Table of Contents
Advertisement
Functional Description
For example, four 4-input multiplexer chips can be put at the connector where the analog
signals first arrive on the printed circuit board. As a result, only four analog signals need to
be shielded from noise as they approach the microcontroller chip, rather than having to
protect 16 analog signals. However, external multiplexer chips may introduce additional
noise and errors if not properly utilized. Therefore, it is necessary to maintain low "on"
resistance (the impedance of an analog switch when active within a multiplexed chip) and
insert a low pass filter (R/C) on the input side of the multiplexed chip.
27.7.2.1 External Multiplexing Operation
The QADC can use from one to four external multiplexer chips to expand the number of
analog signals that may be converted. Up to 16 analog channels can be converted through
external multiplexer selection. The externally multiplexed channels are automatically
selected from the channel field of the CCW, the same as internally multiplexed channels.
The QADC is configured for the externally multiplexed mode by setting the MUX bit in
control register 0 (QACR0).
Figure 27-18 shows the maximum configuration of four external multiplexer chips
connected to the QADC. The external multiplexer chips select one of four analog inputs and
connect it to one analog output, which becomes an input to the QADC. The QADC provides
two multiplexed address signals, MA[1:0], to select one of four inputs. These inputs are
connected to all four external multiplexer chips. The analog output of the four multiplexer
chips are each connected to separate QADC inputs (ANW, ANX, ANY, and ANZ) as shown
in Figure 27-18
27-32
MCF5282 User's Manual
MOTOROLA
Advertisement
Table of Contents
