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Analog/Digital Subsystem

Analog/Digital Subsystem. Converting analog signals to digital values. Analog Input refers to some type of input signal whose value is continuously variable. Measuring analog input is very common requirement for embedded systems

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Analog/Digital Subsystem

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  1. Analog/Digital Subsystem Converting analog signals to digital values CS-280 Dr. Mark L. Hornick

  2. Analog Input refers to some type of input signal whose value is continuously variable Measuring analog input is very common requirement for embedded systems • Sensors are used to convert a physical signal to a time-varying voltage CS-280 Dr. Mark L. Hornick

  3. Some examples of physical quantities that are measured in devices you use • Temperature (HVAC) • Force/pressure/sound (Auto) • Position/Velocity/Acceleration (Robotics, Cameras) • Oxygen (Auto) • Biometrics (ECG, BP) • Signal strength, battery charge (mobile phone) CS-280 Dr. Mark L. Hornick

  4. Analog to Digital Conversion is • The process that discretizes a signal from its continuous (analog) form to its corresponding digital approximation • ADC or A/D CS-280 Dr. Mark L. Hornick

  5. Discretization explanation • Consider an analog signal that will vary between two values – say VL and VH volts • Discretization refers to the “levels” the ADC is able to resolve the analog signal to: • a 2-bit converter can resolve 4 different discrete levels CS-280 Dr. Mark L. Hornick

  6. Resolution of Discretization • The resolution of an ADC is limited by the number of bits within the ADC • What is the resolution of a 2-bit converter that is trying to measure a 0-5v signal? • How can the resolution be improved? CS-280 Dr. Mark L. Hornick

  7. Atmel Atmega32A/D Subsystem CS-280 Dr. Mark L. Hornick

  8. Atmega32 has a built-in 10-bit A/D converter • 1024 discrete levels • PortA pins are used to sense the input analog signal • 8 different signals can be connected simultaneously • You cannot use PortA for output • Measures the voltage on a PortA pins w.r.t. ground • E.g. ADC0 vs. ground • Can also measure relative voltage between pins • E.g. ADC0 vs. ADC1 CS-280 Dr. Mark L. Hornick

  9. The absolute voltage that can be measured is limited • Measured voltage should be no higher than a reference voltage supplied to the AREF pin • VM <= VAref • AREF is tied to a potentiometer • Voltage can be varied between 0v and Vcc (5v) • The measured voltage VM • VM = 0v is converted to a value of 0 • VM = VAref is converted to a value of 1023 • VM > VAref is clipped to a value of 1023 • Don’t try to measure voltages higher than 5v • Otherwise, you’ll fry the chip CS-280 Dr. Mark L. Hornick

  10. Several I/O Registers are used to control the A/D subsystem • ADCSRC (0x06) – A/D Control/Status Register • SFIOR (0x30) – Special Function Register • Used for other subsystems as well as A/D • ADMUX (0x07) – A/D Multiplex Register • ADCH (0x05) – High byte of converted value • ADCL (0x04) – Low byte of converted value CS-280 Dr. Mark L. Hornick

  11. Meaning of the various bits in the register: ADPS0-ADPS2 : ADC clock frequency divider set all to 1 to run ADC at 1/128th of Atmega32 clock ADIE – Enable ADC Interrupt Interrupt is triggered when AD conversion is complete ISR Vector is 0x20 ADIF – automatically set when AD Interrupt occurs Automatically cleared when ISR is executed Or writing 1 to it (SBI) clears it ADATE – set to enable auto-trigger Trigger source is determined by SFIOR register ADSC – Start AD conversion In Free Running Mode, start first conversion automatically returns to 0 when conversion is complete ADEN – Set to enable ADC Enabling the ADC via ADCSRA– A/D control/status register CS-280 Dr. Mark L. Hornick

  12. SFIOR – Special Function Register • Only meaningful if ADATE in ADCSRA is SET • In Free Running Mode, ADC Interrupts will not occur • ADC will just continuously convert as fast as it can CS-280 Dr. Mark L. Hornick

  13. MUX2-MUX0 – Used to pick the PortA pin to sense 000 for PA0; 111 for PA7 MUX4-MUX3 Gain setting; just set to 0 for now REFS1-REFS0 Reference voltage; just clear REFS1, set REFS0 for now ADLAR – see next slide ADMUX – another A/D control/status register CS-280 Dr. Mark L. Hornick

  14. The ADCH and ADCL I/O Registers hold the 10-bit result • If ADLAR in ADMUX is set: • ADCL must be read first, because ADCL and ADCH are automatically reset after ADCH is read • If ADLAR in ADMUX is clear: CS-280 Dr. Mark L. Hornick

  15. A/D conversion takes a relatively long time • How much? • 65 – 250 microseconds • i.e. 1040 - 4000 clock cycles at 16MHz CS-280 Dr. Mark L. Hornick

  16. ADC Results • Discretization Q is a quantum • Q = (Vmax-Vmin)/2n • Q = (5 – 0)/210 = 5/1024 = 4.88 mV • Apply 1.34 volts • Result: 274 (0x112) • ADC “reports” 1.33712 volts CS-280 Dr. Mark L. Hornick

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