identifier "Serial1" is undefined

[clarkbremer]

I have an Arduino Mega 2560. It has 3 serial ports. The standard is "Serial", the others are "Serial1" and "Serial2". VSCode doesn't have a problem with "Serial", but shows an error for "Serial1". Code compiles and uploads just fine, but would be nice not get this error message.

Version: 1.36.1 (user setup)
Commit: 2213894ea0415ee8c85c5eea0d0ff81ecc191529
Date: 2019-07-08T22:59:35.033Z
Electron: 4.2.5
Chrome: 69.0.3497.128
Node.js: 10.11.0
V8: 6.9.427.31-electron.0
OS: Windows_NT x64 10.0.17134

Name: Arduino
Id: vsciot-vscode.vscode-arduino
Description: Arduino for Visual Studio Code
Version: 0.2.27
Publisher: Microsoft
VS Marketplace Link: https://marketplace.visualstudio.com/items?itemName=vsciot-vscode.vscode-arduino

[mMerlin]

Try adding

"defines": ["USBCON"]

to "configurations" in the c_cpp_properties.json file in the .vscode folder. Or adding "USBCON" to the list, if "defines" already exists.

[jimaobian]

If you are using arduino UNO (ATmega328P chip) try:

"defines": [
                "__AVR_ATmega328P__",
                "ARDUINO=10800"
           ]

If you are using arduino Leonardo (ATmega32U4 chip) try:

"defines": [
                "__AVR_ATmega32U4__",
                "ARDUINO=10800"
           ]

If you are using arduino Mega (ATmega2560 chip) try:

"defines": [
                "__AVR_ATmega2560__",
                "ARDUINO=10800"
           ]

Here is the io.h file in AVR library for reference, if you are using other board of AVR.

[Xring-git]

The same for me, but with:

TCCR2A
TCCR2B
TCNT2
OCR2A
WGM21
CS22
CS20

Code compiles and uploads just fine, but with "Not defined" error.

With F12 appear multiple .h files, seems that "AVR_ATmega328P" define is ignored.

[thomasio101]

The problem

I had a similar issue, the issue with Serial was indeed resolved by adding "USBCON" to defines, but I still had issues with identifiers such as DDRD being undefined. (After writing this answer, I discovered that @jimaobian's answer has the same effect.)

TLDR

Add the following properties to c_cpp_properties.json:

"compilerArgs": ["-mmcu=atmega328"],
"defines": ["USBCON"]

Explanation

After some research, I discovered that the definition for DDRD can be found in multiple files (each of which is for a different MCU version). For example, this is a definition from C:\Program Files (x86)\Arduino\hardware\tools\avr\avr\include\avr\iom328p.h;

#define DDRD _SFR_IO8(0x0A)

At the top of that file, I found this comment;

/* This file should only be included from <avr/io.h>, never directly. */

Thus, I took a look at C:\Program Files (x86)\Arduino\hardware\tools\avr\avr\include\avr\io.h. That file included the following comment;

This header file includes the apropriate IO definitions for the
device that has been specified by the -mmcu= compiler
command-line switch. This is done by diverting to the appropriate
file <avr/ioXXXX.h> which should
never be included directly. Some register names common to all
AVR devices are defined directly within <avr/common.h>,
which is included in <avr/io.h>,
but most of the details come from the respective include file.

After finding this, I managed to find the -mmcu argument in this documentation. Based on that documentation I concluded that I need to pass -mmcu=atmega328 to the compiler for it to work with an Arduino with an ATmega328 MCU.

Now, my c_cpp_properties.json looks like this;

{
    "configurations": [
        {
            "name": "Win32",
            "includePath": [
                "${workspaceFolder}/**",
                "C:/Program Files (x86)/Arduino/hardware/arduino/avr/cores/arduino",
                "C:/Program Files (x86)/Arduino/libraries/**",
                "C:/Program Files (x86)/Arduino/hardware/arduino/avr/libraries/EEPROM/src",
                "C:/Program Files (x86)/Arduino/hardware/arduino/avr/libraries/HID/src",
                "C:/Program Files (x86)/Arduino/hardware/arduino/avr/libraries/SoftwareSerial/src",
                "C:/Program Files (x86)/Arduino/hardware/arduino/avr/libraries/SPI/src",
                "C:/Program Files (x86)/Arduino/hardware/arduino/avr/libraries/Wire/src",
                "C:/Program Files (x86)/Arduino/tools/**",
                "C:/Program Files (x86)/Arduino/hardware/arduino/avr/**",
                "C:/Program Files (x86)/Arduino/hardware/tools/avr/avr/include",
                "C:/Program Files (x86)/Arduino/hardware/tools/avr/lib/gcc/avr/7.3.0/include"
            ],
            "forcedInclude": [
                "C:/Program Files (x86)/Arduino/hardware/arduino/avr/cores/arduino/Arduino.h"
            ],
            "compilerArgs": ["-mmcu=atmega328"],
            "intelliSenseMode": "clang-x64",
            "compilerPath": "C:/Program Files (x86)/Arduino/hardware/tools/avr/bin/avr-gcc.exe",
            "cStandard": "c17",
            "cppStandard": "c++17",
            "defines": ["USBCON"]
        }
    ],
    "version": 4
}

[YakovL]

I had problems similar to those of @Xring-git :
OUTPUT, PORTC,
TCCR1A, (but not TCCR1B!), TCNT1, OCR1A, WGM12, CS10, TIMSK1, OCIE1A, TIMER1_COMPA_vect
all give errors like

identifier "DDRC" is undefined

which don't prevent sketch from compiling/uploading.

When I opened ./vscode/c_cpp_properties.json and changed

        "defines": [
            "USBCON"
        ],

to

        "defines": [
            "__AVR_ATmega328P__",
            "ARDUINO=10800",
            "USBCON"
        ],

All those errors went away.

However, I still have errors for noInterrupts, interrupts and ISR functions. They tell specifically:

#define noInterrupts() cli()
Expands to:

__asm__ __volatile__ ("cli" ::: "memory")
identifier "__asm__" is undefined C/C++(20)

and

#define noInterrupts() sei()
Expands to:

__asm__ __volatile__ ("sei" ::: "memory")
identifier "__asm__" is undefined C/C++(20)

and

#define ISR(vector,...) extern "C" void vector (void) __attribute__ ((signal,__INTR_ATTRS)) __VA_ARGS__; void vector (void)
Expands to:

extern "C" void __vector_11 (void) __attribute__ ((signal,used)) ; void __vector_11 (void)
expected a '{'C/C++ (130)

I wonder if there's a more systematic approach to fix these, also without editing c_cpp_properties.json per each sketch.