What you need to notice on the scope
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01
Look for the frame
Every letter travels inside a tidy 10-bit package called a frame: a LOW start bit to wake up the receiver, eight data bits that carry the letter, and a HIGH stop bit to finish. Watch for the first time the line drops from HIGH to LOW - that edge marks the beginning of a new character.
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02
Smallest value first
Right after the start bit comes the least significant bit (worth 1). Then 2, 4, 8, and so on up to 128. UART sends the smallest place value first, so read the bits from left to right and add the place values wherever the trace is HIGH.
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03
Sample in the middle
Both devices agree on speed (called baud rate) before they start. At 9600 baud each bit lasts about 104 microseconds. The receiver checks the voltage near the middle of each bit slot, where the signal is steady and clean.
Digital basics
A wire only knows on and off
Voltage on a digital wire is either near 5 V or near 0 V. Nothing in between counts. Engineers call these two states HIGH and LOW, and they stand for the binary digits 1 and 0.
Every message, photo, song, and video your devices exchange is built from long streams of these two states. That is the entire alphabet of digital electronics.
How letters travel
Letters ride as numbers
The wire cannot carry the letter 'i' directly. It can only carry HIGHs and LOWs. So the sender looks up 'i' in a table called ASCII and finds the number 105.
105 in binary is 01101001. Those eight bits travel down the wire. The receiver looks up 105 in the same table and gets the letter back.
One tidy package
One letter = one 10-bit frame
UART wraps each byte in a neat package called a frame: a LOW start bit to wake the receiver, the eight data bits of the letter, and a HIGH stop bit to finish cleanly.
Press NEXT BIT to step through all ten bits of the frame one at a time. You are watching a single character cross the wire.