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IO in Embedded Systems

IO in Embedded Systems. Martin Schöberl. Overview. Input/Output Digital, Analog Translation Heater control example. Input. Usually simple No or minimal UI Examples Buttons Sensors (e.g. temperature) Camera. Output. Again simple (UI) Output signal needs amplification

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IO in Embedded Systems

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  1. IO in Embedded Systems Martin Schöberl

  2. Overview • Input/Output • Digital, Analog • Translation • Heater control example Embedded IO

  3. Input • Usually simple • No or minimal UI • Examples • Buttons • Sensors (e.g. temperature) • Camera Embedded IO

  4. Output • Again simple (UI) • Output signal needs amplification • (Solid-state) relays (On/Off) • Pulse-Width Modulation (PWM) • Failure on output? • Broken wire • Read back with an input Embedded IO

  5. Digital IO • 1/0 – On/Off • Represented as electrical value • E.g. 0=0V, 1=5V • Translation to the real world • Contact switches • Relay • Several IO bits/pins in one register Embedded IO

  6. Analog IO • Value range • E.g. -20°C … 100°C • Representation as electrical signal • Voltage e.g. 0-20V • Resistance issue • Current 0-20mA • Industry standard • 4-20mA Value, <4mA broken wire Embedded IO

  7. Analog/Digital Conversion • Electrical signal to digital information • Input: Analog/Digital Converter (ADC) • Output: Digital/Analog Converter (DAC) • Resolution in bits • E.g. 8 bits => 0…255 Embedded IO

  8. Translation Example • -20°C … 100°C => 4mA … 20mA • 0mA … 20mA => 0 … 255 • What value is read at 27°C? • Is the temperature sensor linear? Embedded IO

  9. Control • Read input • Calculate output • Write output • Continue this loop forever Embedded IO

  10. Example: Temperature control For (;;) { int temp = readTemp(); if (temp < 27) { heaterOn = true; } else { heaterOn = false; } setHeater(heaterOn); waitForNextPeriod(); } Embedded IO

  11. Heater Example cont. • What happens around 27°C? • On – Off – On – Off …. • Not so good • Heater does not like this • Relay does not like this • Solution • Hysteresis (two thresholds) Embedded IO

  12. Example: Heater revised for (;;) { int temp = readTemp(); if (temp < 27) { heaterOn = true; } else (temp > 30) { heaterOn = false; } else { // we keep the heater state } setHeater(heaterOn); waitForNextPeriod(); } Embedded IO

  13. Summary • IO is very simple • Translation • Physical world to electrical signals • Electrical signals to digital information • Almost no UI • Control runs in a loop • The control loop Embedded IO

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