I2C problem writing to slave

Hi everyone,
Once more I am struggling with I2C. I had two PIC16F877A communicating nicely. I built my code based on the example of master and slave from the website. But now, I have added more code to my slave code and that makes the writing command from master to slave fail. I have a time error on the master and the slave also does not receive the single byte I am trying to write to it.
Problem is that I need this piece of code in the slave.
I tried to add some test to make it loop only on the receive command until I2C has finished transmission but I could not make it work.
Is interrupt the way to go for that?
Any other idea or code example to avoid this type of issue?

Thanks for any help.
regards

You will need to post the modified code before anyone will be able to help trouble shoot the problem.

Hi,

sorry for that, here is the code.
As you will see, the slave i2c part is unchanged, I just add some call to subroutine in the main loop.
The funny thing is that it does not affect the read command from the other pic but only the write command which result in a time error.
It seems to be linked to the time it takes during the main loop as I can make the write command fail by either uncomment the hserout or the call to subroutine.
But why is there no effect on the read command? This one with all the code active always send me the right values.
NOTE: the code is not finished and some change in subroutine (ADC and average computation) is not reflected yet in some other part of the code, so don't be surprised. I was planning to finish to trouble shoot once I could have the 2 pic communicating.

Thanks for your help

Code:

---

' Name        : I2CSLAVE.pbp
' Compiler    : PICBASIC PRO Compiler 2.6
' Assembler  : PM or MPASM
' Target PIC  : 40-pin 16F877A
' Hardware    : Lab-X1
' Oscillator  : 4MHz internal or external
' Keywords    : I2CREAD, I2CWRITE, LCDOUT
' Description : PICBASIC PRO program for I2C slave program - PIC16F877/PIC-X1.
'

DEFINE  LOADER_USED 1  ' Using boot-Loader
Define HSER_CLROERR 1  'to automatically clear any buffer overflow of serial input
Define HSER_BAUD 4800  'Set to NMEA baud rate standard

' Define ADCIN parameters
Define    ADC_BITS    10    ' Set number of bits in result
Define    ADC_CLOCK    3    ' Set clock source (3=rc)
Define    ADC_SAMPLEUS    50    ' Set sampling time in uS

' Alias I2C pins
scl VAR PORTC.3        ' I2C clock input
sda VAR PORTC.4        ' I2C data input

' Define used register flags
SSPIF VAR PIR1.3      ' SSP (I2C) interrupt flag
BF    VAR SSPSTAT.0    ' SSP (I2C) Buffer Full
R_W  VAR SSPSTAT.2    ' SSP (I2C) Read/Write
D_A  VAR SSPSTAT.5    ' SSP (I2C) Data/Address
CKP  VAR SSPCON.4    ' SSP (I2C) SCK Release Control
SSPEN VAR SSPCON.5    ' SSP (I2C) Enable
SSPOV VAR SSPCON.6    ' SSP (I2C) Receive Overflow Indicator
WCOL  VAR SSPCON.7    ' SSP (I2C) Write Collision Detect

' Define constants
I2Caddress CON  $a0      ' Make our address 2

' Allocate alias
qUp    var    PORTA.2
qDown  var    PORTA.5

' Allocate variables
qpot    var    word 'raw potentiometer reading
qpotflt var    word 'raw pot filtered
xqpot  var    word 'used to compute average
qpotnb  var    byte 'filtering number
qpotold var    word
qpotCal var    byte 'calibrated pot reading (20 - 220)
qpos    var    byte  '0 down, 1 up
CalFlag var    byte
x      var    word
y      var    word
QDmax  var    word
QUmax  var    word
Qmin    var    word
PotDA  var    word  'amplitude pot for Down position
PotDOf  var    word 
PotUA  var    word  'amplitude pot for up position
PotUOf  var    word 
QposPerc var    byte 
Qdraft  var    byte 
ind    var    byte
bogus  var    word  'dummy for div32
Qstatus var    byte  'to send status info to Master, 0 acq, 1 cal, 2 mov up, 3 mov down
datain  var    byte  'received data from Master
DataOut var    byte[4]
readcnt var    byte
result  VAR    BYTE      ' ADC result


' Initialize ports and directions
ADCON1 = %10010101       
TRISA = %11111111    ' Set PORTA to all input

' Initialize I2C slave mode
  SSPADD = I2Caddress ' Set our address
  SSPCON2 = 0        ' General call address disabled
  SSPCON = $36        ' Set to I2C slave with 7-bit address

  readcnt = 0        ' Zero counter

  GoTo mainloop      ' Skip over subroutines

  '*****************************QMove******************************************
QMove:
if qpotold > qpot then
    if qpotold-qpot > 4 then
        if qUp=1 then qPos=0
        if qDown=1 then qpos=1
    endif
else
    if qpot-qpotold > 4 then
        if qUp=1 then qPos=1
        if qDown=1 then qpos=0
    endif
endif
return
'*****************************CalProc****************************************
CalPROC:
if qUp=1 and qpos=1 then
  QUmax = QUmax max qpot
endif
if qDown=1 and qpos=0 then
  QDmax = QDmax max qpot 
endif
Qmin = Qmin min qpot

if qdown=1 then calflag=2               
if calflag=2 and qdown=0 then calflag=3 
if calflag=3 and qUp=1 then calflag=4   
if calflag=4 and qUp=0 then             
    PotDA=qdmax-qmin
    x=qmin*220
    y=div32 PotDa   
    PotDOf= 120-y      'offset +100 to avoid any negative number
    PotUA=qdmax-qmin
    x=qmin*220 
    y=div32 PotUa 
    PotUOf= 120-y      'offset +100 to avoid any negative number
    write 0, word potda, word PotDOf, word PotUA, word PotUOf, qpos
    calflag=0          'back to acquisition mode
endif
return


'*****************************Getx*******************************************       
' Subroutine to get pot value
getx:
    ADCIN 0, qpot        ' Read channel 0 to qpot
    xqpot=xqpot+qpot
    qpotnb=qpotnb+1
    if qpotnb= 8 then
        qpotnb=0
        qpotflt=xqpot/8
        xqpot=0
       
        dataout[0]=qpotflt
        dataout[1]=qpotCal
        dataout[2]=Qstatus
        hserout ["$pot,", dec qpotflt]
        hserout ["$qup,", dec qUp]
        hserout ["$qdown,", dec qDown]
        hserout ["$cf,", dec calflag]
        hserout ["$qdm,", dec qdmax]
        hserout ["$qum,", dec qumax]
        hserout ["$qm,", dec qmin]
        gosub QMove
        if calflag=0 then
          if qpos=0 then
            bogus = 220 * qpot
            qpotcal = div32 potda
            qpotcal=qpotcal+PotDOf-100
          else
            bogus = 220 * qpot
            qpotcal = div32 potUa
            qpotcal=qpotcal+PotUOf-100 
          endif
          gosub convert
        else
          gosub CalPROC
        endif
    endif
Return

'*******************************conversion***********************************
convert:
if qpos=0 then
  ' down position
  lookdown2 qpotcal,<=[20,22,26,33,42,55,73,96,127,167,192,206,230],ind
  lookup ind,[67,70,73,77,80,83,87,90,93,97,98,99,100], qposperc
  lookup ind,[166,171,176,180,184,187,191,193,196,198,199,200,200],qdraft
else
  ' up position
  lookdown2 qpotcal,<=[20,21,24,28,33,39,46,53,62,72,83,94,106,119,133,148,163,179,195,204,208,220],ind
  lookup ind,[63,60,57,53,50,47,43,40,37,33,30,27,23,20,17,13,10,7,3,2,1,0],qposperc
  lookup ind,[161,156,150,144,138,132,125,118,111,104,97,89,81,73,65,56,48,39,30,26,23,20],qdraft
endif
'dataout[0]= qposperc
'dataout[0]= qpot
''dataout[1]= qdraft
'dataout[1]= qpot
'dataout[2]= Qstatus
return

i2cslave:              ' I2C slave subroutine
  SSPIF = 0          ' Clear interrupt flag

  IF R_W = 1 Then i2crd  ' Read data from us
  IF BF = 0 Then i2cexit  ' Nothing in buffer so exit
  IF D_A = 1 Then i2cwr  ' Data for us (not address)
  IF SSPBUF != I2Caddress Then i2cexit ' Clear the address from the buffer
  readcnt = 0        ' Mark as first read
  GoTo i2cexit

i2cwr:                ' I2C write data to us
  datain = SSPBUF    ' Put data into array
  if datain=127 then   
    calflag=1        ' goes into calibration mode
    qpos=1            ' calibration is started from keel
  endif
  wflag=0
  GoTo i2cexit

i2crd:                ' I2C read data from us
  IF D_A = 0 Then
      readcnt = 0      ' Mark as first read
  EndIF

  SSPBUF = dataout[readcnt]    ' Get data from array
  CKP = 1                      ' Release SCL line
  readcnt = readcnt + 1        ' Move along read count
  GoTo i2cexit                ' That's it

i2cexit:
  Return

mainloop:              ' Main program loop
  IF SSPIF  ' Check for I2C interrupt flag
        GoSub i2cslave
  EndIF
if calflag>0 then
  Qstatus=1
else
  if qUp=1 then
    Qstatus=2
  elseif qDown=1 then
    Qstatus=3
  else
    Qstatus=0
  endif
endif
gosub getx
'dataout[0]=qpotflt                  '----------------With everything as comment, I2C works to write and read
'dataout[1]=qpotCal                  '---------------- if I uncomment all this, i2c still works with a read command
'dataout[2]=Qstatus                  '---------------- if I uncomment either the hserout or only the call to the subroutine
'hserout ["$pot,", dec qpotflt]      '---------------- read comment is still OK but write command fails
'hserout ["$qup,", dec qUp]
'hserout ["$qdown,", dec qDown]
'hserout ["$cf,", dec calflag]
'hserout ["$qdm,", dec qdmax]
'hserout ["$qum,", dec qumax]
'hserout ["$qm,", dec qmin]
'gosub QMove
'if calflag=0 then
'  if qpos=0 then
'    bogus = 220 * qpot
'    qpotcal = div32 potda
'    qpotcal=qpotcal+PotDOf-100
'  else
'    bogus = 220 * qpot
'    qpotcal = div32 potUa
'    qpotcal=qpotcal+PotUOf-100 
'  endif
'  gosub convert
'else
'  gosub CalPROC
'endif

  GoTo mainloop      ' Do it all forever

  End

---

OOOPS, I posted the wrong code. I tried to modify the code once I had issues and moving around some procedure and did not posted the original one which did not have dupplicate.
I am going to clean that and will post the right one later.