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The Embedded computing platform

The Embedded computing platform. CPU bus Memory I/O devices. CPU bus. Connects CPU to: memory devices Protocol controls communication between entities. Bus protocol. Determines who gets to use the bus at any particular time Governs length, style of communication. Four-cycle handshake.

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The Embedded computing platform

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  1. The Embedded computing platform CPU bus Memory I/O devices

  2. CPU bus • Connects CPU to: • memory • devices • Protocol controls communication between entities

  3. Bus protocol • Determines who gets to use the bus at any particular time • Governs length, style of communication

  4. Four-cycle handshake • Basis of many bus protocols • Uses two wires: • enq (enquiry) • ack (acknowledgment) enq dev1 dev2 data ack

  5. 1 3 2 4 Four-cycle example enq data ack time

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

  7. one rising falling zero 10 ns stable changing Timing diagrams A B C time

  8. Typical bus timing for read • CPU: • set R/W’=1 • asserts address, address enable • Memory: • asserts data • asserts data ready’ • CPU: • deasserts address, address enable

  9. Bus read state diagram Get data Done See ack Adrs Wait

  10. Transaction types • Wait state: • state in a bus transaction to wait for acknowledgment • Disconnected transfer: • bus is freed during wait state • Burst: • multiple transfers

  11. MPC850 example • Basic bus protocol

  12. Bus signals

  13. Bus Arbitration

  14. Arbitration Phase

  15. Bus Arbitration Timing

  16. Single-beat Read flow

  17. Single-beat Read flow, cont’d

  18. Single-beat Write flow

  19. Timers and counters • Very similar: • a timer is incremented by a periodic signal • a counter is incremented by an asynchronous, occasional signal • Rollover causes interrupt

  20. Watchdog timer • Watchdog timer is periodically reset by system timer • If watchdog is not reset, it generates an interrupt to reset the host host CPU interrupt watchdog timer reset

  21. Switch debouncing • A switch must be debounced to multiple contacts caused by eliminate mechanical bouncing:

  22. Encoded keyboard • An array of switches is read by an encoder • N-key rollover remembers multiple key depressions row scan columns

  23. LED • Must use resistor to limit current:

  24. 7-segment LCD display • May use parallel or multiplexed input

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

  26. Touchscreen • Includes input and output device • Input device is a two-dimensional voltmeter

  27. voltage Touchscreen position sensing ADC Vx X position

  28. Digital-to-analog conversion • Use resistor tree: R Vout bn 2R bn-1 4R bn-2 8R bn-3

  29. Flash A/D conversion • N-bit result requires 2n comparators: encoder Vin ...

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