ADC interfacing with PIC.

1. INTERFACING OF ADC, DAC &
SENSOR WITH PIC18F
MICROCONTROLLER

2. Basics of A/D Conversion
•Can convert only electrical voltages to digital
values.
• A transducer is needed to convert a non-electric
quantity into an electrical voltage.
• Different names of transducers are used for
different physical quantities.
• A data acquisition system is used to referred to
those systems that perform A/D conversions.

3. Characteristics of ADC
 Resolution
 Conversion time
 Vref
 Digital data output
Dout = Vin/step size

4. Parallel VS Serial ADC
 Parallel in serial ou
shift register
 Speed is slow
•8 bit & 16 bit o/p line
•Speed is fast

5. The PIC18f A/D Converter
-The PIC18 has a 10-bit A/D converter.
-The number of analog inputs varies
among difference PIC18 devices.
-The contents of these registers vary
with the PIC18 members.
-Early PIC18 (PIC18FXX2) members
have only ADCON0 and ADCON1
registers.

7. A/D Registers
-The A/D converter has the following
registers:
•A/D Result High Register (ADRESH)
•A/D Result Low Register (ADRESL)
•A/D Control Register 0 (ADCON0)
•A/D Control Register 1 (ADCON1)

8. Three control registers are
used to:
•Set up the I/O pins for analog signals from ports A, B,
and E that are used as inputs for A/D conversion.RA5
•Select a channel: AN4
•Set up pins RA2 and RA3 to connect external VREF +
and VREF - if specified in the control register ADCON1.
•Select an oscillator frequency divider through the
control register ADCON1.
•Select an acquisition time through the control register
ADCON2.

9. A/D Control Register 0 (ADCON0)
• Primary function of the ADCON0
register:
– Select a channel for input analog
signal
– Start a conversion
– Indicate the end of the conversion
• Bit2 is set to start the
conversion, and at the end of
the conversion this bit is reset.

10. A/D Control Register 1
(ADCON1)
 ADCON1 is primarily used to set up the
I/O pins either for analog signal or for
digital signals and select VREF voltages.
 An input to be converted must be an
analog input.
 The ADFM bit of the ADCON1 is used for
making it Right-justified or Left-justified
because we need 10 out of 16 bits.

11. ADFM bit
 Alignment to the left – the eight MSB bits
are stored in the ADRESH, and the two LSB
bits are stored in ADRESL. In this case, the
remaining six bits appear as - "0".
• Alignment to the right – the eight LSB bits are
stored in ADRESL, and two MSB bits are stored in
the ADRESH. In this case six highest bits appear
as - "0".

13. A/D Acquisition Requirements
 The A/D converter has a sample-and-hold
circuit for analog input.
 The sample-and-hold circuit keeps the
voltage stable when it is converted.
 The sample-and-hold circuit is shown in
Figure.

14. Automatic Acquisition Time
 For earlier PIC18 members, when the
GO/DONE bit is set, sampling is stopped
and conversion begins.
 The user is responsible for making sure
enough acquisition time is provided.
 For newer PIC18 members, the A/D
module will continue to sample after the
GO/DONE bit is set for the selected
acquisition time.
 The automatic acquisition time makes
A/D programming a little easier.

15. Selecting the A/D Conversion
Clock
 The per bit A/D conversion time is defined
as TAD.
 Each 10-bit A/D conversion takes 12 TAD
to complete.
 For some devices, the options for TAD
are defined in ADCON0. For others, the
options for TAD are defined in ADCON2.
 The length of TAD must be at least 1.6 .

16. EXAMPLE

17. Procedure for Performing A/D
Conversion
 Configure the A/D module
1. Configure analog pins, reference voltages
2. Select A/D input channel
3. Select A/D acquisition time (if available)
4. Select A/D conversion clock
5. Enable A/D module
 Configure A/D interrupt
1. Clear ADIF flag
2. Set ADIE bit (if desired)
3. Set GIE bit (if desired)

18. Procedure contd…..
 Wait for the desired acquisition time
(if required)
 Start conversion by setting the
GO/DONE bit
 Wait for A/D conversion to complete
 Read the A/D result registers; clear
the ADIF flag
 For next conversion, go to step 1 or
step 2.

19. Programming PIC18F458 ADC

20. Programming using interrupt

21. Digital to Analog (D/A, DAC, or
D-to-A) Conversion
 Converting discrete signals into
discrete analog values that represent
the magnitude of the input signal
compared to a standard or reference
voltage
◦ The output of the DAC is discrete analog
steps.
◦ By increasing the resolution (number of
bits), the step size is reduced, and the
output approximates a continuous analog
signal.

22. Digital-to-Analog Conversion
 2 Basic Approaches
◦ Weighted Summing Amplifier
◦ R-2R Network Approach

23. Weighted Sum DAC
 One way to achieve D/A conversion is
to use a summing amplifier.
 This approach is not satisfactory for a
large number of bits because it
requires too much precision in the
summing resistors.
 This problem is overcome in the R-2R
network DAC.

25. R-2R Ladder DAC
 The summing amplifier with the R-2R
ladder of resistances shown produces
the output where the D's take the
value 0 or 1.
 The digital inputs could be TTL
voltages which close the switches on
a logical 1 and leave it grounded for a
logical 0.
 This is illustrated for 4 bits, but can be
extended to any number with just the
resistance values R and 2R.

27. Example
 What will be the analog equivalent of
1001 0001?

28. DAC0808 interfacing with
PIC18
 8 bit DAC, provides 256 discrete
voltage levels.

29. Converting Iout to Voltage in
DAC0808
 In the MC0808 the digital inputs are
converted to current and to convert it
to voltage Iout is connected to a
resistor.
 As the resistance affect the output
voltage so current output is isolated by
connecting it to an op-amp.

30. Generating a sine wave
 To generate a sine wave we first need
a table whose values represent the
magnitude of sine of angle b/w 0 and
360 degrees.
 Sin varies from -1 to +1.
 This method ensures that only integer
numbers are output to the DAC by
PIC18.
 Assume full-scale voltage of 10 V.
 Vout = 5V + (5*sinQ)

31. Vout of DAC for various angles

32. Program for generating sine
wave
Full scale Vout = 10V
256steps/ 10V = 25.6 steps per volt

33. Output sine wave

34. Sensor interfacing
 Sensor is a device which detects or
measures a physical property and
records, indicates, or otherwise
responds to it.
 Transducer is a device that converts
variations in a physical quantity, such
as pressure or brightness, into an
electrical signal, or vice versa.
 We are considering only temperature
sensor.

35. Temperature Sensor
• Transducer that converts temperature into an
analog electrical signal.
• Many are available as integrated circuits, and
their outputs (voltage or current) are, in
general, linearly proportional to the
temperature.
• However, output voltage ranges of these
transducers may not be ideally suited to
reference voltages of A/D converters.
• Therefore, it is necessary to scale the output
of a transducer to range of the reference
voltages of an A/D converter.
• Scaling may require amplification or shifting
of voltages at a different level.

36. Interfacing LM34/LM35 with
PIC18
 The sensor of LM34 series are
precision integrated-circuit
temperature sensors.
 Output voltage is linearly proportional
to fahrenheit temperature.
 Outputs 10mV for each degree of
fahrenhiet temperature.
 In LM35 output voltage is linearly
proportional to celsius temperature.

37. LM34 sensors
LM35 sensors

38. Interfacing LM35 with PIC18
 Temperature calculations
◦ A/D converter has 10-bit resolution
◦ For temperature range 0ºF to +300ºF, the digital
output should be divided into 1024 steps (0 to
3FFH).
◦ Therefore, the digital value per degrees
Fahrenheit is 10.24 (1024/100 = 10.2410).
◦ This 10.24 V is exceeding the permissible max.
value i.e 3V.
◦ If step size is taken 2.5 mV then max Vout will be
2.56 V.
◦ Final value is calculated by dividing output by 4.

39. Interfacing LM35 with PIC18
 LM35 is
connected to
channel
0(RA0 pin).
 The channel
AN3(RA3
pin) is
connected to
the Vref of
2.56 V.

40. Program for reading and
displaying temperature