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hc_sr04: Improve accuracy by measuring microseconds #367

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Feb 28, 2023
Merged

hc_sr04: Improve accuracy by measuring microseconds #367

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27 changes: 13 additions & 14 deletions applications/plugins/hc_sr04/hc_sr04.c
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Original file line number Diff line number Diff line change
Expand Up @@ -3,6 +3,7 @@
// Ported and modified by @xMasterX

#include <furi.h>
#include <furi_hal.h>
#include <furi_hal_power.h>
#include <furi_hal_console.h>
#include <gui/gui.h>
Expand All @@ -26,7 +27,7 @@ typedef struct {
NotificationApp* notification;
bool have_5v;
bool measurement_made;
uint32_t echo; // ms
uint32_t echo; // us
float distance; // meters
} PluginState;

Expand Down Expand Up @@ -72,7 +73,7 @@ static void render_callback(Canvas* const canvas, void* ctx) {

FuriString* str_buf;
str_buf = furi_string_alloc();
furi_string_printf(str_buf, "Echo: %ld ms", plugin_state->echo);
furi_string_printf(str_buf, "Echo: %ld us", plugin_state->echo);

canvas_draw_str_aligned(
canvas, 8, 38, AlignLeft, AlignTop, furi_string_get_cstr(str_buf));
Expand Down Expand Up @@ -110,9 +111,11 @@ static void hc_sr04_state_init(PluginState* const plugin_state) {
}
}

float hc_sr04_ms_to_m(uint32_t ms) {
const float speed_sound_m_per_s = 343.0f;
const float time_s = ms / 1e3f;
float hc_sr04_us_to_m(uint32_t us) {
//speed of sound for 20°C, 50% relative humidity
//331.3 + 20 * 0.606 + 50 * 0.0124 = 0.034404
const float speed_sound_m_per_s = 344.04f;
const float time_s = us / 1e6f;
const float total_dist = time_s * speed_sound_m_per_s;
return total_dist / 2.0f;
}
Expand Down Expand Up @@ -147,27 +150,23 @@ static void hc_sr04_measure(PluginState* const plugin_state) {
furi_delay_ms(10);
furi_hal_gpio_write(&gpio_usart_tx, false);

// TODO change from furi_get_tick(), which returns ms,
// to DWT->CYCCNT, which is a more precise counter with
// us precision (see furi_hal_cortex_delay_us)

const uint32_t start = furi_get_tick();

while(furi_get_tick() - start < timeout_ms && furi_hal_gpio_read(&gpio_usart_rx))
;
while(furi_get_tick() - start < timeout_ms && !furi_hal_gpio_read(&gpio_usart_rx))
;

const uint32_t pulse_start = furi_get_tick();
const uint32_t pulse_start = DWT->CYCCNT;

while(furi_get_tick() - start < timeout_ms && furi_hal_gpio_read(&gpio_usart_rx))
;
const uint32_t pulse_end = DWT->CYCCNT;

const uint32_t pulse_end = furi_get_tick();
//FURI_CRITICAL_EXIT();

plugin_state->echo = pulse_end - pulse_start;
plugin_state->distance = hc_sr04_ms_to_m(pulse_end - pulse_start);
plugin_state->echo = (pulse_end - pulse_start) / furi_hal_cortex_instructions_per_microsecond();
plugin_state->distance = hc_sr04_us_to_m(plugin_state->echo);
plugin_state->measurement_made = true;

//furi_hal_light_set(LightRed, 0x00);
Expand Down Expand Up @@ -270,4 +269,4 @@ int32_t hc_sr04_app() {
delete_mutex(&state_mutex);

return 0;
}
}