8051 Timers
Timers are digital counters incremented by pulses. They are controlled by four SFRs: TMOD, TCON, TH0/TL0, and TH1/TL1.
Timers overflow when they reach their maximum value and reset to 0. Overflow sets bits in the TCON SFR, which can be programmed to generate an interrupt and execute a subroutine.
Timer Operation Types
- Timing operation: Timer registers incremented by internal clock pulses.
- Counting operation: Timer registers incremented by external pulses via P3.4 (T0) for Timer 0 and P3.5 (T1) for Timer 1.
Operating Modes of 8051
| TxM1 | TxM0 | Timer Mode | Description |
|---|---|---|---|
| 0 | 0 | Mode 0 | 13-bit timer |
| 0 | 1 | Mode 1 | 16-bit timer |
| 1 | 0 | Mode 2 | 8-bit auto reload |
| 1 | 1 | Mode 3 | Split timer mode |
Mode 0 – 13-bit Timer Mode
- Uses 13 bits out of 16 (5 bits from TL0/TL1, 8 bits from TH0/TH1).
- TL0/TL1 counts 0–31; on overflow increments TH0/TH1.
- Maximum count: 8192 values (0–8191).
- Works as timer (internal clock) or counter (external clock).
- Selection by D2 (Timer 0) and D6 (Timer 1) bits of TMOD.
- Run control: D4 and D6 of TCON.
- GATE bits (D4 and D7) in TMOD allow external control via INT0/INT1.
- Overflow sets TF1 (Timer 1) or TF0 (Timer 0).
Mode 1 – 16-bit Timer Mode
- Full 16-bit use: TL0/TL1 counts 0–255; overflow increments TH0/TH1.
- Maximum count: 65536 values (0–65535).
- Gating and run control same as Mode 0.
Mode 2 – 8-bit Auto Reload Mode
- TL0/TL1 is incremented; on overflow, TLx is reloaded from THx automatically.
- THx value is constant; TLx is incremented repeatedly.
- Commonly used for baud rate generation in serial communication.
- Gating and run control same as Mode 0.
Example:
If TH0 = FDh, TL0 = FEh → next pulse TL0 = FFh → overflow → TL0 reloaded with FDh.
Mode 3 – Split Timer Mode
- Applicable only to Timer 0.
- Splits Timer 0 into two independent 8-bit timers: TL0 and TH0.
- TL0 counts 0–255; TH0 counts 0–255.
- Real Timer 1 (TH1/TL1) stops and holds count (TR1=0 condition).
- Used when two timers + baud rate generator are needed.
Timer Control and Operation
For timer mode (C/T=0 in TMOD):
- Timer counts FOSC/12 rate.
- Example: If clock = 11.059 MHz → increment rate = 921,583 Hz.
- Delay calculation: For 0.1 sec delay → count value = 0.1 × 921,583 = 92,158.
Steps to Initialize and Use Timer in 8051
- Select timer mode (0–3).
- Initialize TMOD SFR.
- Load initial value into timer registers.
- Start timer by setting TR0/TR1 bit in TCON.
- Check TF0/TF1 or handle overflow via interrupt.
Bit Setting Example
To set TR1 (TCON D6):
MOV TCON, #40h- Or use bit-addressable instruction:
SETB TR1(recommended, no effect on other bits).