TCA9548A i2c multiplexer tutorial: 8 sensors, one address

The idea

How the TCA9548A works: a railway switch for your bus

The TCA9548A sits on your i2c bus at 0x70 (selectable up to 0x77 via the A0–A2 pins) and fans out to eight isolated downstream channels. Whatever channel you open gets electrically connected to the host bus; the other seven don't exist as far as the controller is concerned. Eight BME280s can all keep their default 0x76 — they just live on different channels, and since each channel is isolated, the chip also resets the accumulated pull-up and capacitance load of a big chain. It's the heavy-duty answer from our address-conflict guide, in practice.

Wiring is unremarkable: VCC, GND, SDA, SCL on the host side; each channel exposes its own SDn/SCn pair. One select byte does everything — bit N opens channel N.

Arduino

TCA9548A Arduino code: the four lines that matter

#include <Wire.h>
#define MUX_ADDR 0x70

void muxSelect(uint8_t channel) {       // 0-7
  Wire.beginTransmission(MUX_ADDR);
  Wire.write(1 << channel);             // bitmask: one bit per channel
  Wire.endTransmission();
}

That's the entire driver. Reading four same-address sensors becomes:

for (uint8_t ch = 0; ch < 4; ch++) {
  muxSelect(ch);
  float t = readSensorAt0x76();   // your normal sensor code, unchanged
  Serial.printf("sensor %u: %.2f\n", ch, t);
}

The select is sticky — it persists until the next write — so libraries that know nothing about multiplexers work untouched: select the channel, then call the library exactly as in our Arduino Wire tutorial. One habit worth keeping: initialise each sensor's library object after selecting its channel the first time.

MicroPython

TCA9548A in MicroPython: same trick, Python flavour

from machine import I2C, Pin
i2c = I2C(0, sda=Pin(0), scl=Pin(1))

def mux_select(ch):
    i2c.writeto(0x70, bytes([1 << ch]))

for ch in range(4):
    mux_select(ch)
    print(ch, i2c.scan())   # each scan sees only that channel (plus 0x70)

That scan loop is also the best debugging tool the part has: channel by channel, you see exactly which devices are alive where. (Full MicroPython bus basics in the MicroPython i2c tutorial.)

Gotchas

Three TCA9548A gotchas that bite

The mux answers scans too. 0x70 always appears in a bus scan alongside the selected channel's devices. Don't mistake it for a sensor — and keep 0x70–0x77 free of other parts unless you've moved the mux's address.

Selection is per-controller-reset, not per-transaction. After a reboot the mux comes up with all channels closed — if your code "worked yesterday," check that setup selects a channel before the first sensor call.

Open channels sum their loads. Opening several channels at once reconnects all their pull-ups and wiring capacitance to one bus — long chains can sag back into rise-time trouble. One channel at a time is the reliable default.