Button recognition method experiment

One of the key recognition methods
1. Each time switch SP1 is pressed in the experimental task
, the count value increases by 1, and its binary count value is displayed through P1.0 to P1.3 of the P1 port of the AT89S51 microcontroller.
2. Circuit schematic

Figure 4.8.1
3. Hardware connection on the system board
(1. Connect the P3.7/RD port in the "Single-chip Computer System" area to the SP1 port in the "Independent Keyboard" area;
(2. Connect the P1. The 0-P1.4 port is connected to the "L1-L8" port in the "Eight-way LED Indicator Module" area with an 8-core cable; the requirements are that P1.0 is connected to L1, P1.1 is connected to L2, and P1.2 Connected to L3, P1.3 is connected to L4.
4. Programming method
(1. In fact, it is a complete process from not pressing to pressing and releasing, that is, when we press a button Sometimes, we always hope that a certain command will be executed only once, and there should be no interference during the process of pressing the button, because if there is interference during the pressing process, it may cause a false trigger process. This is not what we want. wanted. So when the button is pressed,

Figure 4.8.2
needs to filter out the interference signals on our hands and the mechanical contact of the keys. Generally, we can use capacitors to filter out these interference signals, but in fact, it will increase the hardware cost and We don’t want the size of the hardware circuit. There must be a way to solve this problem. Therefore, we can use software filtering to remove these interference
signals. Generally speaking, when a button is pressed, it is always at the moment when it is pressed. There is a certain interference signal, and it basically enters a stable state after pressing it. The specific signal diagram of the whole process from pressing to releasing a button is shown in the figure above:
As can be seen from the figure, when we program the design, there is a delay of more than 5ms after the button is recognized and pressed, thus avoiding the problem of In the interference signal area, let's check again to see if the button has really been pressed. If it has been pressed, the output will definitely be low level. If the high level is detected at this time, it proves that it was caused by the interference signal. If there is a false trigger, the CPU will think it is a false trigger signal and abandon the key recognition process this time. Thereby improving the reliability of the system.
Since the command is required to be executed once every time it is pressed, the command will be executed again when the button is pressed again. Therefore, after the button is recognized, we can execute the command this time, so there must be a waiting button to release. The process, obviously the release process, is to restore it to a high level state.
(1. For the instruction of key recognition, we still choose the following instruction JB BIT. The REL instruction is used to detect whether BIT is high level. If BIT = 1, the program will turn to REL to execute the program, otherwise it will continue to execute the program downwards. . Or JNB BIT, the REL instruction is used to detect whether BIT is low level. If BIT = 0, the program will move to REL for execution, otherwise it will continue to execute the program
(2. But the key is pressed during the programming process. The block diagram of the recognition process is shown on the right:

Figure 4.8.3
5． Block diagram

Figure 4.8.4
6. Assembly source program
ORG 0
START: MOV R1, #00H; Initialize R1 to 0, indicating that counting starts from 0
MOV A, R1;
CPL A; Negate the instruction
MOV P1, A; Send the P1 port and display it on the LED
REL: JNB P3 .7, REL; Determine whether SP1 is pressed.
LCALL DELAY10MS; If pressed, delay about 10ms.
JNB P3.7, REL; Determine whether SP1 is really pressed.
INC R1; If pressed, proceed with key processing.
MOV A,R1; Add 1 to the count content and send it out to the P1 port by
CPL A; LED display
MOV P1,A;
JNB P3.7,$; Wait for SP1 to release
SJMP REL; Continue to scan the K1 button
DELAY10MS: MOV R6,# 20 ;Delay 10ms subroutine
L1: MOV R7,#248
DJNZ R7,$
DJNZ R6,L1
RET
END

7. C language source program
#include

unsigned char count;

void delay10ms(void)
{
unsigned char i,j;
for(i=20;i>0;i--)
for(j=248;j>0;j--);
}

void main(void)
{
while(1)
{
if(P3_7==0)
{
delay10ms();
if(P3_7==0)
{
count++;
if(count==16)
{
count=0;
}
P1=~count ;
while(P3_7==0);
}
}
}
}