Manual
74
AT90S/LS8535
1041H–11/01
• ADC9..0: ADC Conversion result
These bits represent the result from the conversion. $000 represents analog ground and
$3FF represents the selected reference voltage minus one LSB.
Scanning Multiple
Channels
Since change of analog channel always is delayed until a conversion is finished, the
Free Running Mode can be used to scan multiple channels without interrupting the con-
verter. Typically, the ADC Conversion Complete interrupt will be used to perform 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 Running 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 AT90S8535 generates EMI that might affect the
accuracy of analog measurements. If conversion accuracy is critical, the noise level can
be reduced by applying the following techniques:
1. The analog part of the AT90S8535 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 AT90S8535 should be connected to the digital V
CC
supply
voltage via an LC network as shown in Figure 50.
4. Use the ADC noise canceler function to reduce induced noise from the CPU.
5. If some Port A pins are used as digital outputs, it is essential that these do not
switch while a conversion is in progress.
Figure 50. ADC Power Connections
GND
VCC
PA0 (ADC0)
PA1 (ADC1)
PA2 (ADC2)
PA3 (ADC3)
PA4 (ADC4)
PA5 (ADC5)
PA6 (ADC6)
PA7 (ADC7)
AREF
AVCC
AGND
PC0
100nF
Analog Ground Plane
AT90S8535
39
35
37
33
32
38
34
36
43 4142 4044
1
10µΗ