Bluetooth Module: The Serial Highways Inside the Radio
UART, SPI, I2C—Why They Still Matter
A Bluetooth module may speak 2.4 GHz on the air, but on the PCB it chats through old-school copper: UART for console confessions, SPI for high-speed registers, I2C for sensor gossip. These buses are the module’s native tongue; pick the wrong one and your “wireless” project becomes a wired headache.
UART: The Lazy Serial Line
TX and RX, usually at 3.3 V, 115200 baud by default. One start bit, eight data bits, no parity, one stop bit—done. UART is perfect for AT-command bridges: send “AT+ADVSTART” and the module replies “OK” without you writing a single line of C. Downsides? No chip-select, no flow control, and baud-rate drift if both sides don’t share the same crystal. Use it when speed is secondary and human readability counts.
SPI: The Speed Demon
Four wires—MOSI, MISO, SCLK, CS—clock up to 32 MHz on modern modules. Registers burst in 8-bit frames, perfect for streaming sensor data or driving a 320×240 TFT. The catch: one master, one slave per chip-select, and a rats-nest of traces if you chain too many devices. Choose SPI when you need raw throughput and don’t mind dedicating a GPIO to every peripheral.
I2C: The Social Bus
Two wires—SDA and SCL—carry addressing, data, and handshaking for up to 127 devices. It’s ideal for temperature sensors, EEPROMs, or RTCs that only need a few kilobits per second. The penalty: shared bus means shared collisions; a single misbehaving slave can hold the line low and crash the whole conversation. Keep I2C for low-speed, multi-drop networks where wiring simplicity beats raw speed.
Real-World Scenario 1: UART Fitness Band
A nRF52832 module rides inside a $40 fitness band. UART at 921600 baud streams raw accelerometer data to the phone; the phone sends AT commands to start/stop heart-rate collection. No custom firmware on the module—just a bridge. Five wires total: TX, RX, VCC, GND, and a reset line. The band lasts five years on a 220 mAh cell because the radio sleeps between bursts.
Real-World Scenario 2: SPI Smart Lock
A nRF52840 module powers a $120 smart lock. SPI at 16 MHz talks to an OLED display and a fingerprint sensor. The module acts as master, clocking 320 kbit/s of display data while simultaneously sampling 128 kbit/s fingerprint frames. AES-256 is hardware-accelerated on the 52840, so the lock unlocks in <200 ms while the radio advertises “open” for 30 s. The same SPI bus also flashes new firmware over-the-air, proving that speed and security can share the same copper.
Choosing Your Highway
Pick UART when you need a quick bridge and don’t care about speed. Pick SPI when you have one fast peripheral and plenty of GPIOs. Pick I2C when you have many slow sensors and want minimal wiring. Whatever you choose, remember: the bus is the bottleneck long before the 2.4 GHz air-interface ever is.