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What should I expect at UART Tx and Rx pins of 8051 when the pins are not connected, i.e hanging. Using multimeter, I found Rx pin high, and Tx pin low. Is it OK?

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    \$\begingroup\$ Why would you measure the voltage of an input pin? \$\endgroup\$ – Ignacio Vazquez-Abrams Aug 6 '13 at 5:26
  • \$\begingroup\$ Are these pins reserved exclusively for uart functionality, or multiplexed with other capabilities as is common on more recent devices? \$\endgroup\$ – Chris Stratton Aug 6 '13 at 12:41
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Tx: Transmit

A stop bit (and therefore an inactive UART Tx pin) is high by default. So the voltage on that pin will be somewhere near Vcc when unloaded and when no data is sent out of the chip.

Rx: Receive

Rx is an input and therefore carries no voltage. Depening on the input circuitry of the chip the input may be pulled low, high or floating high impedance. In either case it can barely source/sink any current.

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  • \$\begingroup\$ I want to know if any MCU that the usart TX pin is high when idle(no connection)? \$\endgroup\$ – lukeluck Dec 17 '19 at 3:46

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