Attention Pin (ATTN) Guide

There are several situations where you may need your embedded application to be alerted the instant the Notecard receives new information. This guide is designed to aid you in creating an interactive example to demonstrate how to leverage this feature of the Notecard. The goal of this example is to solidify and confirm your understanding of the Notecard's attention interrupt and its behavior.

At a high-level, your program will:

1. Respond to a button click.
2. Author a request to the Notecard; instructing it to fire after a few seconds.
3. Respond to interrupts and update an LED to visualize the behavior of the ATTN interrupt.

During the course of this example, you will learn:

1. How to configure the Notecard's attention interrupt.
2. How to use the Notecarrier-AF's built-in button from your program.
3. How to use the Huzzah32's built-in LED from your program.
4. How to write and handle an interrupt service routine (ISR) on the ESP32.

note

Although this guide uses an Adafruit Huzzah32 microcontroller and a Notecarrier-AF to show how the Notecard's attention pin works, you can use any combination of ESP32 microcontroller and Notecarrier to complete this guide. Just note you may need to map the instructions to work with your specific hardware configuration.

You can view or download the full source of this project from GitHub.

Notecard Interrupt

Originally designed for low-power use cases, the Notecard attention interrupt is a latching interrupt. Meaning, once it fires, it stays in the fired position, until it has been manually reset.

The latching behavior enables you to leverage the interrupt in myriad ways:

• The Notecard may idle or delay while waiting for communication from a cellular tower, while simultaneously disabling the host microcontroller with the enable pin.
• When used in a powered setting and connected to an interrupt capable pin on the host MCU, the host MCU can receive and respond to network communication as quickly as possible.
• The host MCU may optimize polling, by querying the logic value of the pin, as opposed to transacting with the Notecard to look for new data.

To learn more about configuring the Notecard ATTN interrupts, read the Handling Notecard Interrupts section of the Notecard guide.

Hardware Setup

Ensure you have access to the following hardware:

• Notecard
• ESP32 microcontroller
• Notecarrier
• Male/male jumper wire
• USB A/micro cable
• momentary tactile push button (built-in to Notecarrier-AF)
• LED (built-in to Adafruit Huzzah32 ESP32 Feather)

The attention, or ATTN, pin is exposed on the Notecarrier-AF, however it is not wired to any pins that are exposed from the Feather socket. To utilize the ATTN pin you must first decide how you would like for it be used (as described above), and then you must wire it the corresponding pin.

1. You must connect the ATTN pin of the Notecard to an interrupt capable GPIO pin on the Huzzah32. Place the male/male jumper wire between the ATTN pin on the Notecarrier-AF 13-Pin Header and pin 5 on the Adafruit Feather 24-Pin Breakout Header.

That's it! To complete the project, you will use the Huzzah32's built-in LED, LED_BUILTIN, and the Notecarrier-AF's built-in button, B0.

note

The female headers pictured above are non-standard hardware. In order to connect your jumper wire between ATTN and 5, either soldering or alligator clips will be required.

Firmware Breakdown

Definitions and Declarations

1. First things first, you will need to include the Notecard library.

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   #include <Notecard.h>

2. Next, you will want to create some defines to make things easier. Take care to note B0 and D5. The pin labels on the Huzzah32 and Notecarrier-AF do not line up 100%, and these defines make it easier to identify the interrupt pins to which you will be programmatically attaching.

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   #ifdef B0
   #undef B0
   #endif
   #define B0 21
   
   #ifdef D5
   #undef D5
   #endif
   #define D5 14
   
   #define LOOP_HZ 20
   #define LOOP_DELAY_MS (1000/LOOP_HZ)
   
   #define serialDebug Serial

3. You will need to instantiate the Notecard class globally, which enables you to configure and interact with your Notecard device in both the setup and loop functions.

   COPY
   Notecard notecard;

4. In order to optimize the interrupt execution (explained here), you need to declare a volatile bool flag. This allows the interrupt and main loop to share state, which enables the interrupt to offload processing onto the main loop.

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   volatile bool notecard_request_to_arm = false;

5. Declare an interrupt to handle the button press event. This ISR will notify the main loop of the request by setting the flag to true, after checking if the ATTN pin is already armed.

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   void IRAM_ATTR armInterrupt() {
     // Take no action when already armed
     if (digitalRead(D5)) {
       notecard_request_to_arm = true;
     }
   }

6. Declare an interrupt to handle the Notecard ATTN pin interrupt. Use the following code to set the Huzzah32's built-in LED to follow the state of the ATTN pin:

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   void IRAM_ATTR attention() {
     // Visualize the attention pin state
     digitalWrite(LED_BUILTIN, digitalRead(D5));
   }

setup Function

1. First, you will start by enabling debug messages for the application.

   COPY
   // Initialize Debug Output
   serialDebug.begin(115200);
   while (!serialDebug) {
     ; // wait for serial port to connect. Needed for native USB
   }

2. Next, you configure and initialize the Notecard.

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   // Initialize Notecard
   notecard.begin();
   notecard.setDebugOutputStream(serialDebug);

3. You have placed a jumper wire betwen pin 5 and ATTN. To register the attention ISR to activity on pin 5, you must use the attachInterrupt API.

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   // Attach Notecard Interrupt
   pinMode(D5, INPUT);
   attachInterrupt(digitalPinToInterrupt(D5), attention, RISING);

4. Button B0 on the Notecarrier-AF connects through the Feather socket to pin 21 of the Huzzah32. To register the armInterrupt ISR to a button press event, you must use the attachInterrupt API.

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   // Attach Button Interrupt
   pinMode(B0, INPUT_PULLUP);
   attachInterrupt(digitalPinToInterrupt(B0), armInterrupt, RISING);

5. The Notecard can be powered seperately, and operates independently, of the Huzzah32. As a result, the Huzzah32 built-in LED and the Notecard's ATTN pin can get out of sync. To ensure alignment, you must initialize the state of the LED to match the state of the ATTN pin.

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   // Debug LED (mirrors `ATTN`)
   pinMode(LED_BUILTIN, OUTPUT);
   digitalWrite(LED_BUILTIN, digitalRead(D5));

loop Function

Due the fact most of the programs logic is executed through event driven code, the loop function is dedicated to servicing the button press event.

When signalled by the interrupt driven flag, notecard_request_to_arm, the MCU will construct a JSON request and send it to the Notecard. If the message is sent successfully, then the Huzzah32's built-in LED will be updated to reflect the state of the armed interrupt. Finally, loop will delay at the frequency specified in LOOP_HZ.

COPY
void loop() {
  // Process arming request
  if (notecard_request_to_arm) {
    notecard_request_to_arm = false;

    // Arm ATTN Interrupt
    J *req = NoteNewRequest("card.attn");
    if (req) {
      JAddStringToObject(req, "mode", "arm");
      JAddNumberToObject(req, "seconds", 3);
      if (notecard.sendRequest(req)) {
        // Visualize the attention pin state
        digitalWrite(LED_BUILTIN, digitalRead(D5));
      } else {
        notecard.logDebug("ERROR: Failed to arm ATTN interrupt!\n");
      }
    }
  }

  // Loop at `LOOP_HZ` Hz
  delay(LOOP_DELAY_MS);
}

More on ESP32 Interrupt Handling

All GPIO pins on the ESP32 are interrupt capable. This is an amazing feature of the ESP32, and is not true of most microcontrollers. However, the ESP32's hardware interrupts require special handling, especially when using the Arduino board support package. Those details, and more, are discussed in this section.

The Notecarrier-AF built-in button B0 is denoted by the orange line (bottom), and the Notecard's ATTN interrupt is shown in white (top). As illustrated by the yellow marker, the rising edge of the button is the trigger. Once the button is released, the program generates and sends a request to arm the attention interrupt (depicted by the red area). Lastly, the Notecard arms the ATTN interrupt, observed as the white line being pulled LOW.

What exactly happens after the button press? Here is the break down:

1. The MCU awaits the remaining frequency delay.
2. The MCU sees the flag indicating the user has made a request to arm.
3. The MCU forms and sends a JSON request to the Notecard to arm the ATTN pin.
4. The Notecard arms the ATTN pin by pulling the line LOW.

In the timing graph, you can see it takes ~80ms to service the request to arm. While 80ms may seem fast, it is quite slow for an MCU, and is precisely why the operation needs to be moved out of the interrupt and into the main loop.

note

ESP32 Interrupt Service Routines should be decorated with IRAM_ATTR(1).

What is IRAM_ATTR?

By flagging a piece of code with the IRAM_ATTR attribute we are declaring that the compiled code will be placed in the Internal RAM (IRAM) of the ESP32.

Otherwise the code is placed in the Flash. And flash on the ESP32 is much slower than internal RAM.

If the code we want to run is an interrupt service routine (ISR), we generally want to execute it as quickly as possible. If we had to ‘wait’ for an ISR to load from flash, things would go horribly wrong.

Example:

COPY
void IRAM_ATTR attention() {
  // Visualize the interrupt
  digitalWrite(LED_BUILTIN, HIGH);
}

warning

Due to a shortcoming in Espressif System's esp32 Arduino Board Package, all interrupts must be configured to fire on the same edge. To successfully observe the Notecard attention pin interrupt, you must monitor the RISING edge. As a result, any other interrupts in your project will also need to fire on the RISING edge.

Additional Resources

• Notecard Datasheet
• Notecarrier-AF Datasheet
• Adafruit Huzzah32 ESP32 Feather
• Handling Notecard Interrupts
• Last Minute Engineers: Handling ESP32 GPIO Interrupts Tutorial
• Espressif: IRAM (instruction RAM)
• GitHub: Espressif arduino-esp32 interrupt behavior issue (#1111)