eslab

• This is the repository for the assignments for Embedded System in 2017 fall.
• Two assignments are in this repository, one is in folder BLE while the other in I2C.

BLE

Description

The goal of this assignment is to establish a BLE service with C, python, or Node.js and transfer data between the central and the peripheral. We choose to use Node.js because there are two powerful libraries: bleno and noble, which can act as peripheral and central respectively. We set up the service to transfer the real time acceleration read from adxl-345(a 3-axis accelerometer) on the peripheral to the central.

• The adxl345 and the peripheral communicate via I2C.
• The adxl345 is manipulated by adxl345-sensor.
• The acceleration given by the adxl345 is in float type, but the read/write between the peripheral and the central seems to be UInt16BE only. Also, the negative numbers have trouble when transfering. We solve the above issues by converting the original data into a set of two numbers - one represents positive or not(1 or 0), one represents the value. Multiply the value by 100 and cast it into UInt16BE without lost of accuracy.

File Description

central

This folder contains the needed component and directory of noble.

• central_acc/central.js
  • When the BLE device is powered on, the central will first scan for BLE service. Once find any, it will stop scanning, connect to it and check the uid of the service and characteristics(the uid of both have been recorded in this file in advance, in order not to connect to wrong services). If there's no characteristics missed, the central will start to read data from the peripheral every two seconds.

peripheral

This folder contains the needed component and directory of bleno.

• peripheral_acc/peripheral.js
  • When the BLE device is powered on, the peripheral will advertising itself and set up the service, waiting for connection. If onReadRequest of AccCharacteristicis triggered, the acceleration will be read out, converted to readable format, and sent back to the request device.

I2C

Description

There are two parts of this assignment, one is to modify the reference code of the communication program for RPi and Arduino using the asynchronous IO techniques, the other is to read the 3D accelerator value periodically in 200 Hz from an ADXL345 module to RPi using C and python, respectively.

File Description

async

The modified communicaiton program let a user to send an integer whenever from the RPi side or the Arduino side via I2C to the other. There are four wires needed - three for I2C, one for interrupt to rpi(connected from Arduino UNO Digital 8 to RPi3 GPIO 17).

i2c_arduino_to_rpi.ino (Arduino side)

• When there is a Serail input from a user, serialEvent() will be called as an ISR and call ISRR().

void ISRR(){
 digitalWrite(8, LOW);
 Serial.println("LOW");
 delayMicroseconds(400);
 digitalWrite(8, HIGH);
}

This function will generate a signal to interrupt the rpi, which will send out a read requset and trigger Wire.onRequest(sendData), which complete the send process.

RPi side

• There are two version of this part, one in for_C, one in for_python.
• When GPIO 17 is triggered in the above procedure, both the ISR in the two version will be execute.

void my_ISR(void){
   if (read(file, (void*) buf, 1) == 1) {
   	temp = (int) buf[0];
   	printf("Received %d\n", temp);
   }
   else printf("Error!");
}

def my_callback(channel):
 print(bus.read_byte(address))

Both will send out a read request to the Arduino.

adxl

• adxl345.c
  • We use the adxl-345 example in Sunfounder_SuperKit_C_code_for_RaspberryPi to read the acceleration.
  • The reading period is manipulated strictly by MicrosecondsNoSleep.

  void MicrosecondsNoSleep(int delay_us){
   long int start_time;
   long int time_difference;
   struct timespec gettime_now;
  
   clock_gettime(CLOCK_REALTIME, &gettime_now);
   start_time = gettime_now.tv_nsec;
   while (1)
   {
   	clock_gettime(CLOCK_REALTIME, &gettime_now);
   	time_difference = gettime_now.tv_nsec - start_time;
   	if (time_difference < 0)
   		time_difference += 1000000000;				
   	if (time_difference > (delay_us * 1000))
   		break;
   }
   return;
  }

  This function compute the remaining time, and wait until this period ends.
• adxl345.py
• We modify the example in adxl345-python to periodically read data.