MicrocontrollersIII.ppt
- 1. CSE 477 Serial Communication 1 Serial Communication RS-232 (standard serial line) Point-to-point, full-duplex Synchronous or asynchronous Flow control Variable baud (bit) rates Cheap connections (low-quality and few wires)
- 2. CSE 477 Serial Communication 2 start bit 8 data bits parity bit stop bit Serial data format Variations: parity bit; 1, 1.5, or 2 stop bits
- 3. CSE 477 Serial Communication 3 all wires active low "0" = -12v, "1" = 12v special driver chips that generate ±12v from 5v RS-232 wires TxD – transmit data TxC – transmit clock RTS – request to send: Handshake CTS – clear to send : Handshake RxD – receive data RxC – receive clock DSR – data set ready: Handshake DTR – data terminal ready: Handshake Ground
- 4. CSE 477 Serial Communication 4 Transfer modes Synchronous clock signal wire is used by both receiver and sender to sample data Asynchronous no clock signal in common data must be oversampled (16x is typical) to find bit boundaries Flow control handshaking signals to control rate of transfer CLK
- 5. CSE 477 Serial Communication 5 baud rate generator terminal RxD TxD RxC TxC baud rate generator async modem RxD TxD DSR DTR CTS RTS RxC TxC phone line interface phone line sync modem RxD TxD DSR DTR CTS RTS RxC TxC phone line interface phone line Typical connections Terminal Asynchronous modem Synchronous modem
- 6. CSE 477 Serial Communication 6 8051 Serial Interface TxD: Port 3, pin 1 Transmit data shifted out RxD: Port 3, pin 0 Receive data shifted in Full duplex: both operate in parallel We will use Mode 1 only asynchronous 10 bit transfer: 1 start, 8 data, 1 stop Look at documentation for other modes Clock for serial shift provided by timer 1 i.e. programmable baud rate takes away a timer from other uses
- 7. CSE 477 Serial Communication 7 Serial Port Control Register (SCON) Configures the serial interface
- 8. CSE 477 Serial Communication 8 Baud Rate Generator Use timer 1 overflow to generate serial data clock serial clock is 16x oversampled, i.e. baud rate x16 SMOD bit (PCON register) 0: divides baud rate by 2 Typical timer 1 setup auto-reload timer reload value determines overflow clock rate Baud rate calculation Clocks between overflows = clocks Overflow frequency = Baud rate (assuming SMOD = 1) Baud rate = Max Baud rate = TH1 value for 9600 baud =
- 9. CSE 477 Serial Communication 9 8051 Serial Interface Transmitter
- 10. CSE 477 Serial Communication 10 Sending Serial Data Transmission is initiated by a write to SBUF start, data and stop bits shifted out automatically TI (transmit interrupt) set when stop bit goes indicates that interface is ready for next character TI can be polled, or used to interrupt must reset it in the software
- 11. CSE 477 Serial Communication 11 8051 Serial Receiver Interface
- 12. CSE 477 Serial Communication 12 Receiving Serial Data Reception is initiated by a 1-0 transition - a start bit data is sampled and shifted in automatically on the stop bit, the 8 data bits are loaded into SBUF same address, but different register and sending SBUF RI (receive interrupt) set when SBUF is loaded indicates a character is ready • next character can start entering before SBUF is read • must read SBUF before next character arrives RI can be polled, or used to interrupt must be reset in the software
- 13. CSE 477 Serial Communication 13 Serial Interface Interrupts RI and TI share the same interrupt Interrupt #4 Interrupt routine must look at RI and TI to see which caused the interrupt Routine must reset RI or TI before returning If both RI and TI are on, another interrupt will happen right away Which bit do you check first?
- 14. CSE 477 Serial Communication 14 Baud Rate Generator Use timer 1 overflow to generate serial data clock serial clock is 16x oversampled, i.e. baud rate x16 SMOD bit (PCON register) 0: divides baud rate by 2 Typical timer 1 setup auto-reload timer reload value determines overflow clock rate Baud rate calculation Clocks between overflows = 12 x (256-TH1) clocks Overflow frequency = Fclk/Clocks-between-overflows Baud rate (assuming SMOD = 1) 1/16 x overflow-frequency Baud rate = 24MHz / (16 x 12 x (256-TH1)) Max Baud rate = 125KHz TH1 value for 9600 baud = 13
- 15. CSE 477 Serial Communication 15 getchar() / putchar() c = getchar() returns the character in the buffer, if there is one returns NULL otherwise could check for error (character overrun) r = putchar(c) sends the character to the serial port, if it is not busy returns c for normal operation, NULL if port was busy Simple operation, no need for interrupts while ((c = getchar) == NULL) { }; while (putchar(c) == NULL) { }; Polling doesn’t allow us to do anything else If we are busy, we might miss a character
- 16. CSE 477 Serial Communication 16 getchar() / putchar() (Part 2) We’ll add a 1-character buffer for both input and output getchar() interrupt when a new character arrives if the buffer is empty, place character in buffer otherwise, set error flag (new function to check for errors) getchar() now looks at the buffer for a character otherwise the same as before putchar() interrupt when a character has been sent if the buffer has a character, send it to the serial port putchar() now puts the character into the buffer otherwise the same as before what if the buffer is empty when interrupt occurs? new character to buffer will not be sent Complication: one interrupt routine for both input and output
- 17. CSE 477 Serial Communication 17 getchar() / putchar() (Part 2) Serial Port input buffer output buffer Main program getchar() putchar() Interrupt routine Interrupt routine Input character Output character
- 18. CSE 477 Serial Communication 18 getchar() / putchar() (Part 3) The 1-character buffer gives us some time to read/write but not a lot Extend the 1-character buffers to 32 characters buffers now we can go away for a long time and not miss incoming characters we can write out lots of characters and not wait for them all to go Each buffer now becomes a queue standard circular queue 33 character vector (why 33?) head, tail pointers initialize on startup getchar() interrupt routine writes characters to buffer, getchar() reads putchar() putchar() writes characters to buffer, getchar() reads
- 19. CSE 477 Serial Communication 19 getchar() / putchar() (Part 3) Serial Port input buffer output buffer Main program getchar() putchar() Interrupt routine Interrupt routine Input character Output character
- 20. CSE 477 Serial Communication 20 +5v device 1 device 2 device n SCL SDA Inter-Integrated Circuit Bus (I2C) Modular connections on a printed circuit board Multi-point connections (needs addressing) Synchronous transfer (but adapts to slowest device) Similar to Controller Area Network (CAN) protocol used in automotive applications
- 21. CSE 477 Serial Communication 21 SDA SCL START STOP Serial data format SDA going low while SCL high signals start of data SDA going high while SCL high signals end of data SDA can change when SCL low SCL high (after start and before end) signals that a data bit can be read
- 22. CSE 477 Serial Communication 22 SDA SCL 1 3 4 5 6 7 8 ack 2 Byte transfer Byte followed by a 1 bit acknowledge from receiver Open-collector wires sender allows SDA to rise receiver pulls low to acknowledge after 8 bits Multi-byte transfers first byte contains address of receiver all devices check address to determine if following data is for them second byte usually contains address of sender
- 23. CSE 477 Serial Communication 23 clk 1 clk 2 SCL Clock synchronization Synchronous data transfer with variable speed devices go as fast as the slowest device involved in transfer Each device looks at the SCL line as an input as well as driving it if clock stays low even when being driven high then another device needs more time, so wait for it to finish before continuing rising clock edges are synchronized
- 24. CSE 477 Serial Communication 24 Arbitration Devices can start transmitting at any time wait until lines are both high for some minimum time multiple devices may start together - clocks will be synchronized All senders will think they are sending data possibly slowed down by receiver (or another sender) each sender keeps watching SDA - if ever different (driving high, but its really low) then there is another driver sender that detects difference gets off the bus and aborts message Device priority given to devices with early 0s in their address
- 25. CSE 477 Serial Communication 25 Inter-Integrated Circuit Bus (I2C) Supports data transfers from 0 to 400KHz Philips (and others) provide many devices microcontrollers with built-in interface A/D and D/A converters parallel I/O ports memory modules LCD drivers real-time clock/calendars DTMF decoders frequency synthesizers video/audio processors