KTU_8255_Programmable Peripheral Interface
- 2. • It is an I/O port chip used for interfacing It is an I/O port chip used for interfacing I/O devices with microprocessor I/O devices with microprocessor system. system. • It is device used to implement parallel It is device used to implement parallel data transfer between processor and data transfer between processor and slow peripheral devices like ADC, DAC, slow peripheral devices like ADC, DAC, keyboard, 7-segment display, lCD etc. keyboard, 7-segment display, lCD etc.
- 3. Features: Features: • It is a programmable device. It is a programmable device. • It has 24 I/O programmable pins like It has 24 I/O programmable pins like PA,PB,PC (3-8 pins). PA,PB,PC (3-8 pins). T T L compatible. T T L compatible.
- 4. • 8255A has three ports • PORT A • PORT B • PORT C • Port A and Port B are 8 bit parallel ports. • Port C can be split into two parts, i.e. PORT C lower (PC0-PC3) and PORT C upper (PC7-PC4) by the control word
- 5. • These three ports are further divided into two groups, – i.e. Group A includes PORT A and upper PORT C. – Group B includes PORT B and lower PORT C • These two groups can be programmed in three different modes.
- 6. • Three operating modes : Three operating modes : – Mode-0(simple I/O port) Mode-0(simple I/O port) – Mode-1(Handshake I/O port) Mode-1(Handshake I/O port) – Mode-2(Bidirectional I/O port) Mode-2(Bidirectional I/O port)
- 7. Mode 0 Mode 0 • In this mode, Port A and B is used as two 8-bit ports and Port C as two 4-bit ports. • Each port can be programmed in either input mode or output mode where outputs are latched and inputs are not latched. • Ports do not have interrupt capability. • Ports in mode 0 is used to interfaces LEDs, Hexa keypad and 7 segment LEDS to the processor.
- 8. Mode 1 Mode 1 • In this mode, Port A and B is used as 8- bit I/O ports. • They can be configured as either input or output ports. • Each port uses three lines from port C as handshake signals. • Inputs and outputs are latched
- 9. • MODE 1 :(Input/output with Hand shake) MODE 1 :(Input/output with Hand shake) • In this mode, input or output is transferred In this mode, input or output is transferred by hand shaking Signals. by hand shaking Signals. • Handshaking signals is used to transfer Handshaking signals is used to transfer data between whose data transfer is not data between whose data transfer is not same. same. Computer Printer DATA BUS STB ACK Busy
- 10. • Example: Example: • The computer send the data to the printer The computer send the data to the printer large speed compared to the printer. large speed compared to the printer. • When computer send the data according to When computer send the data according to the printer speed at the time only, printer the printer speed at the time only, printer can accept. can accept. • If printer is not ready to accept the data then If printer is not ready to accept the data then after sending the data bus , computer uses after sending the data bus , computer uses another handshaking signal to tell printer another handshaking signal to tell printer that valid data is available on the data bus. that valid data is available on the data bus. • Each port uses three lines from port C as Each port uses three lines from port C as handshake signals handshake signals
- 11. Mode 2 Mode 2 • In this mode, Port A can be configured as the bidirectional port and Port B either in Mode 0 or Mode 1. • Port A uses five signals from Port C as handshake signals for data transfer. • The remaining three signals from Port C can be used either as simple I/O or as handshake for port B.
- 12. MODE 2:bi-directional I/O data transfer: MODE 2:bi-directional I/O data transfer: • This mode allows bidirectional data transfer This mode allows bidirectional data transfer over a single 8-bit data bus using over a single 8-bit data bus using handshake signals. handshake signals. • This feature is possible only Group A This feature is possible only Group A • Port A is working as 8-bit bidirectional. Port A is working as 8-bit bidirectional. • PC PC3 3-PC -PC7 7 is used for handshaking purpose. is used for handshaking purpose. • The data is sent by CPU through this port , The data is sent by CPU through this port , when the peripheral request it. when the peripheral request it. • CONTROL WORD FORMATS: CONTROL WORD FORMATS: • In the INPUT mode , When RESET is High all In the INPUT mode , When RESET is High all 24 pins (3-ports) be a input mode. 24 pins (3-ports) be a input mode.
- 13. • i.e all flip flops are cleared and the i.e all flip flops are cleared and the interrupts are rest. interrupts are rest. • This condition is maintained even after This condition is maintained even after RESET goes low. RESET goes low. • This can be avoid by writing single control This can be avoid by writing single control word to the control registers , when word to the control registers , when required. required.
- 15. Function of pins: Function of pins: • Data bus(D Data bus(D0 0-D -D7 7) ):These are 8-bit bi-directional buses, :These are 8-bit bi-directional buses, connected to 8086 data bus for transferring data. connected to 8086 data bus for transferring data. • CS: This is Active Low signal. CS: This is Active Low signal. It stands for Chip Select. A LOW on this input selects the chip and enables the communication between the 8255 and the CPU. • Read: This is Active Low signal, when it is Low the Read: This is Active Low signal, when it is Low the microprocessor reads data from a selected I/O port microprocessor reads data from a selected I/O port of 8255A. of 8255A. • Write: This is Active Low signal, when it is Low the Write: This is Active Low signal, when it is Low the microprocessor writes data into a selected I/O port . microprocessor writes data into a selected I/O port .
- 16. • Address (A Address (A0 0-A -A1 1):This is used to select the ):This is used to select the ports. ports. A1 A1 A0 A0 Select Select 0 0 0 0 PA PA 0 0 1 1 PB PB 1 1 0 0 PC PC 1 1 1 1 Control Control reg. reg.
- 17. • RESET: This is used to reset the device. That RESET: This is used to reset the device. That means clear control registers. means clear control registers. • PA PA0 0-PA -PA7 7:It is the 8-bit bi-directional I/O pins :It is the 8-bit bi-directional I/O pins used to send the data to peripheral or to used to send the data to peripheral or to receive the data from peripheral. receive the data from peripheral. • PB PB0 0-PB -PB7 7:Similar to PA :Similar to PA • PC PC0 0-PC -PC7 7:This is also 8-bit bidirectional I/O pins. :This is also 8-bit bidirectional I/O pins. These lines are divided into two groups. These lines are divided into two groups. 1. 1. PC PC0 0 to PC to PC3 3(Lower Groups) (Lower Groups) 2. 2. PC PC4 4 to PC to PC7 7 (Higher groups) (Higher groups) These two groups working in separately using 4 These two groups working in separately using 4 data’s. data’s.
- 19. Data Bus buffer Data Bus buffer: : • It is a 8-bit bidirectional Data bus. It is a 8-bit bidirectional Data bus. • Used to interface between 8255 data bus Used to interface between 8255 data bus with system bus. with system bus. • The internal data bus and Outer pins D The internal data bus and Outer pins D0 0-D -D7 7 pins are connected in internally. pins are connected in internally. • The direction of data buffer is decided by The direction of data buffer is decided by Read/Control Logic. Read/Control Logic.
- 20. Read/Write Control Logic: Read/Write Control Logic: • This is getting the input signals from This is getting the input signals from control bus and Address bus control bus and Address bus • Control signal are RD and WR. Control signal are RD and WR. • Address signals are A0,A1,and CS. Address signals are A0,A1,and CS. • 8255 operation is enabled or disabled 8255 operation is enabled or disabled by CS. by CS.
- 21. Group A and Group B control: Group A and Group B control: • Group A and B get the Control Signal from CPU and Group A and B get the Control Signal from CPU and send the command to the individual control blocks. send the command to the individual control blocks. • Group A send the control signal to port A and Port C Group A send the control signal to port A and Port C (Upper) PC (Upper) PC7 7-PC -PC4 4. . • Group B send the control signal to port B and Port C Group B send the control signal to port B and Port C (Lower) PC (Lower) PC3 3-PC -PC0 0. . • PORT A PORT A: : • This is a 8-bit buffered I/O latch. This is a 8-bit buffered I/O latch. • It can be programmed by mode 0 , mode 1, mode It can be programmed by mode 0 , mode 1, mode 2 . 2 .
- 22. PORT B: PORT B: • This is a 8-bit buffer I/O latch. This is a 8-bit buffer I/O latch. • It can be programmed by mode 0 and It can be programmed by mode 0 and mode 1. mode 1. • PORT C PORT C: : • This is a 8-bit Unlatched buffer Input and This is a 8-bit Unlatched buffer Input and an Output latch. an Output latch. • It is splitted into two parts. It is splitted into two parts. • It can be programmed by bit set/reset It can be programmed by bit set/reset operation. operation.
- 23. Operation modes in 8255 Operation modes in 8255 • Two operating modes: Two operating modes: – I/O mode( mode 0, mode 1, mode2) I/O mode( mode 0, mode 1, mode2) – Bit set/Reset mode Bit set/Reset mode
- 24. Operation modes: Operation modes: BIT SET/RESET MODE: BIT SET/RESET MODE: • The PORT C can be Set or Reset by sending OUT The PORT C can be Set or Reset by sending OUT instruction to the CONTROL registers. instruction to the CONTROL registers. I/O MODES: I/O MODES: • MODE 0(Simple input / Output): MODE 0(Simple input / Output): • In this mode , port A, port B and port C is used as In this mode , port A, port B and port C is used as individually (Simply). individually (Simply). • Features: Features: • Outputs are latched , Inputs are buffered not Outputs are latched , Inputs are buffered not latched. latched. • Ports do not have Handshake or interrupt capability. Ports do not have Handshake or interrupt capability.
- 25. Control words Control words • Two control words: Two control words: – I/O mode set control word(MSW) I/O mode set control word(MSW) – Bit set/reset control word(BSR) Bit set/reset control word(BSR) • MSW is used to specify I/O functions. MSW is used to specify I/O functions. • BSR is used to set/reset individual pins of BSR is used to set/reset individual pins of Port C. Port C. • Both the control words are written in the Both the control words are written in the same control register. same control register.
- 26. • 8255 ports are programmed by writing control word 8255 ports are programmed by writing control word in the control word in the control register. in the control word in the control register. • For setting I/O functions and mode of operation the For setting I/O functions and mode of operation the I/O mode set control word is send to control register. I/O mode set control word is send to control register. • For setting/ resetting pins of port C, the bit set/reset For setting/ resetting pins of port C, the bit set/reset control word is send to control register. control word is send to control register.
- 27. FOR BIT SET/RESET MODE: FOR BIT SET/RESET MODE: • This is bit set/reset control word format. This is bit set/reset control word format. X X X X X X Don’t care Don’t care Bit select Bit select B B0 0 B B1 1 B B2 2 D7 D6 D5 D4 D3 D2 D1 D0 0 1 2 3 4 5 6 7 0 1 0 1 0 1 0 1 0 0 1 1 0 0 1 1 0 0 0 0 1 1 1 1 BIT SET/RESET 1=SET 0=RESET BIT SET/RESET FLAG =0 Active
- 28. • PC PC0 0-PC -PC7 7 is set or reset as per the status of is set or reset as per the status of D D0. 0. • A BSR word is written for each bit A BSR word is written for each bit • Example: Example: – PC PC3 3 is Set then control register will be is Set then control register will be 0XXX0111. 0XXX0111. – PC PC4 4 is Reset then control register will be is Reset then control register will be 0XXX01000. 0XXX01000. • X is a don’t care. X is a don’t care.
- 29. FORMAT OF I/O MODE: FORMAT OF I/O MODE: The mode format for I/O as shown in figure The mode format for I/O as shown in figure D7 D6 D5 D4 D3 D2 D1 D0 Group A Port C Upper 1=Input 0=Output Port B 1=Input 0=Output Mode selection 00=mode 0 01=mode 1 1x=mode 2 Group B Port C Lower 1=Input 0=Output Port B 1=Input 0=Output Mode selection 0=mode 0 1=mode 1 Mode set 1-I/O mode 0-BSR mode
- 30. • The control word for both mode is same. • Bit D7 is used for specifying whether word loaded in to Bit set/reset mode or Mode definition word. • D7=1=Mode definition mode. • D7=0=Bit set/Reset mode.