the embedded computing platform

© 2000 Morgan Kaufman

Overheads for Computers as Components

The Embedded computing platformCPU bus.Memory.I/O devices.



CPU busConnects CPU to:

memory; devices.

Protocol controls communication between entities.



Bus protocolDetermines who gets to use the bus

at any particular time.Governs length, style of

communication.



Four-cycle handshakeBasis of many bus protocols.Uses two wires:

enq (enquiry); ack (acknowledgment).

dev1 dev2

enq

ack

data



Four-cycle example

time

enq

ack

data

1

2

3

4



Typical bus signalsClock.R/W’: true when bus is reading.Address: a-bit bundle.Data: n-bit bundle.Data ready’.



Timing diagrams

time

A

B

C

zero

one

rising falling

stable

changing

10 ns



Typical bus timing for readCPU:

set R/W’=1; asserts address, address enable.

Memory: asserts data; asserts data ready’.

CPU: deasserts address, address enable.



Bus read state diagram

Getdata Done

Adrs

Wait

See ack



Transaction typesWait state:

state in a bus transaction to wait for acknowledgment.

Disconnected transfer: bus is freed during wait state.

Burst: multiple transfers.



Timers and countersVery similar:

a timer is incremented by a periodic signal;

a counter is incremented by an asynchronous, occasional signal.

Rollover causes interrupt.



Watchdog timerWatchdog timer is periodically reset

by system timer.If watchdog is not reset, it generates

an interrupt to reset the host.

host CPU watchdogtimer

interrupt

reset



Switch debouncingA switch must be debounced to

multiple contacts caused by eliminate mechanical bouncing:



Encoded keyboardAn array of switches is read by an

encoder.N-key rollover remembers multiple

key depressions.row



LEDMust use resistor to limit current:



7-segment LCD displayMay use parallel or multiplexed

input.



Types of high-resolution displayCathode ray tube (CRT)Liquid crystal display (LCD)Plasma, etc.



TouchscreenIncludes input and output device.Input device is a two-dimensional

voltmeter:



Touchscreen position sensing

ADC

voltage



Digital-to-analog conversionUse resistor tree:

R

2R

4R

8R

bn

bn-1

bn-2

bn-3

Vout



Flash A/D conversionN-bit result requires 2n comparators:

encoder

Vin

...



Dual-slope conversionUse counter to time required to

charge/discharge capacitor.Charging, then discharging

eliminates non-linearities.

Vintimer



Sample-and-holdRequired in any A/D:

converterVin

the embedded computing platform

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