MCS51 MCU counter/timer working mode 3 programming example

2 8-bit modes. Mode 3 is only applicable to timer 0. If timer 1 is set to mode 3, timer 1 will be in the off state. 
When T0 is in mode 3, THo and TL0 are divided into 2 independent 8-bit counters . Among them, TL0 can be used as both a timer and a counter, and uses all the control bits of the original T0 and its timer return to zero flag and interrupt source. TH0 can only be used as a timer, and uses the control bit TRl, return to zero flag TFl and interrupt source of T1, see the figure below.

Normally, T0 does not run in working mode 3. It can only be used when T1 is in working mode 2 and does not require interruption. At this time, T1 is often used as a serial port baud rate generator (see 1.4), TH0 is used as a timer, and TL0 is used as a timer or counter. Therefore, mode 3 is specially provided for applications where the microcontroller has an independent timer/counter, a timer, and a serial port baud rate generator. At this time, timer 1 can be used in working mode 2, and timer 0 can be used in working mode 3.

At this time, T1 is often used as a serial port baud rate generator, TH0 is used as a timer, and TL0 is used as a timer or counter. Therefore, mode 3 is specially provided for the application of the microcontroller with an independent timer/counter, a timer, and a serial port baud rate generator. At this time, timer 1 can be used in working mode 2, and timer 0 can be used in working mode 3.

Understanding the content

Timer/Counter and Interrupt Comprehensive Application Example

Example: Clock timing program design. 

The so-called clock timing is the timing in seconds, minutes and hours. This program can be regarded as a typical representative of timer/counter and interrupt application, which can be regarded as a review of these two parts.

⑴ Basic method of realizing clock timing display with MCS-51 single chip microcomputer

① First, calculate the initial value of the count

The key issue of clock timing is the generation of seconds, because seconds are the smallest clock unit. However, when using the timer/counter of MMCS-51 for timing, even in working mode 1, its maximum timing time can only reach 131 milliseconds, which is far from 1 second. Therefore, we implement the second timing by combining hardware timing and software counting, that is, setting the timing time of the timer to 125 milliseconds, so that when the count overflows 8 times, 1 second can be obtained, and the 8-time count can be implemented by software methods.

To get 125ms timing, we can use timer/counter 0 and work in mode 1. When the microcontroller is set to a 6MHz crystal oscillator and the initial value of the design number is X, the following equation is obtained:

(216-X)×2us=125000us 

The calculated initial count value X = 3036,

The binary representation is 0000101111001101 and the hexadecimal representation is 0BCDH.

② The timer timing is completed in an interrupt mode, so that the overflow times (125 milliseconds each time) can be accumulated through the interrupt service program. When 8 times are counted, the second timing is obtained.

③ Realize timing from seconds to minutes and from minutes to hours by accumulating and comparing values ​​in the program 

④ Set the clock display and display buffer

Assume that the clock time is displayed on a six-digit LED digital tube (LED5 to LED0) (hour, minute, and second each occupy two digits). To this end, a display buffer must be set in the internal RAM, with a total of 6 units (79H to 7EH), and the corresponding relationship with the digital tube is:

      LED5→7EH, LED4→7DH, LED3→7CH, LED2→7BH, LED1→7AH, LED0→79H.

That is, the display buffer stores the values ​​of hours, minutes, and seconds from left to right.

⑤ Assume that the LED display program is SMXS and can be called

⑵ Procedure flow

① Main program (MAIN)

The main function of the main program is to initialize the timer/counter and then wait for the 125ms timing interrupt by repeatedly calling the display subroutine.

② Interrupt service routine (PIT0)

The main function of the interrupt service program is to perform timing operations. The program starts by determining whether the count overflows for 8 times. If it is less than 8 times, it means that the minimum timing unit second has not been reached, and the interrupt returns; if it is full 8 times, it means that the minimum timing unit second has been reached, and the program continues to execute and perform timing operations.

③ Add 1 subroutine (DAAD1)

The add 1 subroutine is used to complete the operation of adding 1 to the second, minute and hour. There are three places in the interrupt service program to call this subroutine when adding 1 to the second, minute and hour. The add 1 operation includes the following three contents: 

Composite number

Since each LED display corresponds to an 8-bit buffer unit, the time value represented by the two-bit BCD code occupies one buffer unit each, and only occupies the lower 4 bits. Therefore, before adding 1, the values ​​stored in the two buffer units need to be combined to form a byte, and then the addition operation can be performed. This is where the word "combination" comes from.

Decimal Adjustment 

    Add 1 and make a decimal adjustment

Fraction 

    The time value after adding 1 is split into two bytes and sent back to their respective buffer units. 

⑶ Program List

    The entire reference program list is as follows:

                 ORG 8000H

 START:AJMP MAIN

                ORG 800BH 

                AJMP PITO 

                ORG 8100H 

 MAIN: MOV SP, #60H; establish the stack area 

              MOV R0, #79H ; Display buffer first address 

              MOV R7, #06H; Display digits

 ML1: MOV @R0, #00H; clear the display buffer unit to 0 

              INC R0

              DJNZ R7, ML1

              MOV TMOD, #01H; Timer 0, working mode 1

              MOV TL0, #0CDH; load the initial value of the counter

              MOV TH0, #0BH 

              SETB 8CH; TR0 is set to 1, timing starts

              SETB AFH; EA is set to 1, interrupts are always enabled

              SETB A9H; ET0 is set to 1, timer 0 interrupt is enabled

              MOV 30H, #08H; the required number of count overflows, i.e. the number of loops

 ML0: LCALL SMXS ; Call display subroutine

              SJMP ML0 

PITO: PUSH PSW; interrupt service routine, context protection 

             PUSH ACC

             SETB PSW.3; RS1RS0=01, select 1 group of general registers

             MOV TL0, #0CDH; counter reload

             MOV TH0, #0BH

             MOV A, 30H; the number of cycles decreases by 1

             DEC A

            MOV 30H, A

            JNZ RET0; less than 8 times, go to RET0 and return

            MOV 30H, #08H; after 8 times, start timing operation

             MOV R0, #7AH; Second display buffer unit address

           ACALL DAAD1; add 1 per second

           MOV A, R2; add 1 and the seconds value is in R2

           XRL A, #60H; Check whether 60 seconds have passed

           JNZ RET0 ; if not reached, go to RETO to return 

           ACALL CLR0; clear the display buffer unit to 0 after 60 seconds 

           MOV R0, #7CH; Display buffer unit address

           ACALL DAAD1 ; points plus 1 

           MOV A, R2 

           XRL A, #60H; Determine whether it has reached 60 points

           JNZR 

           ACALL CLR0; At 60 minutes, the minute display buffer unit is cleared to 0

           MOV R0, ＃7EH ; displays the buffer unit address

           ACALL DAAD1 ; add 1

            MOV A,R2] 

            XRL A, #24H; Determine whether it is 24 hours

           JNZR

           ACALL CLR0; At 24 o'clock, the time display buffer unit is cleared to 0

 RET0:POP ACC; scene recovery

            POP PSW

           RETI ;Interrupt return 

DAAD1:MOV A, @R0; add 1 subroutine, send the tens digit to A 

           DEC R0 

           SWAP A; the tens digit occupies the upper 4 digits 

           ORL A, ＠R0; the unit digit occupies the lower 4 digits

           ADD A, #01H; add 1

           DA A ; Decimal adjustment

           MOV R2, A; store the full value in R2 temporarily

           ANL A, #0FH; shield the tens digit and extract the ones digit

           MOV @R0, A ; Send the unit value to the display buffer unit

           MOV A,R2

           INC R0

           ANL A, #F0H; shield the unit digit and extract the ten digit

           SWAP A; make the tens digit occupy the lower 4 digits

           MOV @R0, A ; Send the tens digit to the display buffer unit

           RET ; Return

      CLR0:CLR A; Clear buffer unit subroutine

           MOV @R0, A ; clear the tens digit buffer unit to 0 

           DEC R0

           MOV @R0, A ; clear the single-digit buffer unit to 0