Published February 8, 2022 © GPL3+

Use your Arduino and a X9C10x to control a 555 Timer

Use the X9C104 programmable potentiometer with an Arduino to control the frequency of square waves produced by a 555 timer.

IntermediateFull instructions provided1 hour2,459

Use your Arduino and a X9C10x to control a 555 Timer

Things used in this project

Hardware components

Arduino UNO

555 Timers

Resistor 1k ohm

Capacitor 100 nF

Renesas X9C104

Breadboard (generic)

Software apps and online services

Arduino IDE

Story

The X9C Digitally Controlled Potentiometer (XDCP™) family of chips from Intersil allows for the digital control of a variable resistor via a three wire digital interface.

This project illustrates a simple circuit where a X9C104 IC is controlled by an Arduino and the resulting variable resistance is used to control the frequency of a square wave generated by a 555 timer running in astable mode.

Circuit setup

A simplified block diagram of the circuit shown below:

Block Diagram

The basic principle of operation:

User inputs a variable potentiometer setting via the Arduino serial monitor.
Arduino accepts input and programs X9C104 to setting
X9C104 resistance in 555 timer circuit adjusts frequency of oscillation
The output square wave from 555 timer changes to value set by user in step 1

X9CXXX Digitally Controlled Potentiometer (XDCP)

The X9CXXX integrated circuits are manufactured by Intersil (recently acquired by Renesas corporation) and represent their range of digitally controlled potentiometers. The device includes a resistor array, control section and non-volatile memory. The resistance of the array is controlled by a three wire digital interface.

A block diagram from the Intersil data sheet:

X9CXXX Block Diagram

The device can be used as two terminal variable resistor or a three terminal potentiomenter. It includes non-volatile memory that holds the last resistance value on power down and sets the resistor array to this value on subsequent power up.

Control of the wiper position and hence the resistance is determined by the three digital inputs:

Device is selected by taking CS low
State of Up/Down (U/D )determines whether the wiper moves closer to Vh/Rh or Vl/Rl
The wiper moves one step every time INC is taken low and then returned to high.
The wiper has 100 possible positions (0 – 99), allowing for incremental steps of 1% of the total resistance.
When the wiper reaches the top or bottom ( Vh/Rh or Vl/Rl), it does not move any further nor does it wrap around to the other extreme.

It is important to remember that the wiper moves a single increment for each INC cycle. The wiper cannot jump across multiple steps. This means that moving from step 10 to step 20 requires 10 INC cycles.

Vh and Vl cannot exceed Vcc and Vss.

Complete specifications for the devices can be found at the Renesas website.

Product Description

Datasheet

555 Timer Circuit

The 555 Timer circuit used in this project is a standard astable oscillator. There are multiple articles available that document the workings of this circuit and a quick search will allow access to the explanations.

Schematic

At a high level, operation as follows:

With pin 7 high and blocking, capacitor C1 charges up through R1 and the variable resistor in the X9C104
At a certain threshold voltage, the 555 timer internal flip/flop s1witches and Pin 7 goes low
Capacitor C1 then discharges through the variable resistor in the X9C104
At the lower threshold voltage, the 555 timer internal flip/flop switches back and the cycle repeats

The equations for frequency and duty cycle as follows:

555 Timer Circuit frequency

In the circuit used here, R2 is 100 kΩ (X9C104) and R1 is 1 kΩ. Because of the difference, the frequency and duty cycle of the circuit is effectively set by the value of R2, except at very low values of R2.

Values of R1, R2 and C1 are chosen so that the circuit will generate square waves within the audible frequency range.

Arduino Code

The code makes use of the Fast X9CXXX library for the Arduino. The library is available on GitHub at:

https://github.com/GitMoDu/FastX9CXXX

Make sure that the code is downloaded and included in your Arduino library folder before compiling the code.

The setup part of the program initiates the X9C104 object using pins 3, 4 and 5 from the Arduino. Potentiomenter is set to 0 step.

The main loop waits for an input from the user via the Serial monitor; this input must be between 0 and 99. For a legitimate input, the potentiomenter step is set to this value and the 555 timer circuit will settle to a square wave frequency determined by the step resistance.

The step input and estimated resistance are printed out to the Serial monitor.

Results and Conclusion

Frequency measurements of the circuit were taken at various step values. The results are tabulated in the attached spreadsheet. The first tab shows results using an actual 100 kOhm variable potentiometer. The second tab shows the measured frequency resulting from a XC9104 step compared with a theoretical calculated value.

Frequency versus X9C104 Step

Of course, there are much easier methods to achieve a variable frequency square wave from an Arduino, but this project moved toward a practical use of the digitally controlled potentiometer.

Schematics

Code

Arduino XC9XXX control code

/*

Code used to control the frequency of a 555 timer circuit using the
X9C104 variable potentiometer. User inputs a value between 0 and 99 via the
serial monitor to set the value of the X9C104 and timer circuit frequency adjusts

FastX9CXXX library available at https://github.com/GitMoDu/FastX9CXXX

*/

#include <FastX9CXXX.h>

#define X9_CS_PIN 3  //X9CXXX CS connected to digital pin 3 of Arduino
#define X9_UD_PIN 4  //X9CXXX UD connected to digital pin 4 of Arduino
#define X9_INC_PIN 5  //X9CXXX INC connected to digital pin 5 of Arduino

#define STEP_DELAY_MILLIS 250

// Setup object to control X9C104
FastX9C104 Potentiometer(X9_CS_PIN, X9_UD_PIN, X9_INC_PIN);

uint8_t Step;
String incomingString;

void setup() {
	Serial.begin(9600);
	Serial.println("X9C104 Digital Potentiometer 555 Timer Control.");
  Serial.println();

	// Reset potentiometer to known position (Step = 0)
  // This conicides with max resistance in the circuit and minimum frequency
	Potentiometer.Reset();

	// Plot Header.
	Serial.println(F("Step\t  Result"));
}

void loop() {
	// Wait for User input via Serial Monitor
  if (Serial.available() > 0){
        //Read the incoming string
        incomingString = Serial.readString();
        //Convert to a integer
        Step = incomingString.toInt();
        //Set Potentiomter to step value
        Potentiometer.JumpToStep(Step);
	      
	     // Plot the result.
	     Serial.print(Step);
	     Serial.print('\t');
	     Serial.print("Nominal Resistance in KOhm: ");
       Serial.print((100 * Step / 99.0) ,DEC);
	     Serial.println();

	     delay(STEP_DELAY_MILLIS);    // wait for a bit for circuit to stabilize
  }
}

Credits

kpower

19 projects • 6 followers

Qualified Electrical Engineer with experience in software and hardware development

Use your Arduino and a X9C10x to control a 555 Timer

Things used in this project

Hardware components

Software apps and online services

Story

X9CXXX Digitally Controlled Potentiometer (XDCP)

555 Timer Circuit

Arduino Code

Results and Conclusion

Custom parts and enclosures

Results

Schematics

Schematic

Code

Arduino XC9XXX control code

Credits

kpower

Comments

Embed the widget on your own site

Use your Arduino and a X9C10x to control a 555 Timer

Use your Arduino and a X9C10x to control a 555 Timer

Things used in this project

Hardware components

Software apps and online services

Story

X9CXXX Digitally Controlled Potentiometer (XDCP)

555 Timer Circuit

Arduino Code

Results and Conclusion

Custom parts and enclosures

Results

Schematics

Schematic

Code

Arduino XC9XXX control code

Credits

kpower

Comments

Related channels and tags