C8051F850-C-GM Datasheet, PDF (204/328 Pages) Silicon Laboratories – Low-Cost 8-bit MCU Family with up to 8 kB of Flash

21.4. Port I/O Modes of Operation
Port pins are configured by firmware as digital or analog 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, both
the high and low port I/O drive circuits are explicitly disabled on all crossbar pins. Port pins configured as
digital I/O may still be used by analog peripherals; however, this practice is not recommended and may
result in measurement errors.
21.4.1. Configuring Port Pins For Analog Modes
Any pins to be used for analog functions should be configured for analog mode. When a pin is configured
for analog I/O, its weak pullup, digital driver, and digital receiver are disabled. Port pins configured for
analog functions will always read back a value of ‘0’ in the corresponding Pn Port Latch register. To
configure a pin as analog, the following steps should be taken:
1. Clear the bit associated with the pin in the PnMDIN register to ‘0’. This selects analog mode for the
pin.
2. Set the bit associated with the pin in the Pn register to ‘1’.
3. Skip the bit associated with the pin in the PnSKIP register to ensure the crossbar does not attempt
to assign a function to the pin.
21.4.2. Configuring Port Pins For Digital Modes
Any pins to be used by digital peripherals 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 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 the low-side rail 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 high-side rail to ensure the digital input is at a defined logic state. Weak pull-ups are disabled when the
I/O cell is driven low 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.
To configure a pin as digital input:
1. Set the bit associated with the pin in the PnMDIN register to ‘1’. This selects digital mode for the
pin.
2. Clear the bit associated with the pin in the PnMDOUT register to ‘0’. This configures the pin as
open-drain.
3. Set the bit associated with the pin in the Pn register to ‘1’. This tells the output driver to “drive” logic
high. Because the pin is configured as open-drain, the high-side driver is not active, and the pin
may be used as an input.
Open-drain outputs are configured exactly as digital inputs. However, the pin may be driven low by an
assigned peripheral, or by writing ‘0’ to the associated bit in the Pn register if the signal is a GPIO.
To configure a pin as a digital, push-pull output:
1. Set the bit associated with the pin in the PnMDIN register to ‘1’. This selects digital mode for the
pin.
2. Set the bit associated with the pin in the PnMDOUT register to ‘1’. This configures the pin as push-
pull.
If a digital pin is to be used as a general-purpose I/O, or with a digital function that is not part of the
crossbar, the bit associated with the pin in the PnSKIP register can be set to ‘1’ to ensure the crossbar
does not attempt to assign a function to the pin.
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