C8051T630 Datasheet, PDF (110/221 Pages) Silicon Laboratories – Mixed-Signal Byte-Programmable EPROM MCU

C8051T630/1/2/3/4/5
20.1. Port I/O Modes of Operation
Port pins 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).
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 inputs 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 digital event capture func-
tions, 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/DC+ or GND supply rails based on the
output 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 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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