This code example demonstrates how to interface a pressure sensor with differential output voltage using differential Analog-to-Digital Converter With Computation (ADCC) and Operational Amplifier (OPA) of the PIC16F17146 microcontroller.
- MPLAB® X IDE 6.15 or newer
- Microchip XC8 Compiler 2.45 or newer
- MPLAB® Code Configurator (MCC) 5.3.7 or newer
- MPLAB® Data Visualizer 1.3.1332 or newer
- Microchip PIC16F1xxxx Series Device Support (DFP) 1.22.376 or newer
- PIC16F17146 Curiosity Nano Board
- Curiosity Nano Base Board
- Proto Click
- Resistors (15 KΩ, 820 KΩ) for Gain
- Resistor (2 KΩ) for feedback current
- 2SMPP-02 Pressure Sensor
2SMPP-02 is a MEMS Gauge Pressure Sensor Featuring Small Size and Low Power Consumption. Its sensing method is Piezo-resistance and the pressure medium is air. A constant current is needed to driver this sensor.
Specifications:
- Pressure measurement range: 0 to 37 kPa
- Bridge resistance: 20 ±2 KΩ
- Offset voltage of -2.5±4 mV
- Span voltage of 31.0±3.1 mV
- Overall differential voltage output range: -2.4 to 31.0 mV
- Drive Current: 100 µA
Figure 1. Pressure Sensor Internal Circuit, Pins and kPa vs mV Graph
Pressure Sensor Working
The 2SMPP-02 pressure sensor has a linear relationship between applied pressure and output in mV. A constant current of 100 µA (at Icc) is needed to drive the circuit and then the internal bridge circuit produces the correct output corresponding to applied pressure. The user needs to read Vout+ and Vout- using differential ADCC. As the difference in output is very small (in mV) then an amplifier is must before the signal is fed to differential ADCC.
Below is the implementation circuit overview block diagram using PIC16F17146:
Figure 2. Differential Pressure Sensor Interface Block Diagram
DAC1 is an 8-bit buffered Digital-to-Analog (DAC) that can be used to source pressure sensor which requires a constant excitation current of 100 µA. Op-Amp can be used to amplify a differential signal.
8-bit DAC and Shunt Resistor Calculations:
Pressure Sensor bridge resistance: ~20 KΩ.
Shunt Resistance used to measure circuit current: 2 KΩ.
Total Circuit Resistance: ~22 KΩ.
So, the maximum DAC output voltage (as Bridge resistance decreases as pressure increases) when a 100 µA current is required is: 22 KΩ * 100 µA = 2200 mV
Therefore, the DAC reference voltage must be above 2.048V. When VDD is 3.3V, FVR with 4.096V can not be used.
DAC Reference voltage is set to VDD: 3.3V
DAC step count can give the output resolution of: 3.3/256 = 0.01289V or 12.89 mV
Therefore, DAC step count can vary current in the pressure sensor circuit: 12.89 mV/22 KΩ = 0.586 uA;
which means circuit can be excited with good accuracy (less than 1 µA) using 8-Bit buffered DAC.
12-bit Diff ADC and OPA Gain Calculations for Pressure Measurement:
OP-Amp gain: 1+ (820K/15K) = ~ 55
Pressure extreme range as per data sheet:
Max output: OPA Gain * Extreme Pressure Voltage mV = 55 * 37 = 2035 mV
Resolution per mV: 47 (kPa)/ 2035 (mV) = 0.023 (kPa/mV)
Typical Range:
Max output at Op-Amp: 55 * 31 mV = 1705 mV
Therefore, ADCC positive voltage reference can be set to 2.048V and can be given by FVR.
Pressure Measurement Calculations:
Sensor output (in mV) = 0.84 * Pressure applied (in kPa) – 2.25 [Linear equation from the sensor data sheet graph]
Pressure applied (in kPa) = (2.25 + Sensor output (in mV))/0.84
Pressure applied (in kPa) = 2.97 + Sensor output (in mV) * 1.19 ---------- Eq (1)
But,
Input signal given to ADCC = Sensor output (in mV) * OPA Gain
Sensor output (in mV) = Input signal given to ADCC / OPA Gain
Sensor output (in mV) = (ADCC Count * (ADCC Vref / ADC Max count)) / OPA Gain ------------ Eq (2)
Hence, from Eq (1) and Eq (2),
Pressure applied (in kPa) = 2.97 + ((ADCC Count * (ADCC Vref / ADC Max count)) / OPA Gain) * 1.19
Pressure applied (in kPa) = ~3 + 0.0313 * (ADCC Count)
Figure 3. Hardware Setup
Note: Connection between RC7 and RC2 is not required anymore, as OPA output is connected to ADCC internally.
PIC16F17146 curiosity nano board is used as a development platform. The curiosity nano baseboard is used for connecting click boards to the curiosity nano board. Proto click is used as a general-purpose PCB to connect the 2SMPP-02 pressure sensor, gain resistors (820 KΩ, 15 KΩ), and shunt resistor (2 KΩ) to the PIC16F17146 microcontroller's peripherals. Proto click is placed in slot 3 of curiosity nano baseboard. Refer following sections for connection details.
Hardware Modification
Pin RB4 is used as OPA inverting input. This pin is shared with SDA line of curiosity nano baseboard and it has hardware pull-up soldered by default. This resistor needs to be removed as shown in below diagram. Please refer to PIC16F17146 Curiosity Nano board User Guide for more details.
Figure 4. Hardware Modification
2SMPP-02 Pressure sensor and Proto click connection details:
Figure 5. Proto Click Circuit
| Proto Click | Sensor Pin Name | Function |
|---|---|---|
| INT | (1)Vout (+) | Positive output of differential signal |
| (2)Not Connected | -- | |
| RST | (3)GND | Ground |
| SCK | (4)Vout (-) | Negative output of differential signal |
| (5)N-Sub | Not-connected; but can be short to Sensor pin 6 (Icc) | |
| MISO | (6)Icc | Drive Current (100 µA) |
| AN | 820 K Res | Connected to OPA Output |
| CS | 820 K and 15 K Res | Connected OPA inverting terminal |
| PIC16F17146 Pin | Function | Remarks |
|---|---|---|
| RA21 | DAC Output (sources 100 µA current) | |
| RA41 | Monitors sensor excitation current (ADCC positive channel) | |
| RB41 | OPA inverting input | |
| RC21 | OPA output | Connected to ADCC's positive channel internally |
| RC31 | Sensor Vout+ (OPA non-inverting input) | |
| RC6 | Sensor Vout- (ADCC negative channel) | |
| RB7 | EUSART1 TX | |
| RC1 | Curiosity Nano LED |
1Note: Connections between PIC16F17146 Curiosity Nano board and Click™ Slot 3 are not available for these pins. Jumper cables are used to make the required connections. Connect RA4 to RST3, RB4 to CS3, RA2 to MISO, and RC3 to INT3.
The DAC1 adjusts the output voltage to maintain the constant current (100 µA) across the pressure sensor. The current feedback is taken by ADCC to adjust the DAC output. When the current is maintained at 100 uA the pressure sensor output is read by ADCC by switching the ADCC channels. The Pressure reading is calculated and its percentage value is displayed on the terminal window using UART.
The microcontroller sends data to the PC with a baud rate of 9600 using UART with the help of the virtual serial port feature of the onboard debugger. To see the messages in a terminal window, connect the curiosity nano board to any terminal emulator. Data Visualizer which is available as a plugin to MPLAB X IDE can be used as a terminal emulator.
Figure 6. Terminal Messages
Note: To see messages in a terminal window, the Curiosity Nano board needs to be connected to the terminal emulator. Data Visualizer, which is available as a plugin to MPLAB X IDE, can be used as a terminal emulator. Use a baud rate of 9600.
Also, the user can observe graphical data in Data Visualizer by enabling a macro in the main.c file of the firmware and setting up a variable streamer in Data Visualizer.
Macro:
DV_GRAPH
Figure 7. Macro for Data Visualizer Graph
The image below shows the data graph in Data Visualizer. Import the data_streamer.ds file to configure Data Visualizer for this example. Refer to MPLAB® Data Visualizer User's Guide for more information.
Figure 8. Data Visualizer Graph
This section explains how to configure the peripherals using MPLAB X IDE with the MCC plugin for the recreation of the project.
Refer to Software Used section to install the required tools to recreate this project.
Additional Links: MCC Melody Technical Reference
| Module | Configuration | Usage |
|---|---|---|
| Clock Control | Clock Source - HFINTOSC HF Internal Clock - 4 MHz Clock Divider - 1 |
System clock |
| ADCC | Enable ADC Input Configuration- Differential Mode Operating Mode – Burst Average Result Alignment – Right justified two’s compliment Positive Reference – FVR Clock Source – FOSC Clock Divide – FOSC/16 Threshold Interrupt Mode – Enabled Repeat – 32 Accumulator Right Shift – 5 |
Read Shunt resistor voltage and Pressure sensor output voltage |
| DAC1 | Enable DAC DAC Positive reference selection – VDD DAC Negative reference selection – VSS DAC Output Enable Selection – DACOUT2 Enabled and DACOUT1 Disabled |
To provide 100 uA drive current for sensor |
| OPA1 | Enable OPA Op Amp Configuration – Direct Connection to Pins Positive Channel - OPA1IN+ Positive Source Selection - OPA1IN1+ Negative Channel - OPA1IN- Negative Source Selection - OPA1IN0- |
|
| FVR | Enable FVR FVR buffer 1 Gain (to ADC) – 2x (2.048V) |
|
| TMR0 | Enable Timer Prescaler – 1:8 Postscaler - 1:1 Timer Mode – 8 bit Clock Source – FOSC/4 Request Period – 0.001 s Enable Interrupt |
1 ms interrupt to drive FSM scheduler |
| EUSART1 | UART1 Driver UART PLIB Selector - EUSART1 Requested Baudrate - 9600 Enable Redirect STDIO to EUSART EUSART1 PLIB Disable Receive Enable Serial Port Enable Transmit |
To send messages over terminal |
| Pin Settings | Pin Grid View ADCC ANPx - RA4 ANNx - RC6 OPA1 OPA1INx+ : RC3 OPA1INx- : RB4 EUSART1 TX1 - RB7 DAC1 DCA1OUTx - RA2 Pins RC1 Direction - Output Custom Name - LED_D7 RA4 Custom Name - CurrentP RC6 Custom Name - PressureN RC3 Custom Name - OPA_P_IN RB4 Custom Name - OPA_N_IN RC2 Custom Name - OPA_OUT RB7 Custom Name - UART_TX1 RA2 Custom Name - DCA1OUT |
Note: The onboard debugger on the Curiosity Nano board has a virtual serial port (CDC) that is connected to EUSART on the PIC16F17146 and provides an easy way to communicate with the target application through the terminal software. Refer to the Curiosity Nano board User Guide for more details.
The example demonstrates the usage of DAC1, OPA, and 12-bit differential ADCC to read low-voltage differential signal from pressure sensor and display them over a terminal.