C8051F045 View Datasheet(PDF) - Silicon Laboratories

Part Name

Description

Manufacturer

C8051F045

Mixed Signal ISP Flash MCU Family

Silicon Laboratories 

C8051F040/1/2/3/4/5/6/7

a digital input by setting P3MDOUT.7 to a logic 0, which selects open-drain output mode, and P3.7 to a

logic 1, which disables the low-side output driver.

If the Port pin has been assigned to a digital peripheral by the Crossbar and that pin functions as an input

(for example RX0, the UART0 receive pin), then the output drivers on that pin are automatically disabled.

17.1.4. Weak Pullups

By default, each Port pin has an internal weak pullup device enabled which provides a resistive connection

(about 100 k) between the pin and VDD. The weak pullup devices can be globally disabled by writing a

logic 1 to the Weak Pullup Disable bit, (WEAKPUD, XBR2.7). The weak pullup is automatically deactivated

on any pin that is driving a logic 0; that is, an output pin will not contend with its own pullup device. The

weak pullup device can also be explicitly disabled on Ports 1, 2, and 3 pin by configuring the pin as an

Analog Input, as described below.

17.1.5. Configuring Port 1, 2, and 3 Pins as Analog Inputs

The pins on Port 1 can serve as analog inputs to the ADC2 analog MUX (C8051F040/1/2/3 only), the pins

on Port 2 can serve as analog inputs to the Comparators, and the pins on Port 3 can serve as inputs to

ADC0. A Port pin is configured as an Analog Input by writing a logic 0 to the associated bit in the PnMDIN

registers. All Port pins default to a Digital Input mode. Configuring a Port pin as an analog input:

1. Disables the digital input path from the pin. This prevents additional power supply current from

being drawn when the voltage at the pin is near VDD / 2. A read of the Port Data bit will return

a logic 0 regardless of the voltage at the Port pin.

2. Disables the weak pullup device on the pin.

3. Causes the Crossbar to “skip over” the pin when allocating Port pins for digital peripherals.



Note that the output drivers on a pin configured as an Analog Input are not explicitly disabled. Therefore,

the associated PnMDOUT bits of pins configured as Analog Inputs should explicitly be set to logic 0

(Open-Drain output mode), and the associated Port Data bits should be set to logic 1 (high-impedance).

Also note that it is not required to configure a Port pin as an Analog Input in order to use it as an input to

the ADC’s or Comparators; however, it is strongly recommended. See the analog peripheral’s correspond-

ing section in this datasheet for further information.

Rev. 1.5

207

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