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Analogue Input/Output

Many sensors/transducers produce voltages
representing physical data.
 To
process transducer data in a computer requires
conversion to digital form. Examples:


reading temperature from a thermocouple
processing speech from a microphone
Many output devices require variable control, not just
two digital logic levels
 To
control these devices from a computer requires
conversion from digital to analogue form.
5-1
Analogue Output


Digital to Analogue Converter (DAC)
DAC Characteristics
= 1/2n where n is the number of bits – step size
 Max. digital output = 2n – 1
 output voltage range – determined by reference voltage
(Vref and AGND)
 Step size in volts = resolution x voltage range
 Max output voltage = (2n – 1)/ 2n x voltage range
 uni-polar / bipolar types
 slew rate – rate of change of output.
 interface – parallel (fast) or serial (slower but uses fewer
connections)
 resolution
5-2
DAC principles – Example 4-bit DAC

Sum currents with operational amplifier
1
d3
2R
R
Vref/2
Vo = - Vref(Rf/Rinput)
4R
Vref/4
0
d2
8R
Vref/8
+
1
d1
1
Vref
16R
Vo
Vo = -Vref(digital value/2n)
Example: with 4-bit value = 1011
Vref/16
Vo = -Vref(d3/2 + d2/4 + d1/8 + d0/16)
Vo = -Vref(1/2 + 1/8 + 1/16)
d0
AGND
Vo = -Vref(11/16)
5-3
Digital to Analogue conversion
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Previous design needs many different precise
resistor values
Resisters need to have a tolerance less than the
resolution. E.g. 8-bit
resolution = 1:28 = 1/256 = 0.00390625
resolution = 0.390625%
Alternative is R-2R ladder arrangement
 R-2R
ladder - only requires 2 different resistor values.
5-4
Analogue Input

Main types (methods) of ADC
approximation – good all-rounder
 Flash – fastest type
 Sigma-delta – good for audio
 Dual slope integrating – slow but high resolution with good
noise immunity
 others – Sampling, ramp, charge balancing
 Successive

Characteristics
 resolution
 conversion
method
 conversion time
 input voltage range
 interface – parallel (fast) or serial(fewer connections)
5-5
ADC Block diagram
Interrupt request
Conversion
Control
AN0
AN1
ANn
M
u
t
i
p
l
e
x
e
r
Busy
Start
conversion
Sample
& Hold
Converter
VAREF
Result
Register
VAGND
Reference voltage
5-6
ADC – principle of operation
1.
2.
3.
4.
5.
The voltage is presented to the ADC input.
The ADC is sent a signal to start conversion
While the conversion takes place the input voltage
should remain stable.
The ADC outputs a signal to indicate that it is busy
doing the conversion and should not be disturbed.
When the conversion is completed the ADC makes
the result available and outputs a signal to indicate
that the conversion has completed (e.g remove the
busy signal)
5-7
Multiplexer

To convert several analogue inputs
1. use an ADC for each input or …
2. use one ADC and switch the inputs through a
multiplexer


requires selection of input before each conversion is
started
short delay required before conversion started to allow
switching to occur and signal to settle.
5-8
Sample and Hold Circuit

Sample and Hold (S&H)
 while
conversion takes place voltage must remain stable
 sample voltage – input connected to S&H
 voltage held on a capacitor
 sample time – charging time of capacitor
 input signal disconnected from S&H
5-9
167 ADC


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10-bit resolution, 16 channels using Port 5 which has 16 input
only lines, extra 8 channels using Port 1.
Input voltage range 0 to +5Volts
Conversion modes:
Fixed Channel Single Conversion - produces just one result from the
selected channel

Fixed Channel Continuous Conversion - repeatedly converts the selected
channel

Auto Scan Single Conversion - produces one result from each of a selected
group of channels

Auto Scan Continuous Conversion - repeatedly converts the selected group
of channels

Wait for ADDAT Read Mode - start a conversion automatically when the
previous result is read

Channel Injection Mode - insert the conversion of a specific channel into a
group conversion (auto scan)
5-10
167 ADC – SFR's and Port pins
5-11
ADC channels
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The analog input channels AN15 … AN0 are alternate
functions of Port 5 which is an input-only port.
Port 5 may either be used as analog or digital inputs. For pins
are used as analog inputs it is recommended to disable the
digital input stage via register P5DIDIS. This avoids undesired
cross currents and switching noise while the (analog) input
signal level is between V IL and V IH .
The analog input channels AN23 … AN16 are alternate
functions of Port1 which is an IO port.
The port lines P1L.7-0 may either be used as analog inputs or
digital IOs.
P1DIDIS performs the same function for Port 1 as P5DIDIS
does for Port 5.
5-12
ADCON SFR
5-13
ADCON SFR
5-14
ADDAT – ADC Result Register
5-15
ADC completion



When a conversion completes the ADCIR bit in the
ADCIC SFR is set.
ADCIR may cause an interrupt to occur
Programmer can use either polling or interrupts
– check status of ADCIR bit (could possibly use the
ADBSY bit)
 Interrupts – a future lecture
 Polling
5-16
Fixed Channel Conversion Modes




These modes are selected by programming the mode selection bitfield
ADM in register ADCON to ‘00 B ’ (single conversion) or to ‘01 B ’
(continuous conversion).
After starting the converter through bit ADST the busy flag ADBSY will
be set and the channel specified in bit fields ADCH/ADX will be
converted. After the conversion is complete, the interrupt request flag
ADCIR will be set.
In Single Conversion Mode the converter will automatically stop and
reset bits ADBSYand ADST.
In Continuous Conversion Mode the converter will automatically start
a new conversion of the channel specified in ADCH/ADX. ADCIR will be
set after each completed conversion. When bit ADST is reset by
software, while a conversion is in progress, the converter will complete
the current conversion and then stop and reset bit ADBSY.
5-17
Sample ADC program
/* Filename : fcsc.c
Author
: Alan Goude
Date
: 21/11/02
Version 1.0
Program to read ADC channel 2 using
Fixed channel Single Conversion
*/
#include <stdio.h>
/* standard I/O .h-file*/
#include <reg167.h>
/* SFRs for 167 cpu's */
/* Function declarations */
void serial_init();
void main (void)
{
int adc_value;
serial_init(); /* initialize the serial interface
*/
5-18
(Contd.)
// Initialise ADC for Fixed Channel Single Conversion on chan. 2
// fCPU = 20MHz = 50nS
// ADCTC = 00: fBC = fCPU/4 = 5MHz : Tbc = 200nS
// ADSTC = 00: Sample time = Tbc x 8 = 1600nS = 1.6uS
// Total conversion time = Sample time + 40 x Tbc + 2 x Tcpu
//
= 1.6 + 8 + 0.1 = 9.7uS
ADCON = 0x0002; //0000 0000 0000 0010
while (1)
//do this endlessly!
{
ADCIR = 0; // Reset Interrupt request bit
ADST = 1; //start conversion
// wait for ADC to complete
while(ADCIR == 0);
adc_value = ADDAT & 0x03ff; // mask out top 6 bits
printf("ADC Result = %d\n", adc_value);
printf("ADC voltage = %7.5f\n", 5.0/1024 * adc_value);
}
}//end of main
5-19
ADC function
int read_adc(int channel)
{
int adc_value;
ADCON = (ADCON & 0xfff0) | channel;
ADCIR = 0; // Reset Interrupt request bit
ADST = 1; //start conversion
// wait for ADC to complete
while(ADCIR == 0);
adc_value = ADDAT & 0x03ff; // mask out top 6 bits
return adc_value;
}
Assumes ADCON register initialised elsewhere. The initialisation
could be incorporated into the function.
5-20