CS452 - Real-Time Programming - Spring 2010

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Lecture 17 - Serial I/O: Implementation

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

Transmitting

Initialization
- Enable modem status interrupt (UARTxCtrl)
When bytes arrive to transmit
- Add bytes to end of buffer
- Test modem status
- If OK to send
  - Test transmitter busy: (UARTxFlag at 0x800[bc]0018)
  - If busy
    - enable transmit interrupt (UARTxCtrl at 0x800[bc]0014)
  - else
    - write first byte to hold register
    - If more bytes to transmit
      - enable transmit interrupt (UARTxCtrl at 0x800[bc]0014)
On transmit interrupt
- Test modem status
- If OK to send
  - write first byte to hold register
  - if no more bytes to transmit
    - disable transmit interrupt
- else
  - disable transmit interrupt
On modem status interrupt
- If OK to send
  - Test transmitter busy
  - If busy
    - Enable transmit interrupt
  - else
    - if bytes to transmit
      - write first byte to hold register
      - if more bytes to transmit
        
        enable transmit interrupt.

Receiving

Getting started
- Read data in register and discard it
- Turn on receiver
- Enable Receive interrupt
Receive interrupt
- Test error register
  - If error respond to it

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 only guarantee is that the character has been added to a buffer of characters to be transmitted

How many servers and notifiers?


	one server	two servers	four servers
one notifier	likely queue congestion in server		likely queue congestion in notifer
two notifiers	one per channel? one per direction?	how should they be paired?
four notifiers	certain queue congestion in server	likely queue congestion in server	best performance, most tasks

How should we handle a terminal?

Issues

Line editing
- undefined intermediate states
- well-defined trigger
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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