The Embedded computing platform
description
Transcript of The Embedded computing platform
© 2000 Morgan Kaufman
Overheads for Computers as Components
The Embedded computing platformCPU bus.Memory.I/O devices.
© 2000 Morgan Kaufman
Overheads for Computers as Components
CPU busConnects CPU to:
memory; devices.
Protocol controls communication between entities.
© 2000 Morgan Kaufman
Overheads for Computers as Components
Bus protocolDetermines who gets to use the bus
at any particular time.Governs length, style of
communication.
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Overheads for Computers as Components
Four-cycle handshakeBasis of many bus protocols.Uses two wires:
enq (enquiry); ack (acknowledgment).
dev1 dev2
enq
ack
data
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Four-cycle example
time
enq
ack
data
1
2
3
4
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Typical bus signalsClock.R/W’: true when bus is reading.Address: a-bit bundle.Data: n-bit bundle.Data ready’.
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Timing diagrams
time
A
B
C
zero
one
rising falling
stable
changing
10 ns
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Typical bus timing for readCPU:
set R/W’=1; asserts address, address enable.
Memory: asserts data; asserts data ready’.
CPU: deasserts address, address enable.
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Bus read state diagram
Getdata Done
Adrs
Wait
See ack
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Transaction typesWait state:
state in a bus transaction to wait for acknowledgment.
Disconnected transfer: bus is freed during wait state.
Burst: multiple transfers.
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Timers and countersVery similar:
a timer is incremented by a periodic signal;
a counter is incremented by an asynchronous, occasional signal.
Rollover causes interrupt.
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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
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Switch debouncingA switch must be debounced to
multiple contacts caused by eliminate mechanical bouncing:
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Encoded keyboardAn array of switches is read by an
encoder.N-key rollover remembers multiple
key depressions.row
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LEDMust use resistor to limit current:
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7-segment LCD displayMay use parallel or multiplexed
input.
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Types of high-resolution displayCathode ray tube (CRT)Liquid crystal display (LCD)Plasma, etc.
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TouchscreenIncludes input and output device.Input device is a two-dimensional
voltmeter:
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Touchscreen position sensing
ADC
voltage
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Digital-to-analog conversionUse resistor tree:
R
2R
4R
8R
bn
bn-1
bn-2
bn-3
Vout
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Flash A/D conversionN-bit result requires 2n comparators:
encoder
Vin
...
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Dual-slope conversionUse counter to time required to
charge/discharge capacitor.Charging, then discharging
eliminates non-linearities.
Vintimer
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Sample-and-holdRequired in any A/D:
converterVin