When it goes wrong

SPI has no address, no acknowledge and no error check, so a fault shows up as wrong bytes and nothing else.

Two on MISO

Only the selected peripheral may drive MISO. The W25Q128JV datasheet says its DO pin is high impedance while /CS is high. If two chip selects are low at once, two chips push on one wire and the controller reads whichever wins, and both peripherals also hear the command. Keep exactly one chip select low.

Different modes

The mode is the clock's resting level and the edge the data is read on. A controller in mode 0 reading a part that expects mode 3 gets its bytes shifted by a place. The datasheet says which modes a part takes, and the code has to name one of them.

Too fast

A BME280 takes SPI at up to 10 MHz, while an ESP32 can drive 80. Past what a part can follow, some bits arrive wrong, which gives a reading that is nearly right. The limit is the slowest part on the bus.

Chip select tied low

With one peripheral on the bus, wiring its chip select low for good saves a pin, and the TFT_eSPI driver documents it for an ILI9341 display. What is lost is the chip's only framing signal. A W25Q128JV starts every instruction on a falling /CS, so with /CS held low the next command is taken as more clocks of the read before it, and the answer is the next bytes in memory. A display that slips by one bit can only be put back in step by a chip select pulse, a reset or a power cycle.

The ESP32 side of all this is in SPI and chip select, where this course hands over.

Common mistakes

• Leaving a chip select floating at start-up. A pin not yet driven is at the mercy of the room, and that peripheral may join the transfer.
• Tying chip select low on a part that needs edges. Flash chips and sensors such as the BME280 start and end a transaction on a chip select edge.
• Mixing 3.3 V and 5 V parts without checking. The two families disagree about what a 1 is. Read logic levels, 3.3 V and 5 V before joining them.