C8051F300 Datasheet, PDF (104/176 Pages) List of Unclassifed Manufacturers – Mixed Signal ISP Flash MCU Family

C8051F300/1/2/3/4/5
12.2. Port I/O Initialization
Port I/O initialization consists of the following steps:
Step 1. Select the input mode (analog or digital) for all Port pins, using the Port0 Input Mode
register (P0MDIN).
Step 2. Select the output mode (open-drain or push-pull) for all Port pins, using the Port0 Output
Mode register (P0MDOUT).
Step 3. Set XBR0 to skip any pins selected as analog inputs or special functions.
Step 4. Assign Port pins to desired peripherals.
Step 5. Enable the Crossbar.
All Port pins must be configured as either analog or digital inputs. Any pins to be used as Comparator or
ADC inputs should be configured as an analog inputs. When a pin is configured as an analog input, its
weak pull-up, digital driver, and digital receiver is disabled. This process saves power and reduces noise
on the analog input. Pins configured as digital inputs may still be used by analog peripherals; however this
practice is not recommended.
Additionally, all analog input pins should be configured to be skipped by the Crossbar (accomplished by
setting the associated bits in XBR0). Port input mode is set in the P0MDIN register, where a ‘1’ indicates a
digital input, and a ‘0’ indicates an analog input. All pins default to digital inputs on reset. See SFR Defini-
tion 12.5 for the P0MDIN register details.
The output driver characteristics of the I/O pins are defined using the Port0 Output Mode register
P0MDOUT (see SFR Definition 12.6). Each Port Output driver can be configured as either open drain or
push-pull. This selection is required even for the digital resources selected in the XBRn registers, and is
not automatic. The only exception to this is the SMBus (SDA, SCL) pins, which are configured as open-
drain regardless of the P0MDOUT settings. When the WEAKPUD bit in XBR2 is ‘0’, a weak pull-up is
enabled for all Port I/O configured as open-drain. WEAKPUD does not affect the push-pull Port I/O. Fur-
thermore, the weak pull-up is turned off on an open-drain output that is driving a ‘0’ to avoid unnecessary
power dissipation.
Registers XBR0, XBR1 and XBR2 must be loaded with the appropriate values to select the digital I/O func-
tions required by the design. Setting the XBARE bit in XBR2 to ‘1’ enables the Crossbar. Until the Crossbar
is enabled, the external pins remain as standard digital inputs (output drivers disabled) regardless of the
XBRn Register settings. For given XBRn Register settings, one can determine the I/O pin-out using the
Priority Decode Table; as an alternative, the Configuration Wizard utility of the Silicon Labs IDE software
will determine the Port I/O pin assignments based on the XBRn Register settings.
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