ADMUX
ADMUX Register
REFS1
|
REFS0
|
ADLAR
|
MUX4
|
MUX3
|
MUX2
|
MUX1
|
MUX0
|
REFS1 and REFS0 i.e. bit 7 and
6 are reference selection bits
These bits select the
reference voltage for the ADC. Reference selection table is as shown below.
REFS1
|
REFS0
|
Vref
|
0
|
0
|
AREF Pin as
reference voltage. Set externally using board potentiometer.
|
0
|
1
|
AVCC Pin as reference voltage.
Voltage same as VCC i.e. 5 V.
|
1
|
0
|
Reserved.
(Meaning not used)
|
1
|
1
|
Internal 2.56 V as reference
Voltage. Remains fixed regardless of VCC Value.
|
Genral Value of ADMUX.
With internal 2.56 reference
select, ADLAR High, and single ended input.
ADC0 – 0xE0
ADC1 – 0xE1
ADC2 – 0xE2
ADC3 – 0xE3
ADC4 – 0xE4
ADC5 – 0xE5
ADC6 – 0xE6
ADC7 – 0xE7
ADLAR i.e. bit 5. ADC
left/right adjust results. By default its right adjust.
This bit dictates the result
in ADCH:ADCL be left or right adjust. The AVRs have a 10-bit ADC meaning result
is 10 bit long and cannot be stored into a single byte. So 2 registers are used
to store that is ADCH:ADCL.See table below.
Justification
|
ADCH
|
ADCL
|
||||||||||||||||
Left-Justified
ADLAR = 1
|
D9
|
D8
|
D7
|
D6
|
D5
|
D4
|
D3
|
D2
|
D1
|
D0
|
UNUSED
|
|||||||
Right-Justified
ADLAR = 0
|
UNUSED
|
D9
|
D8
|
D7
|
D6
|
D5
|
D4
|
D3
|
D2
|
D1
|
D0
|
|||||||
MUX4 – MUX0 i.e. bit 4 to bit
0. These are analog channel and gain selection bits. These bits select the gain
for the differential channels and also selects which combo of analog inputs are
connected to ADC.
Is used to select single ended
channels or differential channels. See table Below.
Single Ended Channels.
MUX4
|
MUX3
|
MUX2
|
MUX1
|
MUX0
|
Single Ended
Output
|
0
|
0
|
0
|
0
|
0
|
ADC0
|
0
|
0
|
0
|
0
|
1
|
ADC1
|
0
|
0
|
0
|
1
|
0
|
ADC2
|
0
|
0
|
0
|
1
|
1
|
ADC3
|
0
|
0
|
1
|
0
|
0
|
ADC4
|
0
|
0
|
1
|
0
|
1
|
ADC5
|
0
|
0
|
1
|
1
|
0
|
ADC6
|
0
|
0
|
1
|
1
|
1
|
ADC7
|
See Image below for differential
reference table.
 |
| Differential Input with Gain |
ADCSRA
The ADCSRA register is the
status and control register of ADC. Bits of this control or monitor the
operation of ADC.
ADCSRA Register
ADEN
|
ADSC
|
ADATE
|
ADIF
|
ADIE
|
ADPS2
|
ADPS1
|
ADPS0
|
ADEN. Bit 7. ADC Enable. 1
will enable ADC. 0 will disable ADC.
ADSC. Bit 6. ADC Start
Conversion. 1 to be setted to start each conversion.
ADATE. Bit 5. ADC Auto Trigger
Enable. 1 Auto triggering of ADC is enable. 0 means Auto triggering off.
ADIF. Bit 4. ADC Interrupt
Flag. This is set when conversion is completed and data registers are updated.
ADIE.Bit 3. ADC Interrupt
Enable. Setting 1 enables ADC conversion complete interrupt. 0 disables it.
ADSP. Bit 2 to Bit 0. ADC
Prescaler Select Bits. These bits determine the division between XTAL frequency
and the input clock to ADC.
The Prescaler table is as
shown below.
ADSP2
|
ADSP1
|
ADSP0
|
ADC Clock
|
0
|
0
|
0
|
Reserved
|
0
|
0
|
1
|
CLOCK/2
|
0
|
1
|
0
|
CLOCK/4
|
0
|
1
|
1
|
CLOCK/8
|
1
|
0
|
0
|
CLOCK/16
|
1
|
0
|
1
|
CLOCK/32
|
1
|
1
|
0
|
CLOCK/64
|
1
|
1
|
1
|
CLOCK/128
|
Important note. For the AVR
the ADC requires input clock frequency less than 200khz for the maximum
accuracy. See example below.
Example. For 16MHZ crystal.
ADSP 2:0 = 100 >>>
16MHZ/16 = 1MHZ which is not less than 200khz.
ADSP 2:0 = 101 >>>
16MHZ/32 = 500khz which is again not less than 200khz.
ADSP 2:0 = 110 >>>
16MHZ/64 = 250khz which is close but again not less than 200khz.
So the ADC input clock
frequency for a 16MHZ crystal should have ADSP 2:0 = 111 which will give input
clock – 16MHZ/128 = 125khz which is less than 200khz.