Access Raspberry Pi GPIO with ARM64 assembly

Reading, writing, and arithmetic with the Raspberry Pi in ARM64 assembly language.

In this article, I explore the Raspberry Pi’s general purpose input/output (GPIO) system and look at how to use it to perform some basic input and output tasks with four separate programs that run simultaneously and communicate with each other. Table 1 lists the various programs discussed in this article [1].

Recon

The first program, gpiocount.asm, counts up or down in various number systems: binary, octal, decimal, and hexadecimal – or really, any number system up to base 16. The count mode is changed with a switch that causes an interrupt to a loadable kernel module. That’s the arithmetic portion of the project.

The gpiocount program writes its values to 4 bytes of memory that is shared with gpiomux.asm, which runs in the background, reads the 4 bytes written by gpiocount, and writes the bytes into a four-digit, seven-segment display. All segment lines of the four displays are wired together inside the chip so that, to see the separate digits, you must turn on the four displays one at a time, faster than the eye can detect. That takes care of the reading, writing, and arithmetic.

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