C8051F336_08 Datasheet, PDF (120/227 Pages) Silicon Laboratories – Mixed-Signal Byte-Programmable EPROM MCU

C8051F336/7/8/9
20.1. Port I/O Modes of Operation
Port pins P0.0 - P2.3 use the Port I/O cell shown in Figure 20.2. Each Port I/O cell can be configured by
software for analog I/O or digital I/O using the PnMDIN registers. On reset, all Port I/O cells default to a
high impedance state with weak pull-ups enabled. Until the crossbar is enabled (XBARE = ‘1’), both the
high and low port I/O drive circuits are explicitly disabled on all crossbar pins.
20.1.1. Port Pins Configured for Analog I/O
Any pins to be used as Comparator or ADC input, external oscillator input/output, VREF, or IDAC output
should be configured for analog I/O (PnMDIN.n = ‘1’). When a pin is configured for analog I/O, its weak pul-
lup, digital driver, and digital receiver are disabled. Port pins configured for analog I/O will always read
back a value of ‘0’.
Configuring pins as analog I/O saves power and isolates the Port pin from digital interference. Port pins
configured as digital I/O may still be used by analog peripherals; however, this practice is not recom-
mended and may result in measurement errors.
20.1.2. Port Pins Configured For Digital I/O
Any pins to be used by digital peripherals (UART, SPI, SMBus, etc.), external event trigger functions, or as
GPIO should be configured as digital I/O (PnMDIN.n = ‘1’). For digital I/O pins, one of two output modes
(push-pull or open-drain) must be selected using the PnMDOUT registers.
Push-pull outputs (PnMDOUT.n = ‘1’) drive the Port pad to the VDD or GND supply rails based on the out-
put logic value of the Port pin. Open-drain outputs have the high side driver disabled; therefore, they only
drive the Port pad to GND when the output logic value is ‘0’ and become high impedance inputs (both high
low drivers turned off) when the output logic value is ‘1’.
When a digital I/O cell is placed in the high impedance state, a weak pull-up transistor pulls the Port pad to
the VDD supply voltage to ensure the digital input is at a defined logic state. Weak pull-ups are disabled
when the I/O cell is driven to GND to minimize power consumption, and they may be globally disabled by
setting WEAKPUD to ‘1’. The user should ensure that digital I/O are always internally or externally pulled
or driven to a valid logic state to minimize power consumption. Port pins configured for digital I/O always
read back the logic state of the Port pad, regardless of the output logic value of the Port pin.
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