51 lines
1.6 KiB
C
51 lines
1.6 KiB
C
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/** \file
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\brief Delay routines, ARM specific part.
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*/
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#if defined TEACUP_C_INCLUDE && defined __ARM_STM32F411__
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#include "mbed-stm32f4xx.h" // For __ASM() and ...
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/** Delay in microseconds.
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\param delay Time to wait in microseconds.
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Execution times on ARM aren't as predictable as they could be, because
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there's a code prefetch engine which can change timings depending on the
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position of the code in Flash. We could use the System Tick Timer for this
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task, but this timer is probably better used for more important tasks.
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delay_us() and delay_ms() are used only rarely and not in a way which would
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require high precision.
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Nevertheless, calibrated on the oscilloscope. Measured accuracy:
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delay_us(10) ...(100) ...(1000) ...(10000) ...(65000)
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96 MHz 10.23 us 99.3 us 0.994 ms 9.93 ms 64.5 ms
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CAUTION: this currently works for a 96 MHz clock, only! As other clock rates
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appear, there's more math neccessary, see the AVR version. Or simply
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a second implementation.
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*/
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void delay_us(uint16_t delay) {
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__ASM (".balign 16"); // No gambling with the prefetch engine.
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while (delay) {
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__ASM volatile (
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" nop \n\t" // One nop before the loop slows about 2%.
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" movs r7, #9 \n\t" // One more loop round slows about 20%
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"1: nop \n\t"
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" sub r7, #1 \n\t"
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" cmp r7, #0 \n\t"
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" bne 1b \n\t"
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:
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:
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: "r7", "cc"
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);
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delay--;
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}
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}
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#endif /* defined TEACUP_C_INCLUDE && defined __ARM_STM32F411__ */
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