#787 AT89C2051 Timers

All about using 8051 timer registers with the AT89C2051.

Notes

8051 Timers

The 8051 has two timers Timer0 (T0) and Timer1 (T1), both are 16-bit wide. Since 8051 has 8-bit architecture, each of these is accessed by two separate high/low 8-bit registers (TH0, TL0, TH1, TH0). These registers are used to load timer count.

Each timer may be configured to operate in one of four modes:

• Mode 0: 13-bit timer/counter, 8-bit of THx & 5-bit of TLx
• Mode 1: 16-bit timer/counter, THx cascaded with TLx
• Mode 2: 8-bit timer/counter (auto-reload mode), TLx reload with the value held by THx each time TLx overflows
• Mode 3: Split the 16-bit timer into two 8-bit timers i.e. THx and TLx like two 8-bit timer

Timer Clock

The 8051 microcontrollers provides a clock source to the timers operating at 1/12th of the frequency of crystal attached to the microcontroller (the machine cycle).

For example, with a 16 MHz crystal:

Timer clock frequency
= 16 MHz / 12
= 1.333... MHz
Period T
= 12 / 16 MHz
= 0.75 μs
Clock cycles per millisecond
= 1 ms / 0.75 μs
= 1333.333...

Note that at 16 MHz we don’t get a clean division to 1ms / 1 Hz. This could be a good reason for using a 12 MHz crystal instead, if accurate timing is required.

8051 Timer Registers

8051 has a Timer Mode Register and Timer Control Register for selecting a mode of operation and controlling purpose.

TMOD Register

TMOD is an 8-bit register used to set timer mode of timer0 and timer1.

TCON Register

TCON is an 8-bit control register and contains a timer and interrupt flags.

Note:

• Timer1/Timer0 Overflow Flag: It is cleared through software. In the Timer1 overflow interrupt service routine, this bit will get cleared automatically while exiting from ISR.
• Timer1/Timer0 Run Control Bit. It is set and cleared by software.
• External Interrupt1/Interrupt0 Edge Flag: It is set and cleared by hardware.

Demo Circuit Design

Designed with Fritzing: see Timers.fzz.

The Code

See Timers.c

The core ms_delay function is implemented as follows:

#define CPU_MHZ (16)
#define TIMER_TICKS_PER_MS ((CPU_MHZ * 1000000UL) / 12 / 1000)
#define MAX_STEPS_PER_TIMER_CYCLE (0xFFFF / TIMER_TICKS_PER_MS)

void ms_delay(unsigned int ms) {
    TMOD = 0x01; // timer 0, mode 1, 16 bit mode
    unsigned int counter;
    unsigned int step = MAX_STEPS_PER_TIMER_CYCLE;
    while (ms > 0) {
        if (ms < step) step = ms;
        ms -= step;
        counter = 0xFFFF - step * TIMER_TICKS_PER_MS + 1;
        TL0 = counter & 0xFF;        // Low byte
        TH0 = (counter >> 8) & 0xFF; // High byte
        TR0 = 1;                     // timer on
        while (!TF0);                // wait until overflow
        TR0 = 0;                     // timer off
        TF0 = 0;                     // clear flag
    }
}

Key points to note:

• The timing calculation is based on the clock frequency.
  • If a different crystal is used, just need to change CPU_MHZ accordingly
• We use the timer to wait for TIMER_TICKS_PER_MS machine cycles per ms
• Since we can only count to 0xFFFF (just over 49ms at 16 MHz) per timer cycle,we need to loop over the timer multiple times to reach the desired delay
  • We set the timer for MAX_STEPS_PER_TIMER_CYCLE for n-1 loops
  • Then set the final loop to any remaining time left over

Programming

The Makefile is setup to compile the code using the SDCC compiler .. running on macOS in this instance:

$ make
sdcc -mmcs51 --code-size 2048 Timers.c -o Timers.ihx
packihx Timers.ihx > Timers.hex
packihx: read 18 lines, wrote 25: OK.

Program the chip using at89overlord and the LEAP#394 AT89C2051 Programmer:

$ at89overlord -p /dev/tty.usbserial-2420 -f ./Timers.hex
# Initializing the programmer...
# Initialized!
# Confirming chip ID...
# Confirmed!
# Erasing flash...
# Done!
# Writing flash...
# Done!
# Verifying...
# Done!

Testing

I have the circuit setup on a breadboard with the #780 AT89C2051 Breadboard Adapter:

Monitoring the LED output on the scope, running at 1Hz as expected:

Credits and References

• https://www.electronicwings.com/8051/8051-timers
• https://www.tutorialspoint.com/timers-of-8051