Improved accuracy for hc_sr04 ultrasonic device!

gpio/hc_sr04/README.md

@@ -1,4 +1,26 @@
-## Original version from: [SquachWare hc_sr04](https://github.com/skizzophrenic/SquachWare-CFW/tree/dev/applications/plugins/hc_sr04)
+# HC_SR04
 
+## Description
+The original version is from [SquachWare hc_sr04](https://github.com/skizzophrenic/SquachWare-CFW/tree/dev/applications/plugins/hc_sr04).
 
-## (5V -> VCC) / (GND -> GND) / (13|TX -> Trig) / (14|RX -> Echo)
+This version modifies the code to use furi_hal_cortex_timer_get(0) to
+obtain a high precision counter.  We also tune the calculations based on measured observations.  The display shows 2 digits after the decimal point, displaying in both cm and inches.
+
+## Pins
+- (Flipper -> HC_SR04 device)
+- (5V -> VCC)
+- (GND -> GND)
+- (13|TX -> Trig)
+- (14|RX -> Echo)
+
+## Code changes
+- #include <furi_hal_cortex.h>
+  - This has the definitions for FuriHalCortexTimer and furi_hal_cortex_timer_get(...)
+- FuriHalCortexTimer beginTimer = furi_hal_cortex_timer_get(0);
+  - This gets the current time in beginTimer.start.
+  - The offset is 0 (we could use offset ).
+- furi_string_printf(str_buf, "Distance: %0.2f cm", (double)plugin_state->distance);
+  - This rounds the output to the nearest hundredths place.
+- hc_sr04_duration_to_cm(...)
+  - I have no idea why total_dist needs to be multiplied by 1.588, but doing so seems to make all of the measurements correct.
+  - The code overhead, delays in IN/OUT, etc. seem to take the same time as sound tranvelling for 0.497 cm there and back; so we add this fixed constant.  If you Flipper was faster/slower you would need to adjust this constant.