CS452 - Real-Time Programming - Spring 2008

Lecture 4 - Devices

Questions & Comments

1. Hand in assignment 1
2. Lab should be ready Friday

ISRs

Like function calls without arguments or return values.

1. Save state of interrupted task
2. Turn off source of interrupt
3. Do the work
4. Restore state of interrupted task
5. Return to next instruction of interrupted task

Contrast RT to non-RT

• RT: may need to do a lot of work as quickly as possible
• non-RT: example is a real-time clock for the date command

Devices

Real-time clock

Usually a count-down timer

Programming

• mode: interrupt, continuous
• clock divisor
• cycle length
• start

Reading

• ready bit means timer reached zero
• latch and read gets where we currently are
  • Why do we need to latch?

USART

Universal Synchronous/Asynchronous Receiver/Transmitter

We load a piece of data, usually a byte. It shifts out the bits on a time-multiplexed signal

Programming

• Bit-rate
• Synchronous/asynchronous
• Flow control
• 7 or 8 bit ACSII
• Start and stop bits
• Interrupt
• DMA

Using

• Ready bit means
  • valid data in input buffer on receive
  • output buffer ready on transmit
• Data register to read or write.

  Reading or writing turns off ready bit

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