Beginner
10 min

Experience excellence in resistance measurement with MAX4208 and STM32F429NI

Mastering resistance with Wheatstone's magic

Wheatstone Click with UNI-DS v8

Published Nov 08, 2023

Click board™

Wheatstone Click

Dev Board

UNI-DS v8

Compiler

NECTO Studio

MCU

STM32F429NI

Discover the ultimate tool for precision resistance measurement – our Wheatstone bridge circuit solution is here to empower your journey towards accurate and reliable data.

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Hardware Overview

How does it work?

Wheatstone Click is based on the MAX4208, an ultra-low offset/drift, precision instrumentation amplifier, from Analog Devices. A Wheatstone bridge is an electrical circuit used to measure an unknown electrical resistance by balancing two branches of a bridge circuit, one branch of which includes the unknown component. The primary benefit of the circuit is its ability to provide extremely accurate measurements, in contrast with a simple voltage divider. The R2, R3, and R4 are resistors of known resistance (1k ohm), while the resistance R1 is brought to the terminal block and therefore it is changeable. With no resistance connected to the terminal block, the bridge on this Click board™ is in balance. At this point, the voltage between the two midpoints (IN- and IN+) will be zero. Therefore the ratio

of the two resistances in the known branch (R1 and R3) is equal to the ratio of the two resistances in the unknown leg (R2 and R4). If the external resistance is connected to the terminal block, bridge is unbalanced, and the voltage on the midpoints is proportional to the external resistor value. Wheatstone click is based around the MAX4208 IC, which is connected to an onboard Wheatstone bridge circuit, in order to precisely measure the resistance of an external element. The mentioned IC uses a spreadspectrum, autozeroing technique that constantly measures and corrects the input offset, eliminating drift over time and temperature and the effect of 1/f noise. This technique achieves less than 20μV offset voltage, allows ground-sensing capability, provides ultra-low CMOS input bias current and increased

common-mode rejection performance. It also provides high-impedance inputs, optimized for small-signal differential voltages (±100mV), which makes it ideal for an application such as wheatstone bridge disbalance measurement. This Click board™ also has TPL0501 onboard - 256-Taps, Single-Channel, Digital Potentiometer With SPI Interface, from texas instruments. It is connected to the MAX4208 in a way that it serves an gain adjust instead of with two external resistors The power supply voltage selection for the logic section is done by moving the SMD jumper labeled as VCC SEL to a desired position: left position to select 3.3V, right position to select 5V. This will allow both 3.3V and 5V MCUs to be interfaced with the Click board™ directly.

Wheatstone Click hardware overview image

Features overview

Development board

UNI-DS v8 is a development board specially designed for the needs of rapid development of embedded applications. It supports a wide range of microcontrollers, such as different STM32, Kinetis, TIVA, CEC, MSP, PIC, dsPIC, PIC32, and AVR MCUs regardless of their number of pins, and a broad set of unique functions, such as the first-ever embedded debugger/programmer over WiFi. The development board is well organized and designed so that the end-user has all the necessary elements, such as switches, buttons, indicators, connectors, and others, in one place. Thanks to innovative manufacturing technology, UNI-DS v8 provides a fluid and immersive working experience, allowing access anywhere and under any

circumstances at any time. Each part of the UNI-DS v8 development board contains the components necessary for the most efficient operation of the same board. An advanced integrated CODEGRIP programmer/debugger module offers many valuable programming/debugging options, including support for JTAG, SWD, and SWO Trace (Single Wire Output)), and seamless integration with the Mikroe software environment. Besides, it also includes a clean and regulated power supply module for the development board. It can use a wide range of external power sources, including a battery, an external 12V power supply, and a power source via the USB Type-C (USB-C) connector. Communication options such as USB-UART, USB

HOST/DEVICE, CAN (on the MCU card, if supported), and Ethernet is also included. In addition, it also has the well-established mikroBUS™ standard, a standardized socket for the MCU card (SiBRAIN standard), and two display options for the TFT board line of products and character-based LCD. UNI-DS v8 is an integral part of the Mikroe ecosystem for rapid development. Natively supported by Mikroe software tools, it covers many aspects of prototyping and development thanks to a considerable number of different Click boards™ (over a thousand boards), the number of which is growing every day.

UNI-DS v8 horizontal image

Microcontroller Overview

MCU Card / MCU

default

Type

8th Generation

Architecture

ARM Cortex-M4

MCU Memory (KB)

2048

Silicon Vendor

STMicroelectronics

Pin count

216

RAM (Bytes)

262144

Used MCU Pins

mikroBUS™ mapper

Analog Output
PF9
AN
NC
NC
RST
SPI Chip Select
PB9
CS
SPI Clock
PG13
SCK
NC
NC
MISO
SPI Data IN
PG14
MOSI
Power Supply
3.3V
3.3V
Ground
GND
GND
NC
NC
PWM
NC
NC
INT
NC
NC
TX
NC
NC
RX
NC
NC
SCL
NC
NC
SDA
NC
NC
5V
Ground
GND
GND
1

Take a closer look

Schematic

Wheatstone Click Schematic schematic

Step by step

Project assembly

Fusion for PIC v8 front image hardware assembly

Start by selecting your development board and Click board™. Begin with the UNI-DS v8 as your development board.

Fusion for PIC v8 front image hardware assembly
GNSS2 Click front image hardware assembly
SiBRAIN for PIC32MZ1024EFK144 front image hardware assembly
GNSS2 Click complete accessories setup image hardware assembly
v8 SiBRAIN Access MB 1 - upright/background hardware assembly
Necto image step 2 hardware assembly
Necto image step 3 hardware assembly
Necto image step 4 hardware assembly
NECTO Compiler Selection Step Image hardware assembly
NECTO Output Selection Step Image hardware assembly
Necto image step 6 hardware assembly
Necto image step 7 hardware assembly
Necto image step 8 hardware assembly
Necto image step 9 hardware assembly
Necto image step 10 hardware assembly
Necto PreFlash Image hardware assembly

Track your results in real time

Application Output

After pressing the "FLASH" button on the left-side panel, it is necessary to open the UART terminal to display the achieved results. By clicking on the Tools icon in the right-hand panel, multiple different functions are displayed, among which is the UART Terminal. Click on the offered "UART Terminal" icon.

UART Application Output Step 1

Once the UART terminal is opened, the window takes on a new form. At the top of the tab are two buttons, one for adjusting the parameters of the UART terminal and the other for connecting the UART terminal. The tab's lower part is reserved for displaying the achieved results. Before connecting, the terminal has a Disconnected status, indicating that the terminal is not yet active. Before connecting, it is necessary to check the set parameters of the UART terminal. Click on the "OPTIONS" button.

UART Application Output Step 2

In the newly opened UART Terminal Options field, we check if the terminal settings are correct, such as the set port and the Baud rate of UART communication. If the data is not displayed properly, it is possible that the Baud rate value is not set correctly and needs to be adjusted to 115200. If all the parameters are set correctly, click on "CONFIGURE".

UART Application Output Step 3

The next step is to click on the "CONNECT" button, after which the terminal status changes from Disconnected to Connected in green, and the data is displayed in the Received data field.

UART Application Output Step 4

Software Support

Library Description

This library contains API for Wheatstone Click driver.

Key functions:

  • wheatstone_set_potentiometer - Set potentiometer ( 0 - 100k )

  • wheatstone_read_an_pin_voltage - This function reads results of AD conversion of the AN pin and converts them to proportional voltage level.

Open Source

Code example

This example can be found in NECTO Studio. Feel free to download the code, or you can copy the code below.

/*!
 * \file 
 * \brief Wheatstone Click example
 * 
 * # Description
 * This example demonstrates the use of Wheatstone click board by measuring the input
 * resistance.
 *
 * The demo application is composed of two sections :
 * 
 * ## Application Init 
 * Initializes the driver and logger and sets the default potentiometer (gain) level.
 * 
 * ## Application Task  
 * Reads the AN pin voltage and calculates the input resistance from it.
 * All data are being displayed on the USB UART where you can track their changes.
 * 
 * @note
 * The following formulas you may find useful:
 * AN_PIN(V) = ( ( 1kOhm + R_INPUT(kOhm) ) / ( 1kOhm + 2*R_INPUT(kOhm) ) - 1/2 ) * VCC(V) * GAIN
 * VOUT(V) = AN_PIN(V) / GAIN
 * R_INPUT(kOhm) = ( VCC(V) * GAIN - 2*AN_PIN(V) ) / ( 4*AN_PIN(V) )
 * R_INPUT(kOhm) = ( VCC(V) - 2*VOUT(V) ) / ( 4*VOUT(V) )
 * 
 * \author MikroE Team
 *
 */
// ------------------------------------------------------------------- INCLUDES

#include "board.h"
#include "log.h"
#include "wheatstone.h"

// ------------------------------------------------------------------ VARIABLES

static wheatstone_t wheatstone;
static log_t logger;

// ------------------------------------------------------ APPLICATION FUNCTIONS

void application_init ( void )
{
    log_cfg_t log_cfg;
    wheatstone_cfg_t cfg;

    /** 
     * Logger initialization.
     * Default baud rate: 115200
     * Default log level: LOG_LEVEL_DEBUG
     * @note If USB_UART_RX and USB_UART_TX 
     * are defined as HAL_PIN_NC, you will 
     * need to define them manually for log to work. 
     * See @b LOG_MAP_USB_UART macro definition for detailed explanation.
     */
    LOG_MAP_USB_UART( log_cfg );
    log_init( &logger, &log_cfg );
    log_info( &logger, " Application Init " );

    // Click initialization.
    wheatstone_cfg_setup( &cfg );
    WHEATSTONE_MAP_MIKROBUS( cfg, MIKROBUS_1 );
    wheatstone_init( &wheatstone, &cfg );

    wheatstone_set_potentiometer ( &wheatstone, WHEATSTONE_POT_MAX );

    log_info( &logger, " Application Task " );
}

void application_task ( void )
{
    float an_pin_v = 0;
    float vout = 0;
    float r_kohm = 0;
    if ( WHEATSTONE_OK == wheatstone_read_an_pin_voltage ( &wheatstone, &an_pin_v ) ) 
    {
        vout = an_pin_v / wheatstone.gain;
        if ( 0 != vout )
        {
            r_kohm = ( WHEATSTONE_VCC_5V - 2 * vout ) / ( 4 * vout );
        }
        log_printf( &logger, " VCC     : %.3f V\r\n", WHEATSTONE_VCC_5V );
        log_printf( &logger, " GAIN    : %.3f\r\n", wheatstone.gain );
        log_printf( &logger, " AN_PIN  : %.3f V\r\n", an_pin_v );
        log_printf( &logger, " VOUT    : %.3f V\r\n", vout );
        log_printf( &logger, " R_INPUT : %.3f kOhm\r\n\n", r_kohm );
        Delay_ms( 1000 );
    }
}

void main ( void )
{
    application_init( );

    for ( ; ; )
    {
        application_task( );
    }
}

// ------------------------------------------------------------------------ END

Additional Support

Resources