SPI and push-button code

Using Arduino Networking, Protocols, and Devices
1

Hello everyone,

I am trying to send data from an master Arduino Uno board to a slave Arduino Uno board. I found a great SPI_template header file online that should allow me to send any kind of template struct that I want to send. Here is the code for the header file:

Template

#include <Arduino.h>

template unsigned int SPI_writeAnything (const T& value)
{
const byte p = (const byte) &value;

unsigned int i;

for(i = 0; i < sizeof value; i++)
SPI.transfer(*p++);

return i;
}

template unsigned int SPI_readAnything(T& value)
{
byte p = (byte) &value;

unsigned int i;

for(i = 0; i < sizeof value; i++)
*p++ = SPI.transfer(0);

return i;
}

template unsigned int SPI_readAnything_ISR(T& value)
{
byte p = (byte) &value;

unsigned int i;

*p++ = SPDR;

for(i = 1; i < sizeof value; i++)
*p++ = SPI.transfer(0);

return i;

}

My goal is to be able to send the struct using SPI but I would like to determine when to send the data using a simple push-button. In order to test my code, I included several Serial.println statements in the slave code. But the Serial.println statements are not working, nothing appears on the Serial Monitor before or after I press the push-button. I am using an interrupt for the push-button. Any ideas or recommendations on how to implement an external push-button that start an SPI communication?

Here is my master code:
// Information: This program is being used to test a push button, the action
// of pressing the push button should enable the SPI transfer of data.

#include <SPI.h>
#include "SPI_Template.h"

typedef struct myStruct {
int a;
float b;
long c;
};

// This struct object is used to store and transfer the data values
// that the programmer wishes to use with SPI for the 'on' button.
myStruct structComm;

// This int variable will be used for the push button, Button1
// corresponds to 'send' button.
int Button1 = 2;

// The button press will switch the state.
int toggle_on;

void setup() {
Serial.begin(115200);

pinMode(10, OUTPUT);
SPI.begin();
SPI.setClockDivider(SPI_CLOCK_DIV8);

structComm.a = 44;
structComm.b = 30.0;
structComm.c = 12;

// This code initializes the push-button pin as an input.
pinMode(Button1, INPUT);

attachInterrupt(digitalPinToInterrupt(Button1), pin_ISR1, RISING);
}

void loop() {
if (toggle_on == 1) {
digitalWrite(SS, LOW);
SPI_writeAnything(structComm);
digitalWrite(SS, HIGH);
}
}

// This interrupt function is for the push button.
void pin_ISR1()
{
static unsigned long last_interrupt_time1 = 0;

unsigned long interrupt_time1 = millis();

// After a certain amount of time, it will toggle on.
if (interrupt_time1 - last_interrupt_time1 > 200)
{
toggle_on = 1;
}

last_interrupt_time1 = interrupt_time1;
}

Here is the slave code:

// Information: This program is being used to test a push button, the action
// of pressing the push button should enable the SPI transfer of data.

#include <SPI.h>
#include "SPI_Template.h"

typedef struct myStruct
{
int a;
float b;
long c;
};

volatile myStruct structComm;
volatile bool haveData = false;

void setup()
{
Serial.begin(115200);

pinMode(MISO, OUTPUT);

// This code turns on the SPI in slave mode.
SPCR |= _BV(SPE);

SPI.attachInterrupt();
}

// This void function acts just like the main function.
void loop() {
if (haveData) {
Serial.println();
Serial.println("SPI test");
Serial.println(structComm.a);
Serial.println(structComm.b);
Serial.println(structComm.c);
Serial.println();

delay(2500);

haveData = false;
}
}

// This is a SPI interrupt routine function for the green go button.
ISR(SPI_STC_vect) {
SPI_readAnything_ISR(structComm);
haveData = true;
}

2

I am using an interrupt for the push-button.

Why? The SPI communication is already interrupt driven, so the Arduino has nothing to do but poll the switch.

Resetting toggle_on to 0 at some point seems like a good idea.

Declaring toggle_on to be volatile seems like a good idea, because it is MANDATORY.

3

Hello PaulS,

Thanks for the reply. I am still learning about SPI. Looking at the SPI template, the slave code has an interrupt routine, do you recommend I use that interrupt routine for the green push button? Should I declare an interrupt function in the setup of the slave?

Should I directly connect the green push button to the slave board?

Thanks,
David

4

do you recommend I use that interrupt routine for the green push button?

No. I do not recommend that you use interrupts to read switches pushed by humans.

Should I directly connect the green push button to the slave board?

You should explain why you need a switch at all.

5

Hi Paul,

I am a member of a team of students that is attending an engineering competition. The competition consists of building a robot and the rules state that we have to include a 'green go' push-button that is clearly visible.

The robot has to complete a variety of tasks including digital input from sensors and driving some motors. The team and I decided to use two Arduino boards, the master will interface with the sensors and the slave will drive the motors.

I would like to use SPI to send a integer variable from the master to the slave as soon as I press the push button. With that int variable, the slave will execute the working motor code.

I found the SPI template code that I can customize and send any variables using SPI. I just don't understand how to incorporate the push button yet. I am watching some videos online about SPI right now.

Any advice?

Thanks Paul for the replies and your advice.

David

6

Any advice?

So, the master should read the state of the switch, and send something to the slave when the switch changes state.

The something that it sends should be a single byte. There is no reason to use a complex process that can send a struct containing different kinds of data, when all you want to say is "Hey, slave, break's over. Get to work".

Look at the state change detection example, and SPI.write() and/or SPI.transfer().

7

Hi Paul,

I have been reading the some information on SPI.transfer and I have another question. I understand for the most part, what functions and code I need to include on the master. But I am not sure what code I should include on the slave code.

Right now, I am just trying to send a byte from the master to the slave. In the slave, I want to use the byte with an if statement to serial print statements to the console.

Here is my code so far:
Master:
#include <SPI.h>

// This int variable will be used for the push button, Button1
// corresponds to 'send' button.
int Button1 = 2;

// This is the slave select pin.
const int slavePin = 10;

// The button press will switch the state.
volatile int toggle_on;

SPISettings settings(14000000, MSBFIRST, SPI_MODE1);

const byte masterG = 0x07;

void setup() {
// This code enables the slave communication.
pinMode(slavePin, OUTPUT);

SPI.begin();
SPI.setClockDivider(SPI_CLOCK_DIV8);

// This code initializes the push-button pin as an input.
pinMode(Button1, INPUT);

attachInterrupt(digitalPinToInterrupt(Button1), pin_ISR1, FALLING);
}

void loop() {
SPI.beginTransaction(settings);

if (toggle_on == 1) {
digitalWrite(slavePin, LOW);
SPI.transfer(masterG);
digitalWrite(slavePin, HIGH);
}

toggle_on == 0;

SPI.endTransaction();
}

// This interrupt function is for the push button.
void pin_ISR1()
{
static unsigned long last_interrupt_time1 = 0;

unsigned long interrupt_time1 = millis();

// After a certain amount of time, it will toggle on.
if (interrupt_time1 - last_interrupt_time1 > 200)
{
toggle_on = 1;
}

last_interrupt_time1 = interrupt_time1;
}

Slave:
#include <SPI.h>

byte slaveReceive = 0x00;
byte masterG;

void setup()
{
Serial.begin(115200);

pinMode(MISO, OUTPUT);

SPI.attachInterrupt();
}

// This void function acts just like the main function.
void loop() {
if (slaveReceive == 0x07) {
Serial.println();
Serial.println("SPI test");
Serial.println();
Serial.println("Forward command received.");

delay(2500);
}
}

// This is a SPI interrupt routine function for the green go button.
ISR(SPI_STC_vect) {
slaveReceive = SPI.transfer(masterG);
}

Any advice? What am I missing on the slave unit?

Thanks,
David

8

Hi Paul,

I was able to fix the issue.

Thanks for your advice and help,
David

9