C8051F310 datasheet(129/224 Pages) ETC1

129 / 224 page

Rev. 1.6

129

C8051F310/1/2/3/4/5/6/7

13.2. 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 Port Input Mode

Step 2. Select the output mode (open-drain or push-pull) for all Port pins, using the Port Output

Mode register (PnMDOUT).

Step 3. Select any pins to be skipped by the I/O Crossbar using the Port Skip registers (PnSKIP).

Step 4. Assign Port pins to desired peripherals.

Step 5. Enable the Crossbar (XBARE = ‘1’).

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 pullup, digital driver, and digital receiver are 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 PnSKIP). Port input mode is set in the PnMDIN 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 13.4 for the PnMDIN register details.

The output driver characteristics of the I/O pins are defined using the Port Output Mode registers (PnMD-

OUT). 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

PnMDOUT settings. When the WEAKPUD bit in XBR1 is ‘0’, a weak pullup is enabled for all Port I/O con-

figured as open-drain. WEAKPUD does not affect the push-pull Port I/O. Furthermore, the weak pullup is

turned off on an output that is driving a ‘0’ to avoid unnecessary power dissipation.

Registers XBR0 and XBR1 must be loaded with the appropriate values to select the digital I/O functions

required by the design. Setting the XBARE bit in XBR1 to ‘1’ enables the Crossbar. Until the Crossbar is

enabled, the external pins remain as standard Port I/O (in input mode), 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.

The Crossbar must be enabled to use Port pins as standard Port I/O in output mode. Port output

drivers are disabled while the Crossbar is disabled.

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