[sdcc-devel] [ sdcc-Bugs-1253471 ] PIC16: Signal dispatcher doesn't check **IE bits

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Bugs item #1253471, was opened at 2005-08-07 10:25
Message generated for change (Comment added) made by tecodev
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Category: pic16 target
>Group: fixed
>Status: Closed
>Resolution: Accepted
Priority: 5
Submitted By: Peter Onion (p_onion)
>Assigned to: Raphael Neider (tecodev)
Summary: PIC16: Signal dispatcher doesn't check **IE bits

Initial Comment:
I've found a problem with the scheme used poll the
device interrupt flags .

My C code has these definitions for the signal handlers.

/* Set up the interrupt handlers */

DEF_INTHIGH(high_int)
DEF_HANDLER(SIG_RC, _RC_handler)
DEF_HANDLER(SIG_TX, _TX_handler)
DEF_HANDLER(SIG_TMR2, _TMR2_handler)
DEF_HANDLER(SIG_TMR0, _tmr0_handler)

END_DEF 


The order of the lines above is CRITICAL as it
determines the order the interrupt flags are checked. 
Here is the corresponding code in the .asm file

; ; Starting pCode block
S_picklenet0__high_int  code
_high_int:
                                        
        btfsc _PIR1, 0x5
        goto __RC_handler
        btfsc _PIR1, 0x4
        goto __TX_handler
        btfsc _PIR1, 0x1
        goto __TMR2_handler
        btfsc _INTCON, 0x2
        goto __tmr0_handler
        retfie


The problem concerns the fact that only the TXIE bit
(_PIR1:4) is checked before jumping to the handler.  
The Microchip application note 
Asynchronous Communications with the PICmicro® USART 
AN774  contains example code for using the transmit
interrupt which contains this section

          btfss   PIR1,TXIF       ;test for TXIF
transmit interrupt flag
          bra     LowInt1         ;if TXIF is not set,
done with test
          btfsc   PIE1,TXIE       ;else test if Tx
interrupt is enabled
          bra     PutData         ;if so, go transmit data

;can do special error handling here - an unexpected
interrupt occurred

LowInt1:

[SNIP]

PutData:  btfss   Flags,TxBufEmpty ;check if transmit
buffer is empty
          bra     PutDat1         ;if not then go transmit
          bcf     PIE1,TXIE       ;else disable Tx
interrupt
          bra     EndLowInt

PutDat1:  rcall   GetTxBuffer     ;get data from
transmit buffer
          movwf   TXREG           ;and transmit



When there is nothing more to send, TXIE is cleared to
prevent the transmitter triggering any more interrupts
but TXIF is not reset because no character is written
to TXREG.  When the next character is available all
that is required is to set TXIE and because TXIF is
still set an interrupt will occur.

The sdcc signal handler code does NOT allow for the
possibility that TXIF may be set even though it was not
the USART that triggered the interrupt.  If TXIF is set
it always jumps to the handler.  If the correct handler
is further down the list an interrupt dead lock occures.

My solution is an extended version of the DEF_HANDLER
macro which allows both **IE and **IF bits to be
checked before jumping to a handler.  (Note I've used
the SIG_ prefixes to avoid clashes as I detailed in an
earlier post)

#define SIG_TXIF    0x4

#define SIG1_TX         _PIR1, SIG_TXIF
#define SIG2_TX         _PIE1, SIG_TXIF

#define DEF_HANDLER2(sig1,sig2, handler)           btfss       sig1                 \n    bra        $+8                   \n    btfsc       sig2                 \n    goto        _ ## handler


It is used like this...

DEF_HANDLER2(SIG1_TX, SIG2_TX,_TX_handler)

Peter 





----------------------------------------------------------------------

>Comment By: Raphael Neider (tecodev)
Date: 2005-08-07 20:23

Message:
Logged In: YES 
user_id=1115835

"Fixed" in SDCC 2.5.2 #1083 (provided means to achieve the
desired effect; users have to modify their code).

I added SIG_<src>IE definitions for all signals and added
the proposed DEF_HANDLER2() macro so that one can use
DEF_HANDLER2(SIG_TX, SIG_TXIE, _TX_handler)
to achieve the desired effect.
(Note that for signals outside of PIR/PIE the enable bit
definition cannot be deduced from the flag definition --
this makes the definitions rather ugly...)

----------------------------------------------------------------------

Comment By: Peter Onion (p_onion)
Date: 2005-08-07 10:40

Message:
Logged In: YES 
user_id=1316390

Typo in the above... It shoud read 
"The problem concerns the fact that only the TXIF bit
(_PIR1:4) is checked before jumping to the handler."
Peter 

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