AT90S8535 Datasheet, PDF (74/127 Pages) ATMEL Corporation – 8-bit Microcontroller with 8K Bytes In-System Programmable Flash

Scanning Multiple
Channels
ADC Noise Canceling
Techniques
• 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.
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.
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
74 AT90S/LS8535
1 44 43 42 41 40
39 ^{PA4 (ADC4)}
38 PA5 (ADC5)
37 ^{PA6 (ADC6)}
36 ^{PA7 (ADC7)}
35 ^AREF
34 ^AGND
AVCC
33
32 PC0
1041H–11/01