AT90S/LS2333 and AT90S/LS4433
57
ADC Data Register - ADCL AND ADCH
When an ADC conversion is complete, the result is found in these two registers. In free-run mode, it is essential that both
registers are read, and that ADCL is read before ADCH.
Scanning Multiple Channels
Since change of analog channel always is delayed until a conversion is finished, the Free Run Mode can be used to scan
multiple channels without interrupting the converter. Typically, the ADC Conversion Complete interrupt will be used to per-
form the channel shift. However, the user should take the following fact into consideration:
The interrupt triggers once the result is ready to be read. In Free Run Mode, the next conversion will start immediately
when the interrupt triggers. If ADMUX is changed after the interrupt triggers, the next conversion has already started, and
the old setting is used.
ADC Noise Canceling Techniques
Digital circuitry inside and outside the AT90S2333/4433 generates EMI which might affect the accuracy of analog measure-
ments. If conversion accuracy is critical, the noise level can be reduced by applying the following techniques:
1. The analog part of the AT90S2333/4433 and all analog components in the application should have a separate analog
ground plane on the PCB. This ground plane is connected to the digital ground plane via a single point on the PCB.
2. Keep analog signal paths as short as possible. Make sure analog tracks run over the analog ground plane, and keep
them well away from high-speed switching digital tracks.
3. The AV
CC
pin on the AT90S2333/4433 should be connected to the digital V
CC
supply voltage via an RC network as
shown in Figure 47.
4. Use the ADC noise canceler function to reduce induced noise from the CPU.
5. If some Port C pins are used as digital outputs, it is essential that these do not switch while a conversion is in progress.
Bit
15
14
13
12
11
10
9
8
$05 ($25)
-
-
-
-
-
-
ADC9
ADC8
ADCH
$04 ($26)
ADC7
ADC6
ADC5
ADC4
ADC3
ADC2
ADC1
ADC0
ADCL
7
6
5
4
3
2
1
0
Read/Write
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
Initial value
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0