Digital Sensors, Analog Keypad 1. The use of a digital sensor

Digital Sensors, Analog Keypad 1. The use of a digital sensor
Design with Microprocessors (DMP) – Lab.6
Laboratory 6 – Digital Sensors, Analog Keypad
1. The use of a digital sensor
Most microcontroller based applications use inputs from different analog or digital sensors. Here we
will briefly study the datasheet of digital sensor, in order to see it’s characteristics, if it is suited for the desired
application and how it can be configured and interfaced to the microcontroller.
a. Read the Datasheet
We will use the Bosch BME280, a very precise environment sensor, that provides the temperature,
atmospheric pressure and air humidity. This sensor has two communication interfaces: SPI (Serial Peripheral
Interface) and I2C (Inter-Integrated Circuit). We will use the I2C interface which was studied in the previous
laboratory.
Open the datasheet from the link provide below and search for:
- Supply voltage
- Sensibility and precision of measurement
- I2C interface usage
- How to interpret the provided data
Datasheet: BST-BME280_DS001-11.pdf
b. Electrical Connections
Design with Microprocessors (DMP) – Lab.6
As you could see in the datasheet, the sensor has a maximum operating voltage of 3.6 V, and the
Arduino board is powered at 5V; the HIGH level of the digital pins is also 5V. If one would connect the sensor
directly to the Arduino board, the sensor will be damaged (burned). In order to prevent this, we will use a
Logic Level Shifter between the sensor and the Arduino board, for adapting the voltage of the I2C digital
interface. For powering up the sensor we will use the 3.3 V output from the Arduino board.
c. Read Temperature example
#include <Wire.h>
#define BME280_ADDRESS
(0x77)
// I2C address of the sensor
#define
#define
#define
#define
(0xD0)
(0xF2)
(0xF4)
(0xFA)
//
//
//
//
BME280_REGISTER_CHIPID
BME280_REGISTER_CONTROLHUMID
BME280_REGISTER_CONTROL
BME280_REGISTER_TEMPDATA
ChipID register address
Humidity configuration register address
Measurement configuration register address
Temperature data register address
typedef struct
{
uint16_t dig_T1; int16_t dig_T2; int16_t dig_T3;
uint16_t dig_P1; int16_t dig_P2; int16_t dig_P3;
int16_t dig_P7; int16_t dig_P8; int16_t dig_P9;
uint8_t dig_H1; int16_t dig_H2; uint8_t dig_H3;
} bme280_calib_data;
int16_t
dig_P4;
int16_t
dig_P5;
int16_t
dig_P6;
int16_t
dig_H4;
int16_t
dig_H5;
int8_t
dig_H6;
// Calibration factors. Usually these are read from the ROM memory of the sensor.
bme280_calib_data _bme280_calib = {28398, 26500, 50, 36953, -10772, 3024, 7550, 6, -7, 9900, -10230, 4285,
75, 368, 0, 302, 0, 30};
int32_t t_fine;
// Compensation function from the Datasheet, page 23.
// https://ae-bst.resource.bosch.com/media/_tech/media/datasheets/BST-BME280_DS001-11.pdf
float compensateTemperature(int32_t adc_T)
{
int32_t var1, var2;
adc_T >>= 4;
var1
= ((((adc_T>>3) - ((int32_t)_bme280_calib.dig_T1 <<1))) *
((int32_t)_bme280_calib.dig_T2)) >> 11;
Design with Microprocessors (DMP) – Lab.6
var2
= (((((adc_T>>4) - ((int32_t)_bme280_calib.dig_T1)) *
((adc_T>>4) - ((int32_t)_bme280_calib.dig_T1))) >> 12) *
((int32_t)_bme280_calib.dig_T3)) >> 14;
t_fine = var1 + var2;
float T = (t_fine * 5 + 128) >> 8;
return T/100;
}
void setup() {
Serial.begin(9600);
Wire.begin();
Serial.println("BME280 test");
delay(500);
// Read Register Example
// We read the CHIPID
Wire.beginTransmission((uint8_t)BME280_ADDRESS);
Wire.write((uint8_t)BME280_REGISTER_CHIPID);
Wire.endTransmission();
Wire.requestFrom((uint8_t)BME280_ADDRESS, (byte)1);
uint8_t value = Wire.read();
Serial.print("Registru Chip_Id (0xD0): ");
Serial.println(value);
// Write Register Example
// Set the oversampling factor for humidity to 16x
Wire.beginTransmission((uint8_t)BME280_ADDRESS);
Wire.write((uint8_t)BME280_REGISTER_CONTROLHUMID);
Wire.write((uint8_t)0x05);
Wire.endTransmission();
// Set the oversampling factor for temperature and pressure to 16x + Normal mode (Not sleep)
Wire.beginTransmission((uint8_t)BME280_ADDRESS);
Wire.write((uint8_t)BME280_REGISTER_CONTROL);
Wire.write((uint8_t)0xB7);
Wire.endTransmission();
}
void loop() {
// Read Temperature
Wire.beginTransmission((uint8_t)BME280_ADDRESS);
Wire.write((uint8_t)BME280_REGISTER_TEMPDATA);
Wire.endTransmission();
// The raw temperature is found in regiters 0xFA, 0xFB, 0xFC.
// We can read the registers sequentially; the address is auto incremented
Wire.requestFrom((uint8_t)BME280_ADDRESS, (byte)3);
uint32_t raw_temperature;
raw_temperature = Wire.read();
raw_temperature <<= 8;
raw_temperature |= Wire.read();
raw_temperature <<= 8;
raw_temperature |= Wire.read();
// Raw temperature is encoded on 20 bits
Serial.print("Temperatura raw: ");
Serial.println(raw_temperature >> 4);
// Compensate temperature according to the calibration factors
float temperature = compensateTemperature(raw_temperature);
Serial.print("Temperatura: ");
Serial.println(temperature);
delay (1000);
}
Design with Microprocessors (DMP) – Lab.6
d. Using the Logic Analyzer
The logic analyzer is used to visualize the data traffic between more devices and is especially useful
in problem solving. Follow the next steps for visualizing the traffic towards the sensor:
- Connect the logic analyzer as displayed in the above figures. Check that the SCL line is connected
to CH0 and SDA to CH1.
- Leave the Arduino board disconnected from the PC
- Configure the Sample Rate to 2MS/s and the duration to 5s, as in the figure below:
-
Configure the trigger on falling edge for SCL, Channel 0.
-
Press the start button. The “Logic” will wait for the Trigger
-
Connect the Arduino board to power supply (PC). The recording should start now and after 5 s to
automatically stop.
Design with Microprocessors (DMP) – Lab.6
-
We can see the traffic between the Arduino board and the sensor. Now we can analyze it.
-
From the right part of Logic, section Analyzers, pres ‘+’ and add the I2C analyzer
-
Be sure that SDA is allocated to channel 1 and SCL to channel 0; set the address to 7-bits
-
From the settings of the I2C analyzer, select the Hex mode for display
-
Use Zoom-in zoom-out (mouse scroll), select the first frame from the recording
Design with Microprocessors (DMP) – Lab.6
-
See how the ChipID register is read; compare the decoded data from the Logic analyzer with the
code from the setup function that reads the ChipID register
-
Search for one of the transactions that read the temperature and compare it with the code.
e. Read the pressure and humidity
Read the pressure and humidity by using the compensation functions below. For this you should lookup the addresses for the pressure and humidity registers, from the sensor’s datasheet. Be careful at the width
of these registers.
// Compensation function from the Datasheet, page 23.
float compensatePressure(int32_t adc_P) {
int64_t var1, var2, p;
adc_P >>= 4;
var1 = ((int64_t)t_fine) - 128000;
var2 = var1 * var1 * (int64_t)_bme280_calib.dig_P6;
var2 = var2 + ((var1*(int64_t)_bme280_calib.dig_P5)<<17);
var2 = var2 + (((int64_t)_bme280_calib.dig_P4)<<35);
var1 = ((var1 * var1 * (int64_t)_bme280_calib.dig_P3)>>8) +
((var1 * (int64_t)_bme280_calib.dig_P2)<<12);
var1 = (((((int64_t)1)<<47)+var1))*((int64_t)_bme280_calib.dig_P1)>>33;
if (var1 == 0) {
return 0;
}
p = 1048576 - adc_P;
p = (((p<<31) - var2)*3125) / var1;
var1 = (((int64_t)_bme280_calib.dig_P9) * (p>>13) * (p>>13)) >> 25;
var2 = (((int64_t)_bme280_calib.dig_P8) * p) >> 19;
p = ((p + var1 + var2) >> 8) + (((int64_t)_bme280_calib.dig_P7)<<4);
return (float)p/256;
}
// Functia de compensare din Datasheet, de la pag. 23
float compensateHumidity(int32_t adc_H) {
int32_t v_x1_u32r;
v_x1_u32r = (t_fine - ((int32_t)76800));
v_x1_u32r = (((((adc_H << 14) - (((int32_t)_bme280_calib.dig_H4) << 20) (((int32_t)_bme280_calib.dig_H5) * v_x1_u32r)) + ((int32_t)16384)) >> 15) *
(((((((v_x1_u32r * ((int32_t)_bme280_calib.dig_H6)) >> 10) *
(((v_x1_u32r * ((int32_t)_bme280_calib.dig_H3)) >> 11) + ((int32_t)32768))) >> 10) +
((int32_t)2097152)) * ((int32_t)_bme280_calib.dig_H2) + 8192) >> 14));
Design with Microprocessors (DMP) – Lab.6
v_x1_u32r = (v_x1_u32r - (((((v_x1_u32r >> 15) * (v_x1_u32r >> 15)) >> 7) *
((int32_t)_bme280_calib.dig_H1)) >> 4));
v_x1_u32r
v_x1_u32r
float h =
return h
= (v_x1_u32r < 0) ? 0 : v_x1_u32r;
= (v_x1_u32r > 419430400) ? 419430400 : v_x1_u32r;
(v_x1_u32r>>12);
/ 1024.0;
}
2. Analog Keypad
In the second of the laboratory we will use an analog keypad. This is a first example for using the
analog to digital converter which comes with the AVR microcontroller from the Arduino boards; in
the next laboratory we will use other examples with analog sensors. For now, it is enough to know
that an analog value is read by using the analogRead(pinNb) instruction.
The analog keypad is depicted in the next figure.
Fig 1. Analog Keypad
We will not go into details about the functionality of this component. The important thing to
remember is that when pressing a key from the keypad, a voltage is transmitted on the Vout pin. In
the next figure you can find the nominal Vout voltage for each key.
Fig 2. Voltage values corresponding to each pressed key
If more than one key is pressed, Vout will correspond to the key with the highest numerical value.
For this example, you need an Arduino board with LCD shield and an analog keypad. See the next
image for realizing the setup (connect the VCC and GND keypad pins to 5V and GND on the Arduino
board, and the Vout output pin of the keypad to the A1 input from the Arduino board).
Design with Microprocessors (DMP) – Lab.6
Fig 3. Analog Keypad setup
#include <LiquidCrystal.h>
LiquidCrystal lcd(7, 6, 5, 4, 3, 2);
//use an intermediate value for clearing the LCD screen only when a new keypad is pressed
int valIntermediar;
void setup()
{
//set the reference voltage to 5V
analogReference(DEFAULT); //setarea tensiunii de referinta la tensiunea default
lcd.begin(16, 2);
lcd.setCursor(0,0);
lcd.print("Cititi senzor");
pinMode(A1, INPUT); // set the A1 analogic pin as input
digitalWrite(A1, HIGH); //activate the pull up resistor
//initialize the intermediate value
valIntermediar = 1000;
}
void loop()
{
int val = analogRead(A1); //read the analog value
//get the key corresponding to the analog voltage level
val = getTasta(val);
if(valIntermediar != val)
{
valIntermediar = val;
lcd.clear();//clear screen
lcd.setCursor(0,0);
lcd.print("Cititi senzor");
lcd.setCursor(0,1);//display the new value
lcd.print(valIntermediar);
}
}
/*
This function gets an analog value as parameter and returns the corresponding key for thi
value. Because the analog signal is not filtered nor stabilized it continuously fluctuates.
For obtaining only our desired keypad values we build a hysteresis. These intervals have
been experimentally chosen, based on the voltage levels presented in the keypad’s datasheet.
*/
int getTasta(int val)
{
if(val > 50 && val < 80)return 12;//key 12 is pressed
Design with Microprocessors (DMP) – Lab.6
else
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if(val
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return
> 90 && val < 120) return 11; // key 11 is pressed
> 120 && val < 160) return 10; // and so on
> 160 && val < 200) return 9;
> 200 && val < 240)return 8;
> 240 && val < 290) return 7;
> 290 && val < 320) return 6;
> 320 && val < 370) return 5;
> 370 && val < 410) return 4;
> 410 && val < 450) return 3;
> 450 && val < 490) return 2;
> 490 && val < 530) return 1;
0; //for any other value return 0 – no key is pressed
}
!!! Attention
A problem with the analog sensors is that the read values fluctuate. This can be fixed in hardware by
using a capacitor or a low pass filter for eliminating the spike effects, or it can be fixed in software
by reading more values and computing the average or median of these values.
Individual work
1. Implement and test the examples from this laboratory.
2. Add the functionalities for reading the pressure and humidity from the Bosch BME280 sensor.
Lift the sensor by 50 cm or more above the table and see the change in atmospheric pressure.
Breathe hot air towards the sensor and observe the change in temperature and humidity.
3. Create a simple thermostat. Read the desired temperature and humidity values from the analog
keypad. Turn on the heat, air conditioner or humidifier (3 different LEDs) until you reach the
desired parameters.
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