MMC2114 MOTOROLA [Motorola, Inc], MMC2114 Datasheet - Page 471

• Manufacturer Part Number: MMC2114
• Description: M CORE Microcontrollers
• Manufacturer: MOTOROLA [Motorola, Inc]

19.9.2 External Multiplexing
19.9.2.1 External Multiplexing Operation
MMC2114 • MMC2113 • MMC2112 — Rev. 1.0
MOTOROLA
External multiplexer chips concentrate a number of analog signals onto
a few QADC inputs. This is useful for applications that need to convert
more analog signals than the QADC converter can normally support.
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 of 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 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.
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 19-18
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
Freescale Semiconductor, Inc.
For More Information On This Product,
Queued Analog-to-Digital Converter (QADC)
19-18.
Go to: www.freescale.com
shows the maximum configuration of four external
Queued Analog-to-Digital Converter (QADC)
Functional Description
Advance Information
471