MCS51 8051 Use the Timer

Timer 0，Mode0 : 13 bit counter, not auto reload
clock source可選擇internal crystal (freq/12)或是external trigger (Pin: T0).

TMOD Reg (Not bit addressable)

bit 0 - 3 control Timer0

• bit 0 : M0 (w M1, mode select)
  
• bit 1 : M1 ( --)
• bit 2 : C/T (Counter or Timer ? Select clock source)
• bit 3 : GATE, Timer controllerd by Pin: INT0 ?

bit 4 - 7 are the same as bit 0 - 3, but used to control Timer1

TCON Reg ( bit addressable)

• bit 0 : IT0, external interupt 0 pin, trigger mode (rising or falling )
  
• bit 1 : IE0, external interrupt 0 pin (INT0) enable
• bit 2 : IT1, external interrupt 1 pin, trigger mode (rising or falling )
• bit 3 : IE1, external interrupt 1 pin (INT1) enable
• bit 4 : TR0, Timer 0 enable
• bit 5 : TF0, Timer 0 overflow flag
  
• bit 6 : TR1, Timer 1 enable
• bit 7 : TF1, Timer 1 overflow flag

TH0.TL0 Reg (Counter Register)

• TH0 使用8 bit，代表13bit counter的hi 8 bits
• TL0使用5 bits，代表13bit counter的lo 5 bits

一般的設定code是 (在main)
TH0 = value/32;
TL0 = value%32;Crystal = 12MHz.
因為timer用overflow trigger，所以value = (8192-要counter的次數)。

Example : 使用Timer0 作 5 ms的timer count。

TMOD = 0x00;    //Mode0, C/T 0: use internal, GATE 0: no relation w INT0
TH0=(8192-5000)/32;
TL0=(8192-5000)%32;
IE = 0x82;      // enable interrupt
TR0=1;          // start gtimer

interrupt function

void tmr5ms(void) interrupt 1
{
   TH0=5000/32;
   TL0=5000%32;

   tmrcounter ++;
}