High resolution barometric pressure sensor
A pressure sensor that uses the HP03D to reach a resolution of 1 Pa and 0.01 °C when averaging 100 readings at a sample rate of 10 Hz.
Contents
Development software used
- IDE: µVision3 V3.72
- Compiler: RealView MDK-ARM, version: 3.40
Calibration of the sensor values
The HP03D datasheet is not clear and the formulae are missing important parentheses. The application note with the sample code does not work correctly without floating-point either. This code will work correctly using 32 bit signed integers. The result is multiplied by 10 to get one digit of fractional resolution.
if (D2 >= C5) {
dUT = (D2 - C5)-(((((D2 - C5) * (D2 - C5)) >> 14) * A) >> C);
} else {
dUT = (D2 - C5)-(((((D2 - C5) * (D2 - C5)) >> 14) * B) >> C);
}
OFF = (C2 << 2 ) + (((C4 - 1024) * dUT) >> 12);
SENS = C1 + ((C3 * dUT) >> 10);
X = ((SENS * (D1 - 7168)) >> 14) - OFF;
Pressure = ((X * 10) >> 5) + C7;
Temperature = 250 + (((dUT * C6) >> 16) - (dUT >> D));
LCD interface
The timing is very important, use the hardware timer to make sure the delays are correct. If you use the 4 bit databus you need to read both nibbles, if not you may get the wrong value the next time.
I2C bus
Remember the pull-ups on both the clock and data line. First read the 24C02 EEPROM datasheet and the UM10161 - LPC2101/02/03 User manual carefully. Then read the HP03D datasheet. Clock the bus slowly in the beginning to reduce the chance of any problems. Make sure to differ between a start condition and a repeated start condition.
Reading the EEPROM
This is the bus activity for a random read of the 24C02 EEPROM.
START 0xA0 ACK ADRESS ACK START 0xA1 ACK DATA NOACK STOP
Reading the A/D
This is the bus activity for reading the A/D. Use 0xF0 for pressure and 0xD0 for temperature.
Set XCLR high START 0xEE ACK 0xFF ACK (0xF0 | 0xD0) ACK STOP Pause 40 ms START 0xEE ACK 0xFD ACK START 0xEF ACK AD-MSB ACK AD-LSB NOACK STOP Set XCLR low
Parts used
Suitable parts can be found at Futurlec
ET-BASE ARM2103 controller
- CPU NXP LPC2103
- RAM 8192 bytes
- FLASH 32768 bytes
- Clock rate 59 MHz
- Supply voltage 5 V
- Supply current 45 mA
LCD display Optrex 20434
- 20 x 4 characters
- 4 bit bus
- Supply voltage 5 V
- Supply current 3 mA
Pressure sensor HP03D
- Pressure range 300-1100 hPa
- 16 bit A/D
- I2C Serial interface
- Accurate to 0.5 hPa
- Supply voltage: 2.2 - 3.6 V (provided by controller)
- Supply current 500 µA during A/D conversion
Controller connections
LCD Direction LPC2103 Signal EN <--- P0.17 Enable (10 kOhm pull-up to 5 V on controller) RW <--- P0.18 Direction of data bus RS <--- P0.19 Data/instruction select D4 <--> P0.20 Data 0 D5 <--> P0.21 Data 1 D6 <--> P0.22 Data 2 D7 <--> P0.23 Data 3
HP03D Direction LPC2103 Signal 1 - VSS - GND 2 - VDD - +3.3 V 3 - MCLK <--- P0.7 - MAT2.0 32768 Hz 4 - XCLR <--- P0.4 - PIO Reset (Active low, high during A/D operation) 5 - SDA <--> P0.3 - SDA0 I2C data (10 kOhm pull-up to 3.3 V) 6 - SCLK <--- P0.2 - SCL0 I2C clock (10 kOhm pull-up to 3.3 V)
