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Parallel Input/Output Control
MCF51CN128 Reference Manual, Rev. 6
9-7
Freescale Semiconductor
9.2.2.4
Port x Input Filter Enable Register (PTxIFE)
The pad cells on this device incorporate optional low pass filters on the digital input functions. These are
enabled by setting the appropriate bit in the input filter enable register (PT
x
IFE[n]). When set, a low pass
filter (10MHz to 30MHz bandwidth) is enabled in the logic input path. When cleared, the filter is bypassed.
9.3
Standard GPIO
9.3.1
GPIO Overview
Reading and writing of parallel I/Os are performed through the port data registers. The direction, either
input or output, is controlled through the port data direction registers. The parallel I/O port function for an
individual pin is illustrated in the block diagram shown in
.
The data direction control bit (PT
x
DD[n]) determines whether the output buffer for the associated pin is
enabled, and also controls the source for port data register reads. The input buffer for the associated pin is
always enabled unless the pin is enabled as an analog function or is an output-only pin.
When a shared digital function is enabled for a pin, the output buffer is controlled by the shared function.
However, the data direction register bit continues to control the source for reads of the port data register.
When a shared analog function is enabled for a pin, the input and output buffers are disabled. A value of
0 is read for any port data bit where the bit is an input (PT
x
DD[n] = 0) and the input buffer is disabled.
Writing to the port data register before changing the direction of a port pin to an output ensures that the pin
is not driven momentarily with an old data value in the port data register.
7
6
5
4
3
2
1
0
R
PTxIFE7
PTxIFE6
PTxIFE5
PTxIFE4
PTxIFE3
PTxIFE2
PTxIFE1
PTxIFE0
W
Reset:
0
0
0
0
0
0
0
0
Figure 9-5. Port x Input Filter Enable Register (PTxIFE)
Table 9-10. PTxIFE Field Descriptions
Field
Description
7–0
PTxIFE
n
Port x Input Filter Enable
— Input low-pass filter enable control bits for PTx pins.
0 Input filter disabled
1 Input filter enabled