UART is widely used to connect microcontrollers to sensors, communication modules, and other embedded devices. Although it is normally a point-to-point connection, reliable communication depends on the electrical state of the line even when no data is being transmitted.
Leaving a UART line floating is one hardware detail that can make communication problems look like firmware faults.
UART Needs a Defined Idle State
In standard, non-inverted UART communication, the line remains at logic HIGH when idle. A transition from HIGH to LOW marks the beginning of a start bit.
During normal operation, the transmitter actively drives the line. However, its behavior during reset, startup, or power-down depends on the device.
If the transmitter stops driving the line and nothing holds it HIGH, the receiver may see an undefined voltage. Electrical noise can cause transitions that the receiver interprets as possible start bits, potentially producing invalid bytes or framing errors.
The important question is therefore not only whether TX and RX are connected correctly, but also what holds the receive input at its expected level while the transmitter is inactive.
Figure: UART signal during normal transmission compared with a floating line
Why Startup and Reset Expose the Problem
Two connected devices do not necessarily become ready at the same time. A microcontroller may enable its UART receiver while the other device is still resetting or initializing its pins.
During this interval, the physical connection is present, but the transmitter may not yet be driving a valid idle level.
This situation can occur with different UART-connected devices. Whether it causes a problem depends on their pin behavior, internal biasing, and power sequence. A connection that works reliably after initialization may still behave unpredictably during startup.
An Example: UART Communication with an LTE Module
Consider a microcontroller communicating with an LTE module through AT commands. Firmware may use UART IDLE detection with DMA to receive responses of varying lengths. Some modules also send an early boot message, such as RDY, that firmware waits for before proceeding.
If the module’s TX pin remains in a high-impedance state while the microcontroller is already listening, the connected receive input needs another means of maintaining its idle level.
Without suitable biasing, noise on the line may be interpreted as incoming activity. Unexpected bytes can enter the receive buffer, and noise-induced activity followed by idle periods may trigger receive events at unintended times. This can complicate response handling or interfere with recognition of an expected boot message.
These symptoms can direct attention toward command parsing, DMA configuration, or interrupt handling. However, the underlying cause may be an electrically undefined line during startup.
The LTE module provides one example. The same concern applies whenever a UART transmitter stops driving the line while the receiver on the other connected device remains active.
Figure: Floating TX exposes the MCU’s RX input to noise
The Hardware Fix: Establish a Reliable Idle Level
If the line would otherwise float, an appropriate pull-up resistor can establish the expected idle state.
Connected between the UART signal and the appropriate I/O supply rail, the pull-up holds the receive input HIGH while the transmitter is in a high-impedance state. Once the transmitter becomes active, it drives the signal normally.
First, check the connected devices’ datasheets to determine whether internal biasing already provides a reliable idle level, including during reset. An external pull-up is only needed if the existing arrangement does not adequately define the line.
A resistor around 10 kΩ may be suitable in some designs, but the value should be selected according to input leakage, output drive capability, and the devices’ electrical specifications.
The pull-up must also connect to a compatible I/O voltage rail. Check power sequencing carefully: if that rail remains powered while a connected device is off, the resistor could feed current into an unpowered pin. Follow the device’s specified limits and power-off requirements.
Figure: Pull-up keeps the UART line HIGH when TX is inactive
Check the Line Before Changing the Firmware
Unexpected UART bytes, startup failures, and response-handling problems can lead engineers to look first at drivers, interrupts, DMA configuration, or parsing logic.
The electrical state deserves attention, too. Check whether the transmitter drives a valid idle level throughout startup and resets and whether the receive input remains defined whenever it does not.
This check should cover each direction of the connection independently. Either device may temporarily stop driving its TX pin, leaving the other device’s RX input dependent on the available biasing.
A point-to-point connection alone does not guarantee a stable signal. Reliable UART communication starts with a defined receive level, including the moments before the first valid byte arrives.
If you are working on an embedded product and are experiencing UART communication issues, we are happy to help. Feel free to Contact Us for hardware debugging, firmware development, and embedded system design support.
