Lecture 8 - Serial Port (RS232)

Lecture 8 - Serial Port (RS232)

1. RS232

   • Industry Standard, Asynchronous Serial Communications Protocol
     • No Clock Sent, Receiver must provide clock signal
       • Data rate must be known by transmitter and receiver
   • Data sent as a series of 8-bit serial words
     • LSB First, MSB Last
     • Start bit at the front and Stop bit at the back
   • Data line is high when idle and goes low to signal Start Bit
     • Stop Bit resits it to high

2. RS232 on the MBED

   • UART
     • Universal Asynchronous Receiver/Transmitter
     • One connection for transmitted data
       • TX
     • One connection for received data
       • RX
     • Baud Rate

3. RS232 Transmit Port Operation

   • A Character (Byte) is written into TXREG from Data Bus
     • Data is Transferred into TSR (Transmit Serial Reg) from TXREG
       • But not until TSR is empty
       • Data is then shifted serially out of TSR to TX Pin
         • Program should not send data to TXREG until it is read to receive it
           • This condition is signalled by TXIF Flag
           • TXIF (Transmit Interrupt Flag) is set high when TXREG is empty and cleared low when new data is loaded into it
             • Program should wait for TXIF to go high before sending a character to TXREG
               • Program can poll TXIF in same way as 'car door' program polled input pin, looking to see if the car door was open

4. RS232 Receive Section

   • Data is serially loaded into Receive Shift Register (RSR)
     • When Start Bit, Data Bits and Stop Bit Received Data drops into RX Reg
       • RXIF is set when RX Reg Contains Data

5. MBED Serial Link to PC

   • mbed has three UARTs connecting to external pins
     • LPC1768 has a fourth
       • Reserved for communication back to the USB link