CS452 - Real-Time Programming - Fall 2010

Lecture 17 - Serial I/O: Implementation

Public Service Announcements

1. Final exam: 9.30, December 16, 2010

Easy way to use interrupts

Enable only combined; read UART registers to decide what to do.

Think of the receive and transmit parts of the UART as separate state machines

Handy Features

• Loopback state
• Loopback connector
• Terminal as line analyser

Implementation

The simplest way to handle the interrupts is to turn on only the combined interrupt and then look at the registers of the device.

To identify the current interrupt

Read UARTxIntIDIntClr at 0x800[bc]001c

• Four bits, one for each interrupt
• (Write anything to this register to clear the modem status interrupt)

When service is required, without FIFO.

Transmitting

1. Initialization
   • Set up the line control registers: high byte must be written last
     • low: 0x808[cd]0010:[0-7] LSbyte of baud rate divisor
     • middle: 0x808[cd]000c:[0-7] MSbyte of baud rate divisor
     • high: 0x808[cd]0008:[0,1,2,3,4,56]
       • 0: send a break
       • 1: enable/disable parity
       • 2: even/odd parity
       • 3: one/two stop bits
       • 4: enable/disable FIFO
       • 56: number of bits per frame
   • Set up the control register: 0x808[cd]0014:[0, 7]
     • 0: enable/disable UART
     • 7: loopback
   • Read data and discard
   • Enable receive interrupt
   • Enable modem status interrupt: 0x808c0014:[3].
2. When bytes arrive to transmit
   • Add bytes to end of buffer
   • Test CTS: 0x808c0018:[0]
   • If asserted
     • Test transmitter busy: (UARTxFlag at 0x808[cd]0018:[3])
     • If busy
       • enable transmit interrupt (UARTxCtrl at 0x808[cd]0014:[5])
     • else
       • write first byte to hold register (clears interrupt)
       • If more bytes to transmit
         • enable transmit interrupt (UARTxCtrl at 0x800[bc]0014)
3. On transmit interrupt
   • Test CTS
   • If asserted
     • write first byte to hold register (clears interrupt)
     • if no more bytes to transmit
       • disable transmit interrupt
4. On modem status interrupt
   • If OK to send
     • Test transmitter busy
     • If busy
       • Enable transmit interrupt
     • else
       • if bytes to transmit (clears interrupt)
         • write first byte to hold register
         • if more bytes to transmit
           • enable transmit interrupt.

Receiving

1. Receive interrupt
   • Read data (clears interrupt)

Using the FIFO

Left as an exercise.

Task Structure

We are supposed to support

int Get( int port )

and

int Put( int port, char c )

These are wrappers for sends to one or more serial servers.

• On Get the server blocks the client until data is available
  • Does a non-blocking TryGet give you anything you cannot do with task structure?
• Put is non-blocking. The guarantee is that the character has been added to a buffer of characters to be transmitted, not that it has actually been transmitted.

How many servers and notifiers?

How should we handle a terminal?

Issues

1. Line editing
   • undefined intermediate states
   • well-defined trigger
2. Echo

   Either

   • one server per channel,
   • passing characters from one notifier to the other

   Or

   • courier connecting receive server to transmit server
   • What characteristics does the terminal have? termcap follies

Many other issues come up below as we consider possible task structures.

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