I have written an application that runs on a DS5250/DS5240 based board, a 8052 derivative from Dallas-Maxim, this application has to manage a keybord and a simple protocol to dialogue with an host
I enable two interrupts: interrupt from Timer 0 and interrupt from Serial 0, the first one has a low priority, the second one an high priority.
Interrupt from timer 0 manages a set of virtual timers, interrupt from Serial 0 manages the reception and trasmission using circular buffer.
Sometimes (let’s say after two-three hours that the program runs) happens that interrupt from Timer 0 seems to “freeze” (virtual timer does not work anymore), the rest of the program continue to work well. I was able to break the program in this condition ( I use a Developer’s Kit to debug) and it appears the LIP bits in the STATUS register is always set. This means that a low priority interrupt is active at this point in time. This will prevent any more low priority interrupts from being acknowledged.
The only thing that I can do at this point is to turn off and then to turn on the device.
The only cause I can guess is that Timer 0 interrupt sometimes is terminated by a RET instead of a RETI, but I cant understand how.
The “freeze” disappears if I program the two interrupt with the same priority level (level 0).



The interrupt routines are:


//Timer 0 Interrupt manages a virtual timer table
void timer0 (void) interrupt 1 using 3
{
data unsigned char data *ptabella;
data unsigned char i;

TR0 = 0;

ptabella = (unsigned char data*) &timer10ms;
for( i = 0; i < LENTIMER10MS; i++ )
{
if( (*ptabella) !=0 )
(*ptabella)--;

ptabella++;
}

if( --prescaler_50ms == 0 )
{
prescaler_50ms = VALUE_50MS;
ptabella = (unsigned char data*)&timer50ms;

for( i = 0; i < LENTIMER50MS; i++ )
{
if( (*ptabella) !=0 )
(*ptabella)--;

ptabella++;
}

if( --prescaler_1sec == 0 )
{
prescaler_1sec = VALUE_1SEC;
ptabella =(unsigned char data*)&timer1sec;
for( i = 0; i < LENTIMER1SEC; i++ )
{
if( (*ptabella) !=0 )
{
(*ptabella) --;
}
ptabella++;
}

}

}
TL0 = (unsigned char)LO_T0;
TH0 = (unsigned char)HI_T0;
TR0 = 1;
}


void com_isr (void) interrupt 4 using 2
{
if (RI != 0) /* Received data interrupt */
{
RI = 0;

//inserte a character in a circular buffer
if( r_count < RBUF_SIZE )
{
ACC = SBUF0; //check the parity
if( RB8 == P )
{
r_count++;
rbuf[ r_in++ ] = SBUF0;
if( r_in > RBUF_SIZE-1 )
r_in = 0;

}
}
}

if (TI != 0) // Transmitted data interrupt
{
TI = 0;

//get a character from circular buffer and trasmit it
if( t_count != 0 )
{
ACC = tbuf[ t_out ]; //set the parity
TB8 = P;
SBUF0 = tbuf[ t_out++ ];
t_count--;
if( t_out > TBUF_SIZE-1 )
t_out = 0;
}
else
t_disabled = 1;
}

}