Frame format of 3 Important protocol

These three — UART, I²C, and SPI — are the most commonly asked in embedded interviews. Let’s go step by step and focus on frame format for each:

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UART (Universal Asynchronous Receiver Transmitter)

Frame Format

| Start bit (0) | Data bits (5–9) | Optional Parity bit | Stop bit (1 or 2) |

So, one byte of user data takes around 10 bits to transmit.

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Speed

Parameter	Typical Value
Baud Rate	9600 – 115200 bps (standard)
Max (practically)	~1 Mbps (depends on MCU, cable, noise)

• UART is asynchronous → needs precise baud rate match between TX and RX.
• Slower compared to SPI and I²C.

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Data Amount

• Data is sent byte-by-byte (8 bits + overhead).
• There’s no fixed limit — it depends on how long the receiver keeps reading.
• Commonly used for command-response or logging, not large data streams.

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I²C (Inter-Integrated Circuit)

Frame Format

Each transmission has:

| Start | Address (7/10 bit) | R/W bit | ACK | Data byte | ACK | ... | Stop |

• Data sent in bytes (8 bits).
• Each byte followed by ACK/NACK from receiver.

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Speed

Mode	Max Clock Speed
Standard Mode	100 kHz
Fast Mode	400 kHz
Fast Mode Plus	1 MHz
High-Speed Mode	3.4 MHz
Ultra-Fast Mode	5 MHz (one-direction)

• Speed limited by open-drain design (shared bus with pull-ups).
• Ideal for short-distance, low-speed sensors.

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Data Amount

• Typically used for control data, not bulk streaming.
• Usually 1–32 bytes per transaction (some chips support burst reads up to 255 bytes).
• Max transaction length is protocol-dependent, not hardware-limited.

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SPI (Serial Peripheral Interface)

Frame Format

SPI doesn’t define a fixed frame — it’s purely:

MOSI ↔ MISO synchronized with SCLK (CPOL/CPHA define timing)

• One byte = 8 clock cycles (or configurable to 16/32 bits in some MCUs).
• Full-duplex (send and receive simultaneously).

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Speed

Mode	Typical Speed
Low-End Devices	1–5 MHz
Flash Memory, Displays	10–50 MHz
High-Performance MCUs	100+ MHz possible

SPI is much faster because:

• No ACK/NACK bits.
• Dedicated lines (no bus sharing).
• Continuous streaming possible.

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Data Amount

• Can stream unlimited bytes — depends on chip select and buffer size.
• Used for high-speed data transfer (displays, sensors, memory, ADC/DAC).

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Quick Comparison Summary

Feature	UART	I²C	SPI
Lines Used	2 (TX, RX)	2 (SCL, SDA)	4 (MISO, MOSI, SCLK, SS)
Type	Asynchronous	Synchronous (half-duplex)	Synchronous (full-duplex)
Master/Slave	Peer-to-peer	Multi-master possible	Single master, multi-slave
Data per frame	8 bits + overhead	8 bits + ACK/NACK	Configurable (8/16/32 bits)
Max Speed	~1 Mbps	400 kHz–5 MHz	50–100+ MHz
Typical Use	Logging, Serial Comm	Sensors, EEPROM, RTC	Displays, Flash, ADC/DAC
Distance	Long (meters)	Short (few cm)	Very short (on PCB)
Throughput	Low	Medium	Very High
Frame Format	Start/Data/Parity/Stop	Start/Addr/RW/ACK/Data/Stop	CS + Data (per clock)

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Speed Perspective (approximate)

Let’s see how long it takes to send 1 KB (1024 bytes) of data:

Protocol	Typical Speed	Time to send 1 KB
UART @ 115200 bps	0.115 Mbps	~71 ms
I²C @ 400 kHz	0.4 Mbps	~20 ms
SPI @ 10 MHz	10 Mbps	~0.8 ms

→ So SPI is around 25× faster than I²C and 90× faster than UART at those speeds.

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Rule of Thumb

Application	Choose
Debugging, Serial Consoles	UART
Multiple slow sensors on same bus	I²C
Fast ADCs, Displays, Memory chips	SPI

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