#32 Writting the README file

projects/01bsp_us_ranging/README.md

@@ -1,4 +1,29 @@
-# Ultrasound ranging board support package usage example
+# Ultrasound ranging board support package usage example using HC-SR04 sensor on the DotBot board
+
+
+This is a short example of how to do Ultrasound ranging with HC-SR04 sensor on the DotBot board.
+A trigger pulse for the US ranging is sent every 200 ms with the duration of 10 us. 
+The US range is captured on the US read pin where the pulse duration corresponds to a distance measured.
+We just need to divide the reading in us with 57 to get the distance to an object in mm. 
+
+This code currently works on nRF52840. To use the code on DotBot you need to define different GPIO for US sensor ON and READ pin (trigger and echo) in hc_sr04.c
+
+The US ranging code is developed using GPIOTE, TIMERs and PPI. 
+
+- The GPIOTE is configured with three channels 
+	- Channel 0 for triggering the US (toggling the Trigger pin on HC-SR04 from Low to High)
+	- Channel 1 for the US read pin to create an event when there is a rising edge
+	- Channel 2 for the US read pin to create an event when there is a falling edge
+
+- TIMERs:
+	- TIMER0 is used to control the trigger pulse (COMPARE[0] register is set with the pulse offset (200 ms) and COMPARE[1] with the pulse duration (10 us))
+	- TIMER1 is used to capture the pulse duration on the US read pin, which corresponds to the distance
+
+- PPI chains the TIMER events and GPIOTE tasks to start ranging 
+
+- The result of the US ranging is passed in the us_callback as the us_reading parameter.
+
+
+
 
 
-## Not Implemented