Special Function Registers (SFR) of 8051 Microcontroller in detail

 

Special Function Registers (SFR) of 8051 Microcontroller in detail

            Registers are the memory elements that are used to store user data. In 8051 microcontroller there are almost 20 SFR’s and each one have unique function. The list of SFR is given below with vector location. Formats of typical SFR’s like PSW, TCON, TMOD, IE, IP, SCON, PCON, SBUF are given below and we will discuss SFR’s in detail in upcoming articles. 

S.N.	SFR	Name	Vector Address
1	A	Accumulator	0E0 H
2	B	B register	0F0 H
3	P0	Port 0	80 H
4	P1	Port 1	90 H
5	P2	Port 2	0A0 H
6	P3	Port 3	0B0 H
7	IE	Interrupt Enable	0A8 H
8	IP	Interrupt Priority	0B8 H
9	TMOD	Timer mode	89 H
10	TCON	Timer control	88 H
11	TH0	Timer 0 High	8C H
12	TH1	Timer 1 High	8D H
13	TL0	Timer 0 Low	8A H
14	TL1	Timer 1 Low	8B H
15	SCON	Serial control	98 H
16	SBUF	Serial buffer	99 H
17	PCON	Power control	97 H
18	PSW	Program status word	0D0 H
19	DPTR	Data pointer	DPL-82H DPH-83H
20	SP	Stack pointer	81H

 

 

PSW (Program Status Word)
D7	D6	D5	D4	D3	D2	D1	D0
CY	AC	F0	RS1	RS0	OV	-	P
IE (Interrupt Enable)
D7	D6	D5	D4	D3	D2	D1	D0
EA			ES	ET1	EX1	ET0	EX0
IP (Interrupt Priority)
D7	D6	D5	D4	D3	D2	D1	D0
---	---	---	PS	PT1	PX1	PT0	PX0
TMOD (Timer Mode)
D7	D6	D5	D4	D3	D2	D1	D0
GATE	C/T\	M1	M0	GATE	C/T\	M1	M0
Timer1				Timer0
TCON (Timer Control)
D7	D6	D5	D4	D3	D2	D1	D0
TF1	TR1	TF0	TR0	IE1	IT1	IE0	IT0
SCON (Serial Control)
D7	D6	D5	D4	D3	D2	D1	D0
SM0	SM1	SM2	REN	TB8	RB8	TI	RI
SBUF (Serial Buffer)
D7	D6	D5	D4	D3	D2	D1	D0
PCON (Power Control)
D7	D6	D5	D4	D3	D2	D1	D0
SMOD	-	-	-	GF1	GF0	PD	IDL

 

 

Let us discuss some SFR’s in detail

1.     PSW (Program Status Word)

PSW is used to observe the status of program. Consider we performed arithmetic operation of addition, different flags of PSW indicates result of operation.

PSW (Program Status Word)
D7	D6	D5	D4	D3	D2	D1	D0
CY	AC	F0	RS1	RS0	OV	-	P

 

Bit	Flag	Name	Function
D7	CY	Carry flag	Sets=when carry is generated from MSB
D6	AC	Auxiliary carry flag	Sets=when carry is generated from lower nibble
D5	F0	F0	User definable flag
D4	RS1	Register bank select	Used to select register banks
D3	RS0
D2	OV	Overflow	Sets=when carry is generated from MSB or D6 to D7
D1	-	-	Reserved for future use
D0	P	Parity	Sets=odd parity of 1’s in result

 

RS1	RS0	Bank
0	0	Bank 0
0	1	Bank 1
1	0	Bank 2
1	1	Bank 3

 

2.     IE (Interrupt Enable)

Interrupt enable SFR is used to enable or disable interrupts provided that EA (enable all) bit D7 must be 1.

IE (Interrupt Enable)
D7	D6	D5	D4	D3	D2	D1	D0
EA			ES	ET1	EX1	ET0	EX0

 

Bit	Flag	Name	Function
D7	EA	Enable all	1=allow enable/disable individual interrupt
D6	-	-	Reserved for future use
D5	-	-	Reserved for future use
D4	ES	Enable serial interrupt	1=enable 0=disable
D3	ET1	Enable timer 1 interrupt	1=enable 0=disable
D2	EX1	Enable external interrupt 1	1=enable 0=disable
D1	ET0	Enable timer 0 interrupt	1=enable 0=disable
D0	Ex0	Enable external interrupt 0	1=enable 0=disable

 

3.     IP (Interrupt Priority)

Interrupt priority SFR is used to set or alter the default priority of interrupts of 8051 microcontroller.

IP (Interrupt Priority)
D7	D6	D5	D4	D3	D2	D1	D0
---	---	---	PS	PT1	PX1	PT0	PX0

 

Bit	Flag	Name	Function
D7	-	-	Reserved for future use
D6	-	-	Reserved for future use
D5	-	-	Reserved for future use
D4	PS	Serial interrupt priority	1=enable 0=disable
D3	PT1	timer 1 interrupt priority	1=enable 0=disable
D2	PX1	external interrupt 1 priority	1=enable 0=disable
D1	PT0	timer 0 interrupt priority	1=enable 0=disable
D0	Px0	external interrupt 0 priority	1=enable 0=disable

 

4.     TMOD (Timer Mode)

TMOD SFR is used to select timer and its different modes. 8051 consists of two 16-bit timers i.e. timer 0 and timer 1.

 

TMOD (Timer Mode)
D7	D6	D5	D4	D3	D2	D1	D0
GATE	C/T\	M1	M0	GATE	C/T\	M1	M0
Timer1				Timer0

 

Bit	Flag	Function
D7,D3	GATE	1=external hardware is required to ON/OFF timer
D6,D2	C/T\	1=counter 0=timer
D5,D1	M1	Timer mode selector bit
D4,D0	M0	Timer mode selector bit

 

M1	M0	Mode	Use
0	0	Mode 0	13-bit timer/counter
0	1	Mode 1	16-bit timer/counter
1	0	Mode 2	8-bit auto reload
1	1	Mode 3	Split timer mode (only for timer 0, timer 1 is passive)

 

5.     TCON (Timer Control)

TCON (Timer Control)
D7	D6	D5	D4	D3	D2	D1	D0
TF1	TR1	TF0	TR0	IE1	IT1	IE0	IT0

 

Bit	Flag	Name
D7	TF1	Timer 1 overflow flag
D6	TR1	Timer 1 run control bit
D5	TF0	Timer 0 overflow flag
D4	TR0	Timer 0 run control bit
D3	IE1	External interrupt 1 edge flag
D2	IT1	Timer 1 interrupt bit
D1	IE0	External interrupt 0 edge flag
D0	IT0	Timer 0 interrupt bit

 

6.     SCON (Serial Control)

8051 support 1 full duplex serial port. SCON SFR is used to select different modes of serial communication

SCON (Serial Control)
D7	D6	D5	D4	D3	D2	D1	D0
SM0	SM1	SM2	REN	TB8	RB8	TI	RI

 

Bit	Flag	Name
D7	SM0	Serial mode specifier
D6	SM1	Serial mode specifier
D5	SM2	Serial mode 2
D4	REN	Receive enable
D3	TB8	Transmit bit 8
D2	RB8	Receive bit 8
D1	TI	Transmit interrupt
D0	RI	Receive interrupt

 

SM0	SM1	Mode	Use	Baud rate
0	0	Mode 0	8-bit shift register	Fosc/12
0	1	Mode 1	8-bit UART	Variable
1	0	Mode 2	9-bit UART	Fosc/32 or Fosc/64
1	1	Mode 3	9-bit UART	Variable

 

7.     SBUF (Serial Buffer)

Serial buffer SFR is mainly used during transmission and reception of serial data. It is blank SFR, during transmission of serial data, it must be sent to SBUF first and then sent. During reception of data, it first goes into SBUF.

 

SBUF (Serial Buffer)
D7	D6	D5	D4	D3	D2	D1	D0

 

8.     PCON (Power Control)

8051 supports power saving modes like power down and ideal. In power down mode, onchip oscillator is in frozen state which cuts the frequency to CPU and peripherals. Where in ideal mode, CPU is in sleep mode and peripherals are active.

PCON (Power Control)
D7	D6	D5	D4	D3	D2	D1	D0
SMOD	-	-	-	GF1	GF0	PD	IDL

 

Bit	Flag	Function
D7	SMOD	1= double the baud rate
D6	-	Reserved for future use
D5	-	Reserved for future use
D4	-	Reserved for future use
D3	GF1	General purpose flag 1
D2	GF0	General purpose flag 0
D1	PD	1=Power down  mode
D0	IDL	1= Ideal mode