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41 lines
1.9 KiB
Markdown
41 lines
1.9 KiB
Markdown
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# Benchmark for `periph/gpio_ll`
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This application will generate a square wave on two output pins with a phase
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difference of zero between them using both the `periph/gpio` API (as reference)
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and the `periph/gpio_ll` API. You can use a logic analyzer or scope to verify
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that the square waves are indeed simultaneous (no phase difference) and their
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frequency. Note that with the pin based `periph/gpio` API a phase difference is
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expected, but not for the port based `periph/gpio_ll` API.
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In addition, a timer is used to measure the average frequency over 50,000
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square wave periods. The overhead of the loop is estimated and a compensated
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frequency (that would be achievable only by unrolling the loop) is calculated.
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Both frequencies are printed, in addition to the number of CPU cycles per wave
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period. The optimal value is 2 CPU cycles (signal is 1 cycle high and 1 cycle
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low).
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## Configuration
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Configure in the `Makefile` or set via environment variables the number of
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the GPIO port to use via the `PORT_OUT` variable. The `PIN_OUT_0` and
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`PIN_OUT_1` variables select the pins to use within that GPIO port. If possible,
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choose a GPIO port that is fully broken out to pins of your board but left
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unconnected. That way you can connect a scope or a logic analyzer to verify
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the output.
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Note that the test using `gpio_ll_write()` might cause changes to unrelated pins
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on the `PORT_OUT` GPIO port, by restoring their value to what it was at the
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beginning of the benchmark.
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## FAQ
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Why are 4 functions calls used for `periph/gpio`, but only 2 for
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`periph/gpio_ll`? This isn't fair!
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Since in a port based APIs multiple pins can be accessed at once, only two
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accesses are needed (one for the high and one for the low part of each square
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wave period). In the pin based `periph/gpio` API, two accesses are needed per
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pin. This unfair advantage in speed is one of the reasons we want a low level
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port based API in RIOT - in addition to a more convenient to use and high level
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pin based API.
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