C8051F040/1/2/3/4/5/6/7
Rev. 1.5 221
17.2.1. Configuring Ports Which are Not Pinned Out
Although P4, P5, P6, and P7 are not brought out to pins on the C8051F041/3/5/7 devices, the Port Data
registers are still present and can be used by software. Because the digital input paths also remain active,
it is recommended that these pins not be left in a ‘floating’ state in order to avoid unnecessary power dissi-
pation arising from the inputs floating to non-valid logic levels. This condition can be prevented by any of
the following:
1. Leave the weak pullup devices enabled by setting WEAKPUD (XBR2.7) to a logic 0.
2. Configure the output modes of P4, P5, P6, and P7 to “Push-Pull” by writing PnOUT = 0xFF.
3. Force the output states of P4, P5, P6, and P7 to logic 0 by writing zeros to the Port Data regis-
ters: P4 = 0x00, P5 = 0x00, P6= 0x00, and P7 = 0x00.
17.2.2. Configuring the Output Modes of the Port Pins
The output mode of each port pin can be configured to be either Open-Drain or Push-Pull. In the Push-Pull
configuration, a logic 0 in the associated bit in the Port Data register will cause the Port pin to be driven to
GND, and a logic 1 will cause the Port pin to be driven to
V
DD
. In the Open-Drain configuration, a logic 0 in
the associated bit in the Port Data register will cause the Port pin to be driven to GND, and a logic 1 will
cause the Port pin to assume a high-impedance state. The Open-Drain configuration is useful to prevent
contention between devices in systems where the Port pin participates in a shared interconnection in
which multiple outputs are connected to the same physical wire.
The output modes of the Port pins on Ports 4 through 7 are determined by the bits in their respective
PnMDOUT Output Mode Registers. Each bit in PnMDOUT controls the output mode of its corresponding
port pin (see SFR Definition 17.17, SFR Definition 17.19, SFR Definition 17.21, and SFR Definition 17.23).
For example, to place Port pin 4.3 in push-pull mode (digital output), set P4MDOUT.3 to logic 1. All port
pins default to open-drain mode upon device reset.
17.2.3. Configuring Port Pins as Digital Inputs
A Port pin is configured as a digital input by setting its output mode to "Open-Drain" in the PnMDOUT reg-
ister and writing a logic 1 to the associated bit in the Port Data register. For example, P7.7 is configured as
a digital input by setting P7MDOUT.7 to a logic 0, which selects open-drain output mode, and P3.7 to a
logic 1, which disables the low-side output driver.
17.2.4. Weak Pullups
By default, each Port pin has an internal weak pullup device enabled which provides a resistive connection
(about 100 k) between the pin and
V
DD
. The weak pullup devices can be globally disabled by writing a
logic 1 to the Weak Pullup Disable bit, (WEAKPUD, XBR2.7). The weak pullup is automatically deactivated
on any pin that is driving a logic 0; that is, an output pin will not contend with its own pullup device.
17.2.5. External Memory Interface
If the External Memory Interface (EMIF) is enabled on the High ports (Ports 4 through 7), EMIFLE
(XBR2.5) should be set to a logic 0.
If the External Memory Interface is enabled on the High ports and an off-chip MOVX operation occurs, the
External Memory Interface will control the output states of the affected Port pins during the execution
phase of the MOVX instruction, regardless of the settings of the Port Data registers. The output configura-
tion of the Port pins is not affected by the EMIF operation, except that Read operations will explicitly dis-
able the output drivers on the Data Bus during the MOVX execution. See Section “16. External Data
Memory Interface and On-Chip XRAM” on page 187 for more information about the External Memory
Interface.
