Sr04.cs

//
// Copyright (c) .NET Foundation and Contributors
// See LICENSE file in the project root for full license information.
//

using System;
using System.Threading;
using nanoFramework.Hardware.Esp32.Rmt;

namespace nanoFramework.device.sensor
{
	/// <summary>
	/// This class is used to interface with SR04 devices and measure distance.
	/// HC-SR04 , SN-SR04 etc.
	/// 
	/// Range is 2cm to 450cm
	/// 
	/// When connecting to ESP32 be aware that the output is 5V and a level shifter should be used
	/// As a minimum connect a resistor between SR04 output(ECHO) and ESP32 pin. (10K) to limit current.
	/// For testing it can be connected directly.
	/// 
	/// If a 3.3V SR04 device is used then no resistor/level shifter is required ( HC-SR04P )
	/// </summary>
	public class Sr04
	{
		ReceiverChannel _rxChannel;
		TransmitterChannel _txChannel;
		RmtCommand _txPulse;

		const float _speedOfSound = 340.29F;

		/// <summary>
		/// Create an instance of the SR04 device class
		/// </summary>
		/// <param name="TxPin">GPIO pin number for trigger pin</param>
		/// <param name="RxPin">GPIO pin number of echo pin</param>
		public Sr04(int TxPin, int RxPin)
		{
			// Set-up TX & RX channels
			// We need to send a 10us pulse to initiate measurement
			var txChannelSettings = new TransmitChannelSettings(pinNumber: TxPin)
			{
				// 1us clock ( 80Mhz / 80 ) = 1Mhz
				ClockDivider = 80,
				EnableCarrierWave = true,
				IdleLevel = false,
			};

			_txChannel = new TransmitterChannel(txChannelSettings);
			// we only need 1 pulse of 10 us high
			_txChannel.AddCommand(new RmtCommand(10, true, 0, false));

			// The received echo pulse width represents the distance to obstacle
			// 150us to 38ms
			var rxChannelSettings = new ReceiverChannelSettings(pinNumber: RxPin)
			{
				// 1us clock ( 80Mhz / 80 ) = 1Mhz
				ClockDivider = 80,

				// filter out 200Us / noise
				EnableFilter = true,
				FilterThreshold = 200,

				// 40ms based on 1us clock
				IdleThreshold = 40_000,

				// 100 millisecond timeout is enough
				ReceiveTimeout = TimeSpan.FromMilliseconds(100)
			};

			_rxChannel = new ReceiverChannel(rxChannelSettings);
		}

		
		/// <summary>
		/// Get the distance of object from SR04 device
		/// </summary>
		/// <returns>Distance in meters or -1 if out of range</returns>
		public float GetDistance()
		{
			RmtCommand[] response = null;

			_rxChannel.Start(true);

			// Send 10us pulse
			_txChannel.Send(false);

			// Try 5 times to get valid response
			for (int count = 0; count < 5; count++)
			{
				response = _rxChannel.GetAllItems();
				if (response != null)
					break;

				// Retry every 60 ms
				Thread.Sleep(60);
			}

			_rxChannel.Stop();

			if (response == null)
				return -1;

			// Echo pulse width in micro seconds
			int duration = response[0].Duration0;

			// Calculate distance in meters
			// Distance calculated as  (speed of sound) * duration(meters) / 2 
			return _speedOfSound * duration / (1000000 * 2);
		}
	}
}