A Slight Change Of Plan
I decided to implement support for the HC-SR04 SONAR distance sensors and not delay it to a future phase.
I’ve tested the current code on both an Arduino Uno and an STM32 Pro Black Pill STM32F411CEU6.
Some Implementation Details
Up to 6 HC-SR04 sensors are supported. The distance reports for all sensors consist of a list containing the following:
[PrivateConstants.SONAR_DISTANCE, trigger_pin, distance_value, time_stamp]
The distance value is expressed in centimeters.
Adding A New HC-SR04 Device
The Python set_pin_mode_sonar method is used to add HC-SR04 devices. It takes three parameters, the trigger pin, echo pin, and a mandatory callback function. Currently, all six HC-SR04 sensors share a common callback function. However, this may be changed in the future so that each sensor has its own callback.
When the Arduino core board receives a request to add an additional HC-SR04, the sonar_new function is called. HC-SR04 instances are identified by the trigger pin number specified for the device.
The code for managing the HC-SR04 devices is provided below:
// HC-SR04 Sonar Management
#define MAX_SONARS 6
typedef struct Sonar{
uint8_t trigger_pin;
Ultrasonic *usonic;
};
Sonar sonars[MAX_SONARS];
byte sonars_index = 0; // index into sonars struct
byte last_sonar_visited = 0;
unsigned long sonar_current_millis; // for analog input loop
unsigned long sonar_previous_millis; // for analog input loop
uint8_t sonar_scan_interval = 33 ; // Milliseconds between sensor pings.
// 29ms is about the minimum to avoid crosstalk
void sonar_new(){
if(sonars_index == MAX_SONARS){
byte report_message[1] = {SONAR_MAX_EXCEEDED};
Serial.write(report_message, 1);
return;
}
sonars[sonars_index].usonic = new Ultrasonic((uint8_t)command_buffer[0],
(uint8_t)command_buffer[1],
80000UL);
sonars[sonars_index].trigger_pin = command_buffer[0];
sonars_index++;
}
void scan_sonars(){
unsigned int distance;
if(sonars_index) {
sonar_current_millis = millis();
if (sonar_current_millis - sonar_previous_millis > sonar_scan_interval) {
sonar_previous_millis += sonar_scan_interval;
distance = sonars[last_sonar_visited].usonic->read();
byte report_message[4] = {SONAR_DISTANCE, sonars[last_sonar_visited].trigger_pin,
(byte)(distance>>8), (byte)(distance & 0xff)};
Serial.write(report_message, 4);
last_sonar_visited ++;
if(last_sonar_visited == sonars_index){
last_sonar_visited = 0;
}
}
}
}
Reading The HC-SRO4 Devices
The sensors are read sequentially, with 33 milliseconds between reading each sensor to ensure that they do not interfere with one another.
Please note that the more HC-SR04 devices added, the longer it will be before the first sonar device is reread. If there is only one HC-SR04 device, it will be read every 33 milliseconds, but if there are the full six HC-SRO4 devices enabled, a full scan of all six will take 198 milliseconds.
What Comes Next
I am evaluating using ArduinoJson to serialize the data that is transmitted across the serial link. The decision to use ArduinoJson will depend on the storage space and data transfer rate overhead added by using ArduinoJson. MessagePack with ArduinoJson will also be tested, which should lessen the overhead used with ArduinoJson in pure JSON mode.