what does cmod |= 0x04 mean please it is the | i don't understand

[Chris Reeves]

what does the | do in CMOD |=0x04

/* Use timer0 overflow to clock PCA timer block
      ------------------------------------_------- */
   TMOD = 0x02;                   // T0 mode 2, 8 bit counter auto reload
   TH0  = 0xFF;                   // Counts 1 clock(s), equivalent to 1us
   TL0  = 0xFF;
   TR0  = 1   ;                   // Start timer 0
 
   /* PCA Block clock source T0 overflow, allow interrupt
      --------------------------------------------------- */
   CMOD    |= 0x04;
 

[weaknessforcats]

The | is the binary OR. cmod | 0x04 sets bit 0x04 since the result is true regardless of the value of the 0x04 bit in cmod.
The |= just stores the result back into cmod.
The next effect is that the 0x04 bit in cmod is set in and no other bits are affected.

[Banfa]

This reminds me of micro-processor code with CMOD and all the other variables presenting micro-processor registers. This is born out by the code comments.

If that is the case the operation is setting a bit in a micro-processor register altering the operation of some module, enabling an interrupt from the comment.

[donbock]

CMOD |= 0x04;
is equivalent to
CMOD = CMOD | 0x04;

That is, first a value is read from CMOD, then a new value is computed, then the new value is stored into CMOD. This may be important because you will sometimes run into a microprocessor register cannot be both read from and written to.