xe8802 Semtech Corporation, xe8802 Datasheet - Page 88

• Manufacturer Part Number: xe8802
• Description: Data Acquisition Soc With 16+10 Bit Zoomingadc? Embedded Risc Controller And Lcd Driver
• Manufacturer: Semtech Corporation

13.4 Port D (PD) Operation
The direction of each pin of Port D (input or input/output) can be individually set by using the RegPDDir register. If
PDDir[x] = 1, the output buffer on the corresponding Port D pin is enabled. After reset, Port D is in input only mode
(PDDir[x] are reset to 0). The input buffer is always enabled independently from the RegPDDir contents.
Output data:
Data are stored in RegPDOut prior to output at Port D.
Input data:
The status of Port D is available in RegPDIn (read only). Reading is always direct - there is no digital debounce
function associated with Port D. In case of possible noise on input signals, a software debouncer or an external
filter must be realised.
Pull-up/Snap to Rail:
When configured as an input (PDDir[x]=0), pull-ups are available on every pin. The pull-up function of the pins is
controlled two by two by the PDPullup and PDSnapToRail bits in the register RegPDPullup. When a bit
PDPullup[x] is 0, the pull-ups on the pins PD[2x] and PD[2x+1] are disabled. When a bit PDPullup[x] is set to 1
and the bit PDSnapToRail[x] is set to 0, the pull-up resistor is connected to the pins PD[2x] and PD[2x+1]. When
both PDPullup[x] and PDSnapToRail[x] are 1, the snap-to-rail function is active on the pins PD[2x] and PD[2x+1].
The snap-to-rail function connects a pullup or pulldown resistor to the input pin depending on the value forced on
the input pin. This function can be used for instance when the input port is connected to a tristate bus. When the
bus is floating, the pullup or pulldown maintains the bus in the last low impedance state before it became floating
until another low impedance output is driving the bus. It also reduces the power consumption with respect to a
classic pullup since it selects the pullup or pulldown resistor so that it confirms the detected input state.
The function is summarised in the table below as a function of the different register settings.
At power-on reset, Port D is configured as an input port with all pull-ups active.
© Semtech 2006
PDDir[2x(+1)]
1
0
0
0
0
PDPullup[x]
x
0
1
1
1
Table 13-6: Snap-to-rail and pullup function
XE8802 Sensing Machine Data Acquisition MCU
PDSnapToRail[x]
13-4
0
1
1
x
x
with ZoomingADC™ and LCD driver
PD[2x(+1)] value
(last) externally
forced
x
x
x
0
1
PD[2x(+1)] pull
not connected
not connected
pulldown
resistor
pullup
pullup
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