MMC2107 - Freescale Semiconductor

Freescale Semiconductor, Inc.
Queued Analog-to-Digital Converter (QADC)
Functional Description
18.9 Functional Description
This subsection provides a functional description of the QADC.
18.9.1 QADC Bus Accessing
Coherency of results read across multiple cycles is not guaranteed.
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18.9.2 External Multiplexing
18.9.2.1 External Multiplexing Operation
External multiplexer chips concentrate a number of analog signals onto
a few inputs to the analog converter. This is helpful in applications that
need to convert more analog signals than the A/D converter can
normally provide. External multiplexing also puts the multiplexed chip
closer to the signal source. This minimizes the number of analog signals
that need to be shielded due to the proximity to noisy high speed digital
signals at the microcontroller chip.
For example, four 4-input multiplexer chips can be put at the connector
where the analog signals first arrive on the computer 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.
The QADC can use from one to four or two dual 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 conversion command word (CCW)
table, the same as internally multiplexed channels. The software selects
the external multiplexed mode by setting the MUX bit in control register 0
(QACR0).
MMC2107 – Rev. 2.0
Technical Data
MOTOROLA Queued Analog-to-Digital Converter (QADC) 443
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