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5b600d3a8e
Add an API to search for the frequency supported by a timer that is closest to the given target frequency. This is in fact non-trivial to get right, as pre-scaler registers can be 16 bit or even 32 bit in size, making a naive loop over all possible pre-scalers too expensive (computationally). Co-authored-by: mguetschow <mikolai.guetschow@tu-dresden.de>
91 lines
2.6 KiB
C
91 lines
2.6 KiB
C
/*
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* Copyright (C) 2017 Freie Universität Berlin
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*
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* This file is subject to the terms and conditions of the GNU Lesser
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* General Public License v2.1. See the file LICENSE in the top level
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* directory for more details.
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*/
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/**
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* @ingroup drivers_periph_timer
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* @{
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*
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* @file
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* @brief Shared peripheral timer code
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*
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* @author Hauke Petersen <hauke.petersen@fu-berlin.de>
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*
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* @}
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*/
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#include "periph/timer.h"
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#include "irq.h"
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#ifndef PERIPH_TIMER_PROVIDES_SET
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int timer_set(tim_t dev, int channel, unsigned int timeout)
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{
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unsigned int state = irq_disable();
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int res = timer_set_absolute(dev, channel, timer_read(dev) + timeout);
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irq_restore(state);
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return res;
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}
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#endif
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#ifdef MODULE_PERIPH_TIMER_QUERY_FREQS
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__attribute__((weak))
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uword_t timer_query_channel_numof(tim_t dev)
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{
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(void)dev;
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return TIMER_CHANNEL_NUMOF;
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}
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uint32_t timer_get_closest_freq(tim_t dev, uint32_t target)
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{
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uint32_t freq;
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uword_t idx_max = timer_query_freqs_numof(dev) - 1;
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/* use binary search to find one of the three possibilities:
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* 1. If an exact match is possible: The exact match is found
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* 2. Otherwise the closest frequency is found, which might be either
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* a) the highest frequency below the target, or
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* b) the lowest frequency above the target
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*/
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uword_t idx_lower = 0;
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uword_t idx_upper = idx_max;
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while (idx_lower != idx_upper) {
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uword_t idx_mid = (idx_lower + idx_upper) >> 1;
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freq = timer_query_freqs(dev, idx_mid);
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if (freq > target) {
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idx_lower = idx_mid + 1;
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}
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else {
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idx_upper = idx_mid;
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}
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}
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freq = timer_query_freqs(dev, idx_lower);
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if ((freq < target) && (idx_lower > 0)) {
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/* binary search yielded the closest frequency below the target. But
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* maybe the closest frequency above the target is actually better. */
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uint32_t diff = target - freq;
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uint32_t alternative = timer_query_freqs(dev, idx_lower - 1);
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if (target + diff > alternative) {
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/* the alternative is better */
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return alternative;
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}
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}
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else if ((freq > target) && (idx_lower < idx_max)) {
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/* Got a frequency above the target, maybe the one below would be
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* a closer match */
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uint32_t diff = freq - target;
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uint32_t alternative = timer_query_freqs(dev, idx_lower + 1);
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if (target < alternative + diff) {
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return alternative;
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}
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}
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/* either we got an exact match, or the other candidate was no closer */
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return freq;
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}
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#endif
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