are several ways to achieve the same thing.

    Good luck!

  3. #3
    Join Date
    Nov 2003


    Quote Originally Posted by HenrikOlsson View Post
    ... As soon as the USART is finished sending the character the interrupt fires again and the process repeats.
    This can be very tricky and lead to an endless loop of interrupts in case you do not have to send something for a while.

    I never had a reliable Tx Interrupt routine although Darrel was more than helpful when I was trying to make a succesful Tx Int Routine.


  4. #4
    Join Date
    Oct 2005


    This can be very tricky and lead to an endless loop of interrupts in case you do not have to send something for a while.
    Correct, that's why I said you need to decide if you should keep the interrupt enabled or disable it when you are in the ISR. You need a way to figure out if the character just loaded into the TX-reg was the last character to send or not. You can have an end of line character, fixed length or you need to have a variable indicating the location of the last character in the circular buffer.

    You can also do it in software. Have a DataToSend flag which you set when there's data to send. The main routine checks this flag each iteration, if set AND there's room in TX-reg you GOSUB your transmit routine. The transmit routine loads one character from the buffer into the TX-reg, checks if all data is sent, if it is it resets the DataToSend flag if not it leaves it set. Then it returns to the main routine.

    This aproach works OK as long as you don't have a lot of pause statements or other commands which "hangs" the MCU in which case the serial output will slow down - which might or might not matter.

    Out of curiosity Ioannis, did you end up in that endless loop or what happend?


  5. #5
    Join Date
    Nov 2003


    It just kept on triggering the INT and after many changes (which led to a spaggeti program) I gave up.

    I just used the good ol' array transmit like this: Hserout [str array\n].

    When I feel like in a good mood I may try it again and see how it goes.

    As for the Rx Buffer, you ideas are good.

    I once tried the index pointer with a long array and no end of fle check. If characters kept coming then just over writen the previous characters in the array.


  6. #6
    Join Date
    Oct 2005


    I just made a quick and dirty test here using DT_Ints. The Main routine waits for PortB.0 going low, when it does it enables the TX interrupt. The TX interrupt service routine extracts characters from the buffer array and puts them in the TX register of the USART. When it sees a CR it disables the interrupt.
    '*  Name    : HWTX.BAS                                          *
    '*  Author  : Henrik Olsson                                     *
    '*  Notice  : Copyright (c) 2010 Henrik Olsson 2010             *
    '*          : All Rights NOT Reserved - use as you see fit.     *
    '*  Date    : 2010-11-16                                        *
    '*  Version : 0.0                                               *
    '*  Notes   : Test for interrupt based USART transmit using     *
    '*          : DT_ints-18 on a 18F4520                           *
    DEFINE LOADER_USED 1                ' We're using a bootloader
    DEFINE OSC 20                       ' 20Mhz x-tal is used
    DEFINE HSER_BAUD 38400              ' Baudrate is 38400
    CMCON = 7                           ' Comparators OFF 
    ADCON1 = %00001111                  ' No analog inputs
    TRISC.7 = 1                         ' RxPin is input
    TRISC.6 = 0                         ' TxPin is output
    TRISB.0 = 1                         ' PortB.0 is input
    TxBuffer VAR BYTE[128]              ' Transmit buffer array.
    TxPointer VAR Byte                  ' Pointer into the above array
    INCLUDE "DT_INTS-18.bas"            ' Include Darrels interrupt engine.
    INCLUDE "ReEnterPBP-18.bas"         ' And the stub needed for interrupts in PBP 
    ASM                                 ; Set up the USART transmit interrupt.
    INT_LIST  macro     ; IntSource,   Label,    Type, ResetFlag?
            INT_Handler    TX_INT,   _USART_TX,   PBP,    no
        INT_CREATE                      ; Creates the interrupt processor
      HSEROUT ["Program start",13]        ' A dummy HSEROUT here to init the USART and show we're alive.
      ' Now load the array with something we want to send. The string is, in this case, terminated with a CR (13).
      ArrayWrite TxBuffer,["This is the TX Array Buffer and we're sending it with an interupt based routine",13]
      If PortB.0 = 0 then                 ' PortB.0 going low is what
        TxPointer = 0                     ' Initialize pointer to beginning of array
    @   INT_ENABLE   TX_INT               ; Enable USART TX interrupt  
        Pause 100                         ' Simple debounce for the button
      Goto Main                           ' And do it again.
      TXREG = TxBuffer[TxPointer]       ' Load character from array into USART TX register
      If TxBuffer[TxPointer] = 13 then  ' If that character was a CR we're done so we.....
    @   INT_DISABLE  TX_INT             ; ...disable the any further USART TX interrupts.
      ELSE                              ' If the character was not a CR we.....
        TxPointer = TxPointer + 1       ' ...increase prepare to send the next character.
    @ INT_RETURN                        ; And exits the interrupt.
    Seems to work here, try it out if you like.


  7. #7


    Thanks for all the replies very useful.

    Can my ring buffer RX routine below coexist with your TX routine I wonder?
    Can you have two interrupt driven things going on?

    '        PBP BCM CODE 8mhz 1963 Words
    'MPASM Config
    @ __config _CONFIG1, _INTRC_IO & _WDT_OFF & _LVP_OFF & _MCLR_OFF & _CP_OFF
    @ __config _CONFIG2, _IESO_OFF & _FCMEN_OFF
    DEFINE OSC 8 			'Set oscilator speed to 8mHz 
    OSCCON = %01111000 		'8 Mhz
    CMCON  = %00000111 		'COMPARATORS OFF
    CCP1CON = 0             	'COMPARATORS OFF
    CVRCON = 0              	'COMPARATORS OFF
    INTCON = 0 		       	'Disable interrupts
    TRISB = %00000100 		'SET PORTB RB2(RX) as input, others to OUTPUT
    TRISA = %11110111 		'SET PORTA AS INPUTS except PORTA.3
    ANSEL  = %00000001      	'AN0 Analog Input 
    ADCON0 = %01000001		'Turns A/D on, AN0  Fosc/16
    ADCON1 = %11000000      	'Right justified and Fosc/16
    INCLUDE "DT_INTS-14.bas"
    INCLUDE "ReEnterPBP.bas"     	'Include if using PBP interrupts
    INT_LIST  macro    ;IntSource,        Label,       Type,    ResetFlag?
            INT_Handler    RX_INT,      _INT_Serin,     PBP,        no
        INT_CREATE               	;Creates the interrupt processor
    @   INT_ENABLE   RX_INT     	;Enable external (INT) interrupts
    'Setup the hardware USART for 9600 baud
    DEFINE HSER_BAUD 9600		'Set Baud rate to 9600bps
    DEFINE HSER_BITS 9		'Set to 9 bit mode
    DEFINE HSER_EVEN 1		'Set Even Parity
    DEFINE HSER_CLROERR 1 		'Clear overflow error automatically
    'Debug is used as it is the fastest and smallest serial routine to drive the Lcd display
    DEFINE DEBUG_REG PORTB		'Set Debug pin port B
    DEFINE DEBUG_BIT 4		'Set Debug pin bit for Lcd Display PORTB.4
    'DEFINE DEBUG_BAUD 38400	'Set Debug baud rate 38400 approx 3840 cps or 0.26ms per character
    DEFINE DEBUG_BAUD 9600		'Set Debug baud rate 9600 approx 960 cps or 1ms per character
    DEFINE DEBUG_MODE 0		'Set Debug mode: 0 = true, 1 = inverted (T=True Idle High)(N=Inverted Idle Low)
    DEFINE DEBUG_PACING 1000	'Set Debug Character Pacing mode to 1ms between bytes
    include "modedefs.bas"		'Allows the use of the standard serin/serout command  
    'Variables Constants and I/O definitions
    '------------------------ Variables 16bit --------------------------------------
    Amps 	  var WORD		'Amps Word Variable
    Soc	  var WORD		'Soc Word Variable
    TempH	  var WORD		'High Temperature Word Variable
    TempL	  var WORD		'Low Temperature Word Variable
    Gen1      var WORD      	'Gen1 General variable & Key input up/down/left/right/centre
    Watts	  var WORD		'IMA Power in Watts (Battery Volts x Battery Amps)
    MaxWatts  var WORD		'Maximum IMA Power in Watts during trip
    Errors    var WORD		'Number of RS485 Data Checksum Errors
    Packets   var WORD		'Number of RS485 Data Packets
    '------------------- Variables 8bit --------------------------------------------
    Volts 	  var BYTE		'Volts Byte Variable (0-255)
    BCM87 	  var BYTE[11]		'Define BCM87 as a Byte array (12 Bytes 0-11) (BCM87 = BCM Data)
    BCMAA	  var BYTE[11]		'Define BCMAA as a Byte array (12 Bytes 0-11) (BCMAA = BCM Data)
    Cnt 	  var BYTE		'General Byte Counter Variable (0-255)
    Text 	  var BYTE		'General Byte Text Variable (0-255)
    Gen0	  var BYTE		'Gen0 is a Byte variable (0-255)
    Config    var BYTE		'Gauge Config Byte (8 bits)
    AmpSign   var BYTE      	'Amps Sign Byte Variable (43 = +) (45 = -)
    TempHSign var BYTE		'TempH Sign Byte Variable (43 = +) (45 = -)
    TempLSign var BYTE		'TempL Sign Byte Variable (43 = +) (45 = -)	
    CheckSum  var BYTE		'BCM Data Checksum calculation Byte
    MaxVolts  var BYTE		'Maximum Voltage during trip
    MinVolts  var BYTE		'Minimum voltage during trip
    MaxAsst	  var BYTE		'Maximum Assist Amps during trip
    MaxRegen  var BYTE		'Maximum Regen Amps during trip
    MaxTemp   var BYTE		'Maximim Temp during trip
    MinTemp	  var BYTE		'Minimum Temp during trip
    MaxSoc	  var BYTE		'Maximum OEM Soc during trip
    MinSoc	  var BYTE		'Minimum OEM Soc during trip
    Adr1      VAR BYTE      	'Number for start of menu data
    Adr2      VAR BYTE      	'Number for end of menu data
    Display   VAR BYTE      	'Display Mode 0 = Normal, 1 =  Binary, 2 = Max/Min, 3 = Splash Screen
    'PortA Input Variables
    OverV  VAR PORTA.0		'Define PortA.0 Pin as OverV  (Over Voltage Input)  (Low = Over V)
    UnderV VAR PORTA.7		'Define PortA.7 Pin as UnderV (Under Voltage Input) (Low = Under V)
    'PortA Output Variables
    Spare VAR PORTA.3          	'Define PortA.3 Pin as Data Out 
    'PORTB Input Variables
    BcmIn Var PORTB.2       	'Define PortB.2 Pins as RS485 In
    'PORTB Ouput Variables
    Led1  VAR PORTB.0		'Define PortB.0 Pin as Led1
    Led2  VAR PORTB.1		'Define PortB.1 Pin as Led2
    Piezo VAR PORTB.3       	'Define PortB.3 Pin as Piezo
    BcmOut Var PORTB.5       	'Define PortB.5 Pins as RS485 Out
    ;-- Place a copy of these variables in your Main program -------------------
    ;--   The compiler will tell you which lines to un-comment                --
    ;--   Do Not un-comment these lines                                       --
    ;wsave   VAR BYTE    $20     SYSTEM      ' location for W if in bank0
    ;wsave   VAR BYTE    $70     SYSTEM      ' alternate save location for W 
                                             ' if using $70, comment wsave1-3
    ' --- IF any of these three lines cause an error ?? ------------------------
    '       Comment them out to fix the problem ----
    ' -- Which variables are needed, depends on the Chip you are using -- 
    ;wsave1  VAR BYTE    $A0     SYSTEM      ' location for W if in bank1
    ;wsave2  VAR BYTE    $120    SYSTEM      ' location for W if in bank2
    ;wsave3  VAR BYTE    $1A0    SYSTEM      ' location for W if in bank3
    'Alias & Vars
    RCIF VAR PIR1.5             	'Receive  interrupt flag (1=full , 0=empty)
    TXIF VAR PIR1.4             	'Transmit interrupt flag (1=empty, 0=full)
    OERR VAR RCSTA.1            	'Alias OERR (USART Overrun Error Flag)
    CREN VAR RCSTA.4           	'Alias CREN (USART Continuous Receive Enable)
    BufMax CON 48              	'Sets the size of the ring buffer
    Buffer VAR BYTE[BufMax]    	'Array variable for holding received characters
    Ptr_in VAR BYTE             	'Pointer - next empty location in buffer
    Ptr_out VAR BYTE            	'Pointer - location of oldest character in buffer
    Bufchar VAR BYTE            	'Stores the character retrieved from the buffer
    ErrFlag VAR BYTE            	'Holds error flags
    ptr_in = 0                  	'Clear buffer pointer (in)
    ptr_out = 0                 	'Clear buffer pointer (out)
    Errflag = 0                 	'Clear error flag
    'Skip over the interupt handler & subroutines
    	debug 254,1                    		'Clear Screen   
    	goto MainLoop
    INT_Serin:                  			'Serial Interrupt routine
    	IF OERR Then usart_error 		'Check for USART errors
    	ptr_in = (ptr_in + 1)       		'Increment ptr_in pointer (0 to 63)
    	IF ptr_in > (BufMax-1) Then ptr_in = 0 	'Reset pointer if outside of buffer
    	IF ptr_in = ptr_out Then Buffer_Error	'Check for buffer overrun
    	HSerin [buffer[ptr_in]]     		'Read USART and store data in next empty location
    	IF RCIF Then INT_Serin 			'Check for another character while we're here
    @ INT_RETURN           				;Return to program
    	Buffer_Error:               		'Error - Run out of buffer space
    	errflag.1 = 1               		'Set the error flag for software
    'Move pointer back to avoid corrupting the buffer.
    'MIN insures that it ends up within the buffer. 
    	ptr_in = (ptr_in - 1) MIN (BufMax - 1) 
    	HSerin [buffer[ptr_in]]     		'Overwrite the last data stored (clear the int.)
    USART_Error:                			'Error - USART has overrun (data too fast?)
    	errflag.0 = 1               		'Set the error flag for hardware 
    @ INT_RETURN           				;Return to program
    ' Subroutines
    GetBuf:     					'Move the next character in buffer to bufchar
    @ INT_DISABLE  RX_INT       			;Dont want to interupt this, so Disable interupts
    ptr_out = (ptr_out + 1)     			'Increment ptr_out pointer (0 to 63)                           
    IF ptr_out > (BufMax-1) Then ptr_out = 0  	'Reset pointer if outside of buffer 
    bufchar = buffer[ptr_out]   			'Read buffer location
    @ INT_ENABLE  RX_INT        			;All done, Enable interupts
    ' Error Handler
    ErrorHandler:					'Display error message if buffer has overrun
    	IF errflag.1 Then               	'Determine the error
        	DEBUG 254,128,"Buffer Overflow" 	'Display buffer error
        	DEBUG 254,128,"USART Overrun"   	'Display usart error
    	errflag = 0 				'Reset the error flag
    	CREN = 0 				'Disable continuous receive to clear overrun flag
    	CREN = 1            			'Enable continuous receive
    	GoTo MainLoop       			'Errors cleared, time to work.
    ' RS485 Passthru Test
    	IF errflag Then ErrorHandler    	'Handle error if needed
    	IF ptr_in = ptr_out Then MainLoop   	'loop if nothing in buffer
    	GoSub GetBuf   				'Get a character from buffer (stored in bufchar) 
    	HSEROUT [BufChar]   			'Send the character
    	goto MainLoop  				'Loop forever
    Last edited by retepsnikrep; - 16th November 2010 at 23:08.

  8. #8
    Join Date
    Oct 2005


    Yes, you can have multiple interrupts and I can't see why the transmit interrupt routine wouldn't work with your code. Just remember that you need a way to determine when the last character has been sent. In my example I used the CR character but as said before it might be better to maintain a pointer (like you are doing in the receive buffer) and stopping after n number of bytes.


  9. #9
    Join Date
    Nov 2003


    Hi Henrik.

    Thanks for the snippet. I test it on a 877 at 4MHz and works very good. As expected...

    My mistake when I tried the TX interrupts must have been the Reset flag that I was set it to YES.


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