Insight: Peripheral Confusion Handbook – You Thought It Was SPI, But It Was GPIO All Along
Insight: Peripheral Confusion Handbook – You Thought It Was SPI, But It Was GPIO All Along
It starts out simple. You wire up your sensor. You look at the pinout. You pick SPI mode 0, maybe mode 3. You fire up your code, hit run, and wait for that beautiful stream of hex data to roll across your serial monitor.
Nothing.
You get zeroes. Or worse—every response is 0xFF. Maybe it hangs. Maybe it crashes. Maybe it kind of works, but not in a way you trust. You stare at the wiring diagram. You stare at the datasheet. You go back to your code and change the mode again. Still nothing.
Welcome to Peripheral Confusion—where SPI, I2C, UART, PWM, and GPIO all pretend to be friends… until one of them stabs you in the back.
Nothing.
You get zeroes. Or worse—every response is 0xFF. Maybe it hangs. Maybe it crashes. Maybe it kind of works, but not in a way you trust. You stare at the wiring diagram. You stare at the datasheet. You go back to your code and change the mode again. Still nothing.
Welcome to Peripheral Confusion—where SPI, I2C, UART, PWM, and GPIO all pretend to be friends… until one of them stabs you in the back.
When a Peripheral Gets Stolen by the System
On platforms like Raspberry Pi, ESP32, and many other dev boards, certain pins are multifunctional. GPIO18 might be SPI_CLK. Or PWM. Or UART TX. It depends on what overlay or firmware initialization decides to claim it.
So you think you’ve got SPI working—but Linux decided that pin is now audio out. Or Bluetooth claimed the UART you thought was free. On microcontrollers, it’s usually less dramatic, but you’ll still run into conflicts—especially if the pins you picked for SPI are also tied to JTAG or onboard flash.
And heaven help you if the documentation says “these pins are shared with onboard peripherals.” That’s dev board code for “these pins are haunted.”
Pin Multiplexing and the Curse of Defaults
Many platforms use pin multiplexers (MUX) to assign functions. But here’s the twist: you often have to explicitly set the function. And if you don’t? You’re toggling a GPIO that’s just sitting there looking confused.
Some boards default to I2C. Some default to nothing. Some get configured by device tree overlays or SDK functions you haven’t called yet. And some just sit in tristate purgatory until you properly initialize them.
Even worse, on some boards the pin might technically be available—but it’s being dragged low or high by a soldered-on pull resistor or another component on the board. So you’re debugging SPI, but your real issue is a passive resistor you didn’t know existed.
On platforms like Raspberry Pi, ESP32, and many other dev boards, certain pins are multifunctional. GPIO18 might be SPI_CLK. Or PWM. Or UART TX. It depends on what overlay or firmware initialization decides to claim it.
So you think you’ve got SPI working—but Linux decided that pin is now audio out. Or Bluetooth claimed the UART you thought was free. On microcontrollers, it’s usually less dramatic, but you’ll still run into conflicts—especially if the pins you picked for SPI are also tied to JTAG or onboard flash.
And heaven help you if the documentation says “these pins are shared with onboard peripherals.” That’s dev board code for “these pins are haunted.”
Pin Multiplexing and the Curse of Defaults
Many platforms use pin multiplexers (MUX) to assign functions. But here’s the twist: you often have to explicitly set the function. And if you don’t? You’re toggling a GPIO that’s just sitting there looking confused.
Some boards default to I2C. Some default to nothing. Some get configured by device tree overlays or SDK functions you haven’t called yet. And some just sit in tristate purgatory until you properly initialize them.
Even worse, on some boards the pin might technically be available—but it’s being dragged low or high by a soldered-on pull resistor or another component on the board. So you’re debugging SPI, but your real issue is a passive resistor you didn’t know existed.
The Fix Is Simple, If You Know Where to Look
- Read the pin map. Not just the schematic—look at the board-specific notes or overlays.
- Check which subsystems are enabled. On Raspberry Pi, look in /boot/config.txt. On ESP32, check your menuconfig or board initialization.
- Don’t assume default functionality. Just because MISO is printed on the silkscreen doesn’t mean it’s ready to go.
- Use a logic analyzer or scope. See if the signal’s even getting out of the pin.
- When in doubt—start with bare GPIO. Toggle it manually, see if it responds, and then initialize the peripheral driver.
Conclusion: Know Thy Pins
Peripheral confusion is one of the most maddening bugs in early-stage embedded dev. You think you’ve got a software bug—but your hardware is silently sabotaging you. Or your board is silently reassigning your pins. Or the silkscreen is silently lying.
So if your SPI device won’t speak, your UART won’t talk, or your I2C won’t bus, remember: the pin might not be what you think it is. Trace it. Check the docs. Pull it low and see what happens.
And if all else fails—pick a new pin, name it something absurd, and try again. That’s not superstition. That’s embedded engineering.
Peripheral confusion is one of the most maddening bugs in early-stage embedded dev. You think you’ve got a software bug—but your hardware is silently sabotaging you. Or your board is silently reassigning your pins. Or the silkscreen is silently lying.
So if your SPI device won’t speak, your UART won’t talk, or your I2C won’t bus, remember: the pin might not be what you think it is. Trace it. Check the docs. Pull it low and see what happens.
And if all else fails—pick a new pin, name it something absurd, and try again. That’s not superstition. That’s embedded engineering.
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Image: Gemini
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