C8051F70X_0910 Datasheet, PDF (166/290 Pages) Silicon Laboratories – Mixed Signal ISP Flash MCU Family

C8051F70x/71x
26.1. Port I/O Modes of Operation
Port pins P0.0 - P6.5 use the Port I/O cell shown in Figure 26.2. 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.
26.1.1. Port Pins Configured for Analog I/O
Any pins to be used as Comparator or ADC input, Capacitive Sense input, external oscillator input/output,
VREF output, or AGND connection should be configured for analog I/O (PnMDIN.n = 0). When a pin is
configured for analog I/O, its weak pullup, digital driver, and digital receiver are disabled. Port pins config-
ured 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.
26.1.2. 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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