
MC68F375
QUEUED ANALOG-TO-DIGITAL CONVERTER MODULE-64
MOTOROLA
REFERENCE MANUAL
Rev. 25 June 03
5-9
Port B pins are referred to as PQB when used as an 8-bit input-only digital port. Port
B can also be used for non-multiplexed AN[51:48]/AN[3:0] and multiplexed ANz, ANy,
ANx, ANw analog inputs.
ister for register and bit descriptions.
5.6.2 Port Data Direction Register
The port data direction register (DDRQA) is associated with the port A digital I/O pins.
These bidirectional pins may have somewhat higher leakage and capacitance speci-
information.
Any bit in this register set to one configures the corresponding pin as an output. Any
bit in this register cleared to zero configures the corresponding pin as an input. Soft-
ware is responsible for ensuring that DDRQA bits are not set to one on pins used for
analog inputs. When a DDRQA bit is set to one and the pin is selected for analog con-
version, the voltage sampled is that of the output digital driver as influenced by the
load.
NOTE
Caution should be exercised when mixing digital and analog inputs.
This should be minimized as much as possible. Input pin rise and fall
times should be as large as possible to minimize AC coupling effects.
Since port B is input-only, a data direction register is not needed. Read operations on
the reserved bits in DDRQA return zeros, and writes have no effect. Refer to 5.12.4 5.7 External Multiplexing Operation
External multiplexers 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 sig-
nals than the A/D converter can normally provide. External multiplexing also puts the
multiplexer closer to the signal source. This minimizes the number of analog signals
that need to be shielded due to the close proximity of noisy, high speed digital signals
near the MCU.
NOTE
The main QADC64 treats the AMX as an external multiplexer. It is
recommended that full external multiplexing not be used on the
MC68F375. Mixed internal and external multiplexing should be used.
The QADC64 can use from one-to-four external multiplexers to expand the number of
analog signals that may be converted. Up to 32 analog channels can be converted
through external multiplexer selection. The externally multiplexed channels are auto-
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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