MCS-51 notes (for personal use)

interrupt

1. Pin usage

2. Interrupt entry address

3. Register

3.1 TCON register (bit addressable)

Bit	D7	D6	D5	D4	D3	D2	D1	D0
name	TF1	TR1	TF0	TR0	IE1	IT1	ie0	IT0

timer

• TF: timer/counteroverflow interruptAfter overflow, it is set to 1 by hardware; after entering the interrupt service routine, it is cleared to 0 by hardware or software.
• TR: timer/counterStart and stop position

external interruption

• IE：interrupt flagHardware cleared to 0
• IT：Trigger mode1: falling edge trigger; 0: low level trigger

3.2 SCON register (bit addressable)

• D1: TI serial port transmit buffer (SBUF)nullInterrupt request flag
• D0: RI serial port receive buffer (SBUF)FullInterrupt request flag
• Must be cleared to "0" by software during interrupt service

3.3 Interrupt enable register IE (bit addressable)

Bit	D7	D6	D5	D4	D3	D2	D1	D0
name	EA	–	–	IS	ET1	EX1	ET0	EX0

• EA: total allowed

• ES: serial port

• ET: timer

• EX: external interrupt

  Write 1 to indicate permission

3.4 Interrupt priority register IP (bit addressable)

Bit	D7	D6	D5	D4	D3	D2	D1	D0
name	–	–	–	PS	PT1	PX1	PT0	PX0

• Write 1 to indicate high priority interrupt

• High-priority interrupts can interrupt low-priority interrupts, but the same level cannot interrupt them.

• Query sequence when multiple peer interruptions occur:INT0>>T0>>INT1>>T1>>Serial

4. Response conditions

• For an interrupt request from an interrupt source to be responded to, the following necessary conditions must be met:

  a. Interrupt request flag=1

  b. Interrupt enable bit=1

  c. EA=1

  d. No interrupt of the same level or higher is being executed.

• Interrupt responses are blocked when one of the following three conditions is encountered:

  a. The CPU is processing an interrupt of the same level or higher priority

  b. The machine cycle queried is not the last machine cycle of the instruction currently being executed.

  c. The instruction being executed is RETI or an instruction to access IE or IP.

5. External interrupt response time

More than 3 machine cycles, less than 8 machine cycles

6. Interrupt function

General form of interrupt service function

void 函数名（void） interrupt n using m

n: interrupt number

m: Working register area selection (if the using option is not selected, the contents of all working registers in the interrupt function will be saved to the stack)

No return value, no parameters passed, cannot be called directly

7. Example

void main()
{
    EX0=1; // 允许外部中断0中断
    EX1=1; // 允许外部中断1中断
    IT0=1; // 选择外部中断0为跳沿触发方式
    IT1=1; // 选择外部中断1为跳沿触发方式
   	PX0=0; // 外部中断0为低优先级
	PX1=1; // 外部中断1为高优先级
    EA=1; // 总中断允许
    while( 1 )
    {
		;
    }
}

void int0_isr(void) interrupt 0	// 外中断0的中断服务函数
{
    ;
}

void int1_isr(void) interrupt 2	// 外中断1的中断服务函数
{
    ;
}

timer

1. Two working modes

Timer: for system clock signal12 frequency dividerThe internal pulse signal count after. (Internal pulse signal period = machine period)

Counter: Count the external pulses applied to the two pins T0 (P3.4) and T1 (P3.5)

2. Register

2.1 Working mode register TMOD (Not bit addressable）

Bit	D7	D6	D5	D4	D3	D2	D1	D0
name	GATE	C/~T	M1	M0	GATE	C/~T	M1	M0
timer	T1	T1	T1	T1	T0	T0	T0	T0

• GATE: Gate control bit, controls startup mode

  0, only controlled by TR (TR=1, start)

  1. High level on external interrupt pin (INT0 or INT1) + TRx (TR=1, and INT0 or INT1 pin is high level, start)

• C/~T: Mode selection bit (1, counter; 0, timer)

• M1, M0: working mode selection bit

Method 1, after entering the interrupt service routine, first write the initial value

Method 2, TH and TL write initial values. After the TL count overflows, the initial value written for the first time in TH is automatically entered into TL to achieve automatic reloading.

Mode 3, T0 is divided into two independent 8-bit counters TL0 and TH0

TL0, use the status control bits C/T, GATE, TR0, TF0 of T0

TH0, fixed as an 8-bit timer, cannot be used as an external counting mode. It uses the status control bit TR1 of T1 and occupies the interrupt request source TF1 of timer T1.

​ ① Generally, when T1 is used as the baud rate generator of the serial port, T0 only works in mode 3.
② When T0 is in working mode 3, T1 can be set to mode 0, mode 1 and mode 2, used as a baud rate generator for the serial port, or does not require interruption occasion.

2.2 TCON (bit addressable)

Bit	D7	D6	D5	D4	D3	D2	D1	D0
name	TF1	TR1	TF0	TR0	IE1	IT1	ie0	IT0

• TF
• TR

3. Initial value of counting

Take mode 1, 16-bit timer/counter as an example

1 machine cycle = 12 clock cycles = 12 / crystal oscillator frequency (such as a 12MHz crystal oscillator, the machine cycle is 1us)

Timing time = (65536 - X) * 12 / crystal oscillator frequency

X = 65536 − Timing time * Crystal oscillator frequency / 12

TH = X / 256;
TL = x % 256;

4. External input counting signal

Since it takes time to confirm a negative transition2 machine cycles, so the highest frequency of the externally input counting pulse is the system oscillator frequency1/24

The input signal must be maintained for at least one machine cycle

5. Example

Timer 0, works in mode 1, the timing time is set to 50ms, and the LED turns on and off every 1s (12MHz crystal oscillator)

void main(void)
{
    
    
    TMOD = 0x01;				//设置定时器T0为方式1定时,0000 0001
    TH0 = (65536-50000) / 256;	//定时器T0赋初值
    TL0 = (65536-50000) % 256;
    ET0 = 1;					//T1中断允许
    EA = 1;						//总允许
    TR0 = 1;					//T0启动
    while(1)
    {
    
    
        ;
    }
}

void T0_interserve(void) interrupt 1	//定时器T0中断服务子程序,50ms
{
    
    
    static unsigned char T0_counter = 0;
    TH0 = (65536-50000) / 256;	//定时器T0赋初值
    TL0 = (65536-50000) % 256;
    T0_counter++;
    if(T0_counter >= 20)	//1s=50ms*20
    {
    
    
        LED = ~LED;
        T0_counter = 0;
    }
}

Serial communication

1. Register

1.1 SCON (bit addressable)

Bit	D7	D6	D5	D4	D3	D2	D1	D0
name	SM0	SM1	SM2	REN	TB8	RB8	OF	RI

1. SM0, SM1: working mode selection bits

   

2. SM2: Multi-machine communication control bit

   Multi-machine communication can only be achieved in mode 2 and mode 3

3. REN: Allow serial reception bit

   Write 1 to allow the serial port to receive data

4. TB8: The 9th bit of data sent

   Set to "1" or clear to "0" by software

   ① In dual-computer serial communication, TB8 is generally used as a parity bit.
   ② In multi-machine communication, it is used to indicate whether the host is sending an address frame or a data frame. TB8=1 is an address frame, and TB8=0 is
   data. frame.

5. RB8: The 9th bit of data received

   

6. TI: Transmit interrupt flag bit (SBUFnull）

   Set to "1" by hardware and must be cleared to "0" by software

7. RI: receive interrupt flag bit (SBUFFull）

   Set to "1" by hardware and must be cleared to "0" by software

1.2 PCON

Bit	D7	D6	D5	D4	D3	D2	D1	D0
name	SMOD	–	–	–	–	–	–	–

SMOD: baud rate selection

​

Overflow rate of timer T1 = 1 / overflow time of timer T1 = 1 / (256 - X) * 12 / crystal oscillator frequency

2. Example

A machine sends

#include <reg51.h>
#define UCHAR unsigned char
UCHAR Temp=0;

void main()
{
    
    
    TMOD=0x20; // 设置定时器T1为方式2
    TH1=0xfd; // 波特率9600
    TL1=0xfd;
    
    SCON=0x40; // 串口设置为方式1,只发送
    PCON=0x00; // SMOD=0
    
    ES = 1;
    EA = 1;
    TR1 = 1; // 启动T1
    
    P1=0xff; // P1口为输入
    Temp=P1; // 读入P1口开关的状态数据
    SBUF=Temp; // 数据送串行口发送
    while(1)
    	； // 循环等待
}

void sci_int(void) interrupt 4
{
    
    
    if(TI)
    {
    
    
        TI=0; // 清TI
        Temp=P1; // 读入P1口开关的状态数据
        SBUF=Temp; // 数据送串行口发送
    }
    if(RI)
    	RI = 0;
}

Machine B accepts

#include <reg51.h>
#define UCHAR unsigned char
UCHAR Temp;

void main( )
{
    
    
    TMOD=0x20; // 设置定时器T1为方式2
    TH1=0xfd; // 波特率9600
    TL1=0xfd;
    
    SCON = 0x50; // 方式1接收，REN=1
    PCON = 0x00; // SMOD=0
    
    ES = 1;
    EA = 1;
    TR1 = 1; // 启动T1
    while(1)
    	;
}

void sci_int(void) interrupt 4
{
    
    
    if(RI)
    {
    
    
        RI=0; // 接收到数据，清RI
        Temp = SBUF;
        P1=Temp;
    }
    if (TI)
    	TI = 0;
}

memory expansion

1. Commonly used memory address space allocation methods

1.1 Line selection method

1.2 Decoding method

• Chip select is implemented using high-order address lines.

8255

There are two ways to address commonly used I/O ports, one is independent addressing and the other is unified addressing. The AT89S51 microcontroller uses a unified addressing method.

1. Internal structure:

• On-chip address: PA: PB:1 PC:2 Control port: 3

2. Mode control word register

a. Working mode selection control word (D7=1)

Bit	D7	D6	D5	D4	D3	D2	D1	D0
name	1	A way	A way	AInput/Output	C high 4 input/output	B way	B input/output	C low 4 input/output

82C55 has 3 basic working modes:

• Mode 0—Basic input/output; (unconditional transmission)
• Mode 1—Strobe input/output; (conditional transmission)
• Mode 2—bidirectional transmission (only the PA port has this working mode)

PA port can work in mode 0, 1 and 2

The PB port can only work in modes 0 and 1

The PC port is divided into two parts, the PA port is called group A, and the PB port is called group B.

Example: AT89S51 microcontroller writes the working mode control word 95H to the control word register of 82C55 (assuming the port address is FF7FH)

#include <absacc.h>
#define COM8255 XBYTE[0xff7f] // 0xff7f为82C55的控制寄存器地址
#define UCHAR unsigned char
……
void init8255( void )
{
    COM8255 = 0x95; // 工作方式选择控制字写入82C55的控制寄存器
    ……
}

b. PC port bit set/reset control word (D7=0)

Bit	D7	D6	D5	D4	D3	D2	D1	D0
name	0	–	–	–	PC port selection	PC port selection	PC port selection	1 set/0 reset