CS452 - Real-Time Programming - Spring 2009

Lecture 17 - Serial IO: Programmer's Model

Practical Details

Split serial I/O channels

• One BW, the other interrupts

Serial Hardware

DMA

Direct Memory Access

HDLC

High-level Data Link Control

• Derived from IBM SLDC
• Synchronous, with synchronized clocks at each end of a connection
• Many stages, many modes
• Supports ABM (Asynchronous Balanced Mode)

Serial Interrupts

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)

What service is required

Transmitting

1. Getting started
   • Enable modem status interrupt
2. Bytes arrive to transmit

   If transmitter is busy (UARTxFlag at 0x800[bc]0018)

   • enable transmit interrupt in UARTxCtrl at 0x800[bc]0014

   If transmitter is not busy & fewer than 16 bytes to transmit

   • write bytes to FIFO

   If transmitter is not busy & more than 16 bytes to transmit

   • write 16 bytes to FIFO
   • enable transmit interrupt
3. Modem status interrupt
   • Check modem status

   EITHER

   • Clear interrupt
   • Disable transmitter
   • Enter stalled state: accumulate bytes in buffer

   OR

   • Clear interrupt
   • Enable transmitter
   • Enter transmit state
4. Transmit interrupt

   If there are more than 8 bytes to transmit

   • Write 8 bytes to FIFO
   • Leave interrupt enabled

   If there are fewer than 8 bytes to transmit

   • Write remaining bytes to FIFO
   • Disable transmit interrupt in UARTxCtrl at 0x800[bc]0014
   • Start again when more bytes to transmit

Receiving

1. Getting started
   • Clear any junk from FIFO
   • Turn on receiver
   • Enable Receive and Receive Time Out interrupts
2. Receive interrupt
   • Read 8 bytes from buffer
3. Receive Time Out interrupt
   • Read from FIFO until empty

Typical Task Structure

Most simple

1. Separate server/notifier pair for Receive and for Transmit
2. Reparate Receive/Transmit quartet for each channel

Receive

1. Server provides
   • Getc for clients

     If character in buffer

     • Reply next byte in buffer

     If no characters in buffer

     • Block client until buffer is available
   • Fill buffer for notifier
     • Fill buffer must not be non-blocking. Why?
2. Notifier provides
   • Initialization of device
   • Sending characters to server in blocks as they arrive
   • Managing a buffer that creates blocks for the server
3. Questions.
   • What if server buffer gets full?
   • What if notifier buffer gets full?

Transmitter

1. Server provides
   • Putc for clients,
     • Putc is non-blocking, server replies immediately
   • Fill buffer from Notifier
     • Fill buffer must not be non-blocking. Why?
2. Notifier provides
   • Management of stalled/transmit state
   • Managing an output buffer to hold characters when transmission is stalled
   • Emptying buffer into UART when transmission is not stalled
3. Questions
   • What if server buffer gets full?
   • What if notifier buffer gets full?

Issues

1. Should it be possible to allocate a server?
2. What should transmit/receive granularity be?
3. Should servers be specialized to match devices on the end of their channel?
4. How should echo be handled?

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• Bill Cowan's lecture notes for CS452 in Spring 2009
• Bill Cowan's Spring 2009 CS452 page
• Bill Cowan's CS452 page
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