Programming and application of timer/counter mode 1

Mode 0 of the timer / counter is basically the same as mode 1, except that the counter has a different number of bits. Mode 0 is a 13-bit counter and Mode 1 is a 16-bit counter. Since mode 0 is designed to be compatible with MCS-48, and its initial value calculation is complicated, in practical applications, mode 0 is generally not used, and mode 1 is adopted.

Example 1: Assuming a system clock frequency of 6 MHz, a square wave with a period of 2 ms is output on the P1.0 pin, as shown in the figure to the right.


The basic idea: the period of the square wave is determined by the timer T0, that is, an initial value is set in T0, and the count is performed on the basis of the initial value, and the overflow occurs every 1 ms, that is, T0 generates an interrupt every 1 ms. After the CPU responds to the interrupt, it inverts P1.0 in the interrupt service routine. In this way, a square wave with a period of 2 ms can be output on the P1. pin as shown in the figure to the right. To do this, do the following steps.

(1) Calculate the initial value of the count X
Machine cycle
Let the initial value of T0 need to be loaded into X, then there is

Convert X to a hexadecimal number, that is, X=FE0CH=1111111000001100B. Therefore, the initial value of T0 is TH0=FEH, TL0=0CH.

(2) Initialization program design. This example works in the timer interrupt mode. The initialization program includes timer initialization and interrupt system initialization, mainly to correctly set the corresponding bits of the registers IP, IE, TCON, TMOD, and send the initial value of the count to the timer.

(3) Programming. In addition to completing the required work of generating a square wave, the interrupt service routine also pays attention to reloading the initial value into the timer to prepare for the next interrupt generation. In this example, the main program is replaced with a short jump instruction that goes to itself.

The reference procedure designed according to the above requirements is as follows:

Program Description: When the microcontroller resets, jump from program entry 0000H to the main program MAIN to execute the program. The T0 initialization subroutine PT0M0 is called. After the subroutine returns, the program executes the "AJMP HERE" instruction, and the loop waits. When the microcontroller responds to the T0 timer interrupt, it jumps to the T0 interrupt entry, and then jumps from the T0 interrupt entry to the ITOP label to execute the T0 interrupt service routine. After executing the instruction "RETI" returned by the interrupt, it returns to the breakpoint and continues to execute the loop instruction "AJMPHERE". In the actual program, the "AJMP HERE" instruction is actually a main program. When the 1 ms timer interrupt of the next timer T0 occurs, it jumps to the T0 interrupt entry, thereby repeating the above process.

If the CPU does not do other work, it can also be controlled by query mode, the program is much simpler.

The reference procedure for the query method is as follows:

Although the program of the query mode is simple, the CPU must constantly query the TF0 flag, and the work efficiency is low.

Example 2: Assuming that the system clock is 6 MHz, the timer T0 is programmed to generate a 1 s timing program.

The basic idea: use the timer mode of operation. Due to the long timing, it is first determined which working mode to use. According to the characteristics of various working modes of the timer described above, it can be calculated that the maximum working time of the timer can be calculated under the condition that the clock is 6 MHz:

Mode 0 can be timed up to 16.384ms;
Mode 1 can be up to 131.072ms longest;
Mode 2 can be set to a maximum of 512 μs.

As can be seen from the above, Option 1, interrupt every 100 ms, and interrupt 10 times for 1 s.

(1) Calculate the initial value X of the count. because
. therefore

(2) Implementation of 10 counts. For the count of 10 interrupts, the B register is used as the interrupt counter.

(3) Programming. The reference procedure is as follows:

Program Description: Return to the "SJMP HERE" command regardless of whether the 1s timing has expired. In the actual program, the "SJMP HERE" instruction is actually a main program. In this main program, by judging the F0 flag, it can be known whether the timing of 1s has arrived, and then the specific processing is performed.

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