1. The Silicon Laboratories C8051F020
Enhanced 8051 Part 4
Analog to Digital Conversion (ADC)
ECE/CS-352: Embedded Microcontroller Systems

2. Analog to Digital Conversion
Digital value
time
time
Analog
A/D
Converter
Digital (8-bit)
GND
ECE/CS-352: Embedded Microcontroller Systems

3. Analog to Digital Conversion
DAC n
Basic idea is to compare analog input to value produced by DAC and use logic to adjust the digital output so that it properly represents the analog input.
A/D converters classified based on what logic is used.
ECE/CS-352: Embedded Microcontroller Systems

4. ECE/CS-352: Embedded Microcontroller Systems
Counting Converter
DAC n
Count from 0 to 2n, when DAC output is higher than Input, then flag changes.
ECE/CS-352: Embedded Microcontroller Systems

5. Successive Approximation
DAC n
Starting at MSB, set each bit to 1, and if it trips the comparator, reset to 0. If not, hold at 1. Repeat for all bits.
.....
Example... first 3 bits
analog
ECE/CS-352: Embedded Microcontroller Systems

6. Analog to Digital Conversion Precision
A/D
Converter n
Analog
Digital
Analog
0.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
2.25
2.50
2.75
3.00
3.25
3.50
3.75
4.00
Digital
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
1110
1111
Examples:
Analog Range: 0V-4V
Digital: n=4
Precision:
4 Volts
16 values
.25 V/bit
ECE/CS-352: Embedded Microcontroller Systems

7. ECE/CS-352: Embedded Microcontroller Systems
C8051F020 A/D Converters
ADC0
12-bit Successive Approximation (SAR)
100ksps (kilo-samples per second)
ADC1
8-bit SAR
500ksps
ECE/CS-352: Embedded Microcontroller Systems

8. ECE/CS-352: Embedded Microcontroller Systems
ADC0
Analog source options
SFRs
Reference voltage options
Conversion sync options
ECE/CS-352: Embedded Microcontroller Systems

9. AMX0CF – AMUX0 Configuration Register
Differential pair inputs: + - V
V can be positive or negative
0 – single-ended analog inputs
1 – differential pairs
AMX0CF
ECE/CS-352: Embedded Microcontroller Systems

10. AMX0SL – AMUX0 Channel Select Register
AMUX0 address bits
What is MUXed in
depends on AMUX0CF
AMX0SL
ECE/CS-352: Embedded Microcontroller Systems

11. ECE/CS-352: Embedded Microcontroller Systems
AMUX0SL Possibilities
ECE/CS-352: Embedded Microcontroller Systems

12. ADC0CF – ADC0 Configuration Register
Bit7-Bit3:
SAR Conversion Clock Period Bits
Bit2-Bit0:
Internal Amplifier Gain
000: Gain = 1
001: Gain = 2
010: Gain = 4
011: Gain = 8
10x: Gain = 16
11x: Gain = 0.5
ADC0CF
ECE/CS-352: Embedded Microcontroller Systems

13. ADC0CN – ADC0 Control Register
AD0EN = disabled
1 = enabled
AD0TM Track mode bit
(low power mode)
AD0INT
(must be cleared by software)
0 = conversion not completed
1 = conversion has been completed
AD0BUSY
0 = conversion not in progress
1 = conversion in progress
ADC0CN
ECE/CS-352: Embedded Microcontroller Systems

14. ADC0CN – ADC0 Control Register
Bit3-2: ADC0 Start of Conversion Mode Select.
If AD0TM = 0 (tracking mode on):
00: ADC0 conversion initiated on every write of ‘1’ to AD0BUSY.
01: ADC0 conversion initiated on overflow of Timer 3.
10: ADC0 conversion initiated on rising edge of external CNVSTR.
11: ADC0 conversion initiated on overflow of Timer 2.
If AD0TM = 1: Same as above except conversion takes 3 SAR clock cycles longer
Bit1: ADC0 Window Compare Interrupt Flag.
This bit must be cleared by software.
0: ADC0 Window Comparison Data match has not occurred since this flag was last cleared.
1: ADC0 Window Comparison Data match has occurred.
Bit0: ADC0 Left Justify Select.
0: Data in ADC0H:ADC0L registers are right-justified.
1: Data in ADC0H:ADC0L registers are left-justified.
ECE/CS-352: Embedded Microcontroller Systems

15. ECE/CS-352: Embedded Microcontroller Systems
Selecting VREF, Gain
Maximum VREF = 3.3V (Vdd)
Suppose analog range is about 0-3V
No Gain needed
External VREF of 3V 0r
Gain = .5 (so analog range becomes 0-1.5V)
VREF from DAC0 of 1.5V
Suppose analog range is low: V
Use max gain of 16 (so range is V)
VREF from DAC0 or external, 0.16V)
ECE/CS-352: Embedded Microcontroller Systems

16. ECE/CS-352: Embedded Microcontroller Systems
ADC0 Example + -
Vin
2.0V
Differential input (can be negative or positive)
on AIN0 and AIN1 (channel select AIN0)
Convert on timer 2 overflow
Amplify by 4
Right justified data
Interrupt each conversion
External Vref = 2.0V
ECE/CS-352: Embedded Microcontroller Systems

17. ECE/CS-352: Embedded Microcontroller Systems
ADC0 Example
Data Conversion:
Gain = 4
VREF = 2.0V
Note: Vin limited to range of -.5V to +.5V
Vin
ADC0H: ADC0L
+.5V
07FFh :
+.25V
0400h :
0000h :
-.25V
FC00h :
-.5V
F800h :
n = 12 for single-ended inputs, n = 11 for differential
Values are sign-extended.
ECE/CS-352: Embedded Microcontroller Systems

18. Conversion Calculations
analog range
Vref – 1 lsb
000h
digital range
FFFh
(4095)
There are 4096 different digital values corresponding to 0-Vref.
Find the voltage per least significant bit:
Now find analog value corresponding to some digital value Xd
ECE/CS-352: Embedded Microcontroller Systems

19. ECE/CS-352: Embedded Microcontroller Systems
Become AD Converts....
ECE/CS-352: Embedded Microcontroller Systems