Serial Communication
- 1. 8051 SERIAL PORT PROGRAMMING Processor/C ontroller RS 232 Transreceiver MAX 232 PC By A.Usha Rani,Lecturer, Department of Physics and Electronics St.Ann’s College for Women
- 2. Computers transfer data in two ways: Parallel 8 or more lines (wire conductors) are used to transfer data. Serial The data is sent one bit at a time Sender Receiver Sender Receiver Serial Communication Parallel Communication
- 3. Parallel communication All bits of data are sent at a time. Data transfer speed is high. It is not preferred for long distance communication as it requires more number of wires and becomes expensive. Used for short distance communication Serial communication Only one bit is sent at a time. Data transfer speed is low. Used for long distance communication.
- 4. In serial communication, At the transmitting end, the byte of data is converted into serial bits(using a parallel-in- serial-out shift register) At the receiving end, the serial data is received and packed into byte(using a serial-in-parallel- out shift register)
- 5. There are three types of communication systems Simplex – Data can be transmitted only in one direction at a time. Half Duplex – Data can be transmitted only in one direction at a time. Full duplex – Data can be transmitted in both directions at a time.
- 7. In synchronous communication, common clock is used for transmitter and receiver. A separate link is used to transmit the clock signal. In Asynchronous communication, the synchronization is achieved only at the beginning of the transmission. To transfer data in serial communication, the receiver and sender agree on set of rules called protocol. Synchronous and Asynchronous serial communication
- 8. Serial data communication uses two methods: Synchronous method - Block of data is transferred at a time. – Block-oriented data transfers use the synchronous method Asynchronous method – – A single byte of data is transferred at a time. – Is widely used for character-oriented transmissions Instructions can be written to use either of these methods.
- 9. Format of Asynchronous serial data frame A non-divisible group of bits which can be transmitted at a time is called frame. A frame consists of the actual data and extra bits(called overhead bits) to maintain synchronization of the data. A frame has one start bit, data bits, parity bits and stop bits. Each character is placed in between start and stop bits, this is called framing The start bit is always one bit The stop bit can be one or two bits The start bit is always a 0 (low)
- 11. Data Transfer Rate In serial data communication data transfer rate is defined in bps (bits per second) Another widely used terminology is baud rate – the number of signal changes per second Baud rate and number of signals per second should not be considered to be same. Example: Let two signals be encoded in single transition and consider that the signal is changing after every 5ms. Baud rate = (1/5ms) = 500 Number of bits per second = 2x200=400bits /s
- 12. RS 232 Standard In serial communication for long distances, special devices were developed to use telephone lines for communication between two computers which are at a long distance. These devices are called Data Communication Equipment(DCE). Ex- Modem The devices that send or receive data are called Data Transmission Equipment(DTE). Ex –Computers For the maintenance of compatibility between DCE’s manufactured by different manufacturers and to establish communication signals between DTE and DCE an interfacing standard RS232 wad developed. RS stands for Recommended standard.
- 14. Since not all pins are used in PC cables, IBM introduced the DB-9 version of the serial I/O standard Image source: Google
- 15. Data Communication equipment Classification Data communication equipment is classified as: DTE (Data Terminal Equipment)-Terminal and computers that send and receive data DCE (Data Communication Equipment) - Communication equipment, such as modems The simplest connection between a PC and microcontroller at least requires - three pins, TxD, RxD, and ground Image source: Google
- 16. RS232 Pins DTR (data terminal ready) When terminal is turned on, it sends out signal DTR to indicate that it is ready for communication DSR (data set ready) When DCE is turned on and has gone through the self-test, it asserts DSR to indicate that it is ready to communicate RTS (request to send) When the DTE device has byte to transmit, it asserts RTS to signal the modem that it has a byte of data to transmit
- 17. CTS (clear to send) When the modem has room for storing the data it is to receive, it sends out signal CTS to DTE to indicate that it can receive the data now DCD (data carrier detect) The modem asserts signal DCD to inform the DTE that a valid carrier has been detected and that contact between it and the other modem is established RI (ring indicator) An output from the modem and an input to a PC indicates that the telephone is ringing It goes on and off in synchronous with the ringing sound
- 18. Handshaking (Co-ordination) process between DTE and DCE When the power is turned ON, DTE sends Data Terminal Ready(DTR) signal to inform DEC that is ready . When DCE is ready, it sends Data Set Ready(DSR) signal to DTE. After receiving DSR signal, DTE sends request to use data channel by sending RTS signal to start transmission.
- 19. DCE at other end is dialed. The DCE at receiving end replies by sending signal. After receiving the signal, DCE sends Data Carrier Detect(DCD) to DTE. Then DCE sends Clear to Send(CTS) signal. Now DTE on transmitter side starts sending signal.
- 20. RS232 Standards An interfacing standard set by the Electronics Industries Association (EIA) in 1960. Since it was set long before the advent of the TTL logic family, its input and output voltage levels are not TTL compatible In RS 232 a logic high is voltage between -3V to -25 V and logic low is voltage between +3V to +25 V. In TTL a logic high is +5V and logic low is 0V Hence to connect TTL compatible microcontroller to RS 232 system, a voltage converter is needed. MAX232 and MAX 233 are most widely used voltage converters.
- 21. MAX 232
- 22. MAX 232 has two set of line drivers for transferring and receiving data. Line drivers used for TxD( transmission)are called T1 and T2. Line drivers used for RxD(receiving) are called R1 and R2. T1 line driver has designation T1in and T1out. T1in pin is the TTL side and is connected to TxD of microcontroller while T1out is RS232 side that is connected to RxD pin of RS 232 DB connector. R1 line driver has designation R1in and R1 out. The R1in pin is RS232 side connected to TxD pin of RS232 DB and R1 out is TTL side connected to RxD pin of
- 23. MAX233 To save board space, some designers use MAX233 chip from Maxim MAX233 performs the same job as MAX232 But eliminates the need for capacitors MAX233 and MAX232 are not pin compatible
- 24. Serial Data Buffer(SBUF) Register SBUF is an 8-bit register used for serial communication. For a byte data to be transferred using serial mode, it must be placed in the SBUF register. When a byte is written into SBUF, it is framed with the start and stop bits and transferred serially via the TxD line SBUF also holds the byte of data when it is received by 8051 RxD line. When the bits are received serially via RxD, the 8051 deframes it by eliminating the stop and start bits, and then placing the byte in SBUF
- 25. Serial Control(SCON) Register This register is used to configure baud rate, synchronization clock source. It contains status bits which indicate whether the data is transmitted completely and any new data is received.
- 26. SCON Register SM0, SM1-They determine the framing of data by specifying the number of bits per character, and the start and stop bits SM2 - This enables the multiprocessing capability of the 8051 SM0 SM1 Mode 0 0 Serial mode 0 0 1 Serial mode 1 1 0 Serial mode 2 1 1 Serial mode 3
- 27. Serial Port Modes SM0 SM1 Mode Description Baud Rate 0 0 Serial mode 0 8-bit shift register 1/12 of the clock frequency 0 1 Serial mode 1 8-bit UART Determined by timer 1 1 0 Serial mode 2 9-bit UART 1/12 of clock frequency 1 1 Serial mode 3 9- bit UART Determined by timer 1
- 28. SCON Register REN (receive enable) When it is high, it allows 8051 to receive data on RxD pin When it is low, the receiver is disabled. TI (transmit interrupt) When 8051 finishes the transfer of 8-bit character it raises TI flag to indicate that it is ready to transfer another byte TI bit is raised at the beginning of the stop bit
- 29. RI (receive interrupt) When 8051 receives data serially via RxD, it gets rid of the start and stop bits and places the byte in SBUF register It raises the RI flag bit to indicate that a byte has been received and should be picked up before it is lost RI is raised halfway through the stop bit TI and RI are set by hardware and must be cleared by the software.
- 30. Programming Serial Data Transmitting In programming the 8051 to transfer character bytes serially TMOD register is loaded with the value 20H(indicating the use of timer 1 in mode 2 (8-bit auto-reload) to set baud rate) TH1 is loaded with one of the values to set baud rate for serial data transfer SCON register is loaded with the value 50H(indicating serial mode 1, where an 8-bit data is framed with start and stop bits)
- 31. TR1 is set to 1 to start timer 1 TI is cleared by CLR TI instruction The character byte to be transferred serially is written into SBUF register The TI flag bit is monitored with the use of instruction JNB TI, xx to see if the character has been transferred completely.
- 32. Importance of TI Flag The transmission of a character through TxD, involves following steps The byte character to be transmitted is written into the SBUF register The start bit is transferred The 8-bit character is transferred on bit at a time The stop bit is transferred During the transfer of the stop bit that 8051 raises the TI flag indicating that the last character was transmitted
- 33. By monitoring the TI flag, we make sure that SBUF is not overloaded. If another byte is loaded into the SBUF before TI is raised, the untransmitted portion of the previous byte will be lost. By checking the TI flag bit, we know whether or not the 8051 is ready to transfer another byte
- 34. It must be noted that TI flag bit is raised by 8051 itself when it finishes data transfer It must be cleared by the programmer with instruction CLR TI If we write a byte into SBUF before the TI flag bit is raised, we risk the loss of a portion of the byte being transferred.
- 35. Programming Serial Data Receiving RI is cleared by CLR RI instruction The RI flag bit is monitored with the use of instruction JNB RI,xx to see if an entire character has been received yet When RI is raised, SBUF has the byte, and its contents are moved into a safe place
- 36. Importance of RI Flag In receiving the data through RxD pin the following steps are involved At first start bit is received indicating that the next bit is the first bit of the character byte it is about to receive The 8-bit character is received one bit at a time The stop bit is received When receiving the stop bit 8051 makes RI = 1,indicating that an entire character byte has been received and must be picked up before it gets overwritten by an incoming character
- 37. By checking the RI flag bit when it is raised we can know that a character has been received completely and is in SBUF register. The SBUF contents are shifted to a safe place in some other register or memory before it is lost After the SBUF contents are copied, the RI flag bit must be forced to 0 by CLR RI in order to allow the next received character byte to be placed in SBUF Failure to do this causes loss of the received character
- 38. Doubling Baud Rate There are two ways to increase the baud rate of data transfer To use a higher crystal frequency. To change a bit in the PCON register PCON register is an 8-bit register It is not a bit-addressable register When 8051 is powered up, SMOD is zero We can set it to high by software and thereby double the baud rate
- 39. Thank you