CS452 - Real-Time Programming - Fall 2010

Lecture 3 - I/O Devices

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Assignment 0 Deadline
More about serial I/O: pdf

The Hardware

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Linux systems
- cross compiler: runs on 86_64, produces code for ARM
- GNU toolchain: compiler, assembler, link editor
  - You will notice that my makefile separates
    - compilation to assembly code,
    - assembling the assembly code, and
    - link editing.
- need to login explicitly to linux.student.cs
TFTP servers
- need to type IP number explicitly

TS-7200

`COM' ports

Connected to UARTs

RS-232

Only really two

Ethernet port

Reset switch

EP-9302

System on chip

ARM 920T core, implementing ARM v4T instruction set
Two co-processors
1. System controller, MMU
2. Maverick Crunch floating point unit, mainly used for signal processing
Peripherals
1. UARTs
2. Timers
3. DIO
4. A/D
5. etc.

Memory

Byte addressable, word size 32 bits

32 Mbytes of RAM, starting at 0x00000000
- lowest RAM has code to access exception handlers
- above that is RedBoot, which you do not want to overwrite
4 Mbytes of flash RAM, starting at 0x60000000
- Contains RedBoot, which is loaded into RAM at startup
Device registers above 0x80000000

Separate instruction and data caches

Not turned on for RedBoot

The Software

Compiler

GNU tool chain

when you are getting started optimizing is a bad idea
software multiplication, division, floating point from libgcc.a
Makefile
target.ld

RedBoot

Partial implementation

fconfig :: NOT
load (tftp)
examine, copy, fill memory

Returns when program terminates

Busy-wait IO

COM2 uses monitor; COM1 goes to train

initialization/configuration
output
input

For assignment 0 you need to increase the functionality.

Bug in one function

How Timers Work

Set them up

how far to count, which is the length of the counting cycle
calculate from their own clock speed and the tick rate you want

Start them counting

once started they count forever

Whenever they count through zero they trigger an interrupt.

You have interrupts turned off, but
You could enable interrupts in the timer and look for a bit set in the ICU

A better solution for this assignment

Let the timer count continuously and test for roll-over

Read the documentation carefully.

Data Communication using RS-232

Communication protocol existing since forever.

description of a wire, and the signals on it.

Why do we still use it in the age of USB, etc.

simple
inexpensive
robust
easy to implement, especially by doing busy-wait

When you get a brand new board

Using JTAG
- blink the LEDs on the board
- put in a primitive loader comunicating by serial I/O
Using the primitive loader and serial I/O
- configure a boot loader like RedBoot
- download the boot loader using serial I/O
- use the capabilities of the running boot loader to put the boot loader into Flash
Reset puts you into the boot loader
- configure the ethernet and use it for loading
- when you start up the image you loaded you are completely on your own
- prove that your code is running using serial I/O

Serial Hardware: the Universal Asynchronous Receiver Transmitter (UART)

Really nothing but two shift registers

Parallel to serial for output
Serial to parallel for input

Also must

Add start and stop bits
Provide a clock
Detect error conditions

Finally there is a bus interface

Very simple RS-232

Three wires

ground
transmit
receive

Wired like this

GND ---------- GND

XMIT --\ /---- XMIT
        X
RCV----/ \---- RCV

What could be simpler?

BUT, ...
This is actually called a null modem.
To see why we need to look at the pins on the connector.

Less Simple RS-232

Distinguish Data Terminal Equipment (DTE) from Data Circuit-terminating Equipment (DCE)

On the DE-9 connector we have for DTE

5 -- GND
3 -- XMIT
2 -- RCV

and for DCE

5 -- GND
3 -- RCV
2 -- XMIT

A very simple `straight through' cable connects these perfectly. But when we conect DTE to DTE we need the cable above, which is called a `null modem'.

Train RS-232

Three wire works fine as long as both ends of the wire are much faster than the wire itself. But the train controller is very slow. Why?

Wait for mechanical sensors.
High electrical noise

The train controller needs to be able to say STOP, I can't take any more

How is this done?

Software Flow Control

XOFF, XON

Hardware Flow Control

CTS, RTS or DTR, DSR

The train interface uses CTS/RTS

Wired like this

CTS----\ /-----CTS
        X
RTS----/ \-----RTS

You write CTS when you can't handle more input
You read RTS to find out if it is safe to send more output

What Happens if You Omit Flow Control

Bytes get dropped
Bytes get corrupted
The train controller hangs

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