Basic Infos

• This issue complies with the issue POLICY doc.
• I have read the documentation at readthedocs and the issue is not addressed there.
• I have tested that the issue is present in current master branch (aka latest git).
• I have searched the issue tracker for a similar issue.
• If there is a stack dump, I have decoded it.
• I have filled out all fields below.

Platform

• Hardware: [ESP8285 device]
• Core Version: [f2de9e1]
• Development Env: [Arduino IDE]
• Operating System: [Ubuntu]

Settings in IDE

• Module: [Generic ESP8285 Module]
• Flash Mode: [dout]
• Flash Size: [1MB]
• lwip Variant: [v2 Lower Memory]
• Reset Method: [nodemcu]
• Flash Frequency: [40Mhz]
• CPU Frequency: [80Mhz]
• Upload Using: [SERIAL]
• Upload Speed: [115200] (serial upload only)

Problem Description

I have seen in several places where ets_printf() is the recommended function call for debug printing from an ISR; however, from what I see in the current master I don't think this is true anymore.

ets_printf() uses the character print function installed by ets_install_putc1(). At the start of setup() the print function address resides in IRAM. Once Serial.begin() is called that print function address resides in flash. The MCVE below shows the function address under various situations and the resulting output across Serial and Serial1.

It looks to me that of these options, the safest ISR debug print solution is to use ets_uart_printf() and/or ets_putc(). Both of these functions reside in ROM and neither uses the print function installed by ets_install_putc1(). They use an internal ROM function for printing, which requires a prior call to uart_buff_switch() to select a UART.

MCVE Sketch

#include <ESP8266WiFi.h>
#include <ets_sys.h>
#include <osapi.h>

// Boot ROM print functions
extern "C" int ets_uart_printf(const char *format, ...)  __attribute__ ((format (printf, 1, 2)));
extern "C" void ets_write_char(char c);
extern "C" void uart_buff_switch(uint8_t);
typedef void (*fn_putc1_t)(char);

// Boot ROM data area reference
// Location where ets_install_putc1 saves function address
#define installed_putc1 ((fn_putc1_t)(*(void **)0x3fffdd48))

void print_test() {
  char c;
  size_t i;
  os_printf("  os_print()\n");
  os_printf_plus("  os_print_plus()\n");
  ets_printf("  ets_printf()\n");
  for (i=0; (c="  installed_putc1()\n"[i]); i++) installed_putc1(c);
  for (i=0; (c="  ets_write_char()\n"[i]); i++) ets_write_char(c);

  ets_uart_printf("  ets_uart_printf()\n");
  for (i=0; (c="  ets_putc()\n"[i]); i++) ets_putc(c);
}

void setup() {
  fn_putc1_t original_putc1 = installed_putc1; 
  WiFi.mode(WIFI_OFF);

  Serial.begin (115200);
  Serial1.begin(115200);

  Serial.printf ("\nOriginal value at setup()     putc1 = %p\n", original_putc1);
  Serial1.printf("\nOriginal value at setup()     putc1 = %p\n", original_putc1);

  Serial.printf ("\nAfter Serial.begin(115200)    putc1 = %p\n", installed_putc1);
  Serial1.printf("\nAfter Serial.begin(115200)    putc1 = %p\n", installed_putc1);
  print_test();
        
  Serial.setDebugOutput(true);
  Serial.printf ("\nSerial.setDebugOutput(true)   putc1 = %p\n", installed_putc1);
  Serial1.printf("\nSerial.setDebugOutput(true)   putc1 = %p\n", installed_putc1);
  print_test();
  
  Serial1.setDebugOutput(true);
  Serial1.printf("\nSerial1.setDebugOutput(true)  putc1 = %p\n", installed_putc1);
  Serial.printf ("\nSerial1.setDebugOutput(true)  putc1 = %p\n", installed_putc1);
  print_test();

  Serial1.setDebugOutput(false);
  Serial.printf ("\nSerial1.setDebugOutput(false) putc1 = %p\n", installed_putc1);
  Serial1.printf("\nSerial1.setDebugOutput(false) putc1 = %p\n", installed_putc1);
  print_test();

  uart_buff_switch(0);
  Serial.printf ("\nuart_buff_switch(0)           putc1 = %p\n", installed_putc1);
  Serial1.printf("\nuart_buff_switch(0)           putc1 = %p\n", installed_putc1);
  print_test();

  uart_buff_switch(1);
  Serial.printf ("\nuart_buff_switch(1)           putc1 = %p\n", installed_putc1);
  Serial1.printf("\nuart_buff_switch(1)           putc1 = %p\n", installed_putc1);
  print_test();

  uart_buff_switch(0);
  Serial.printf ("\nuart_buff_switch(0)           putc1 = %p\n", installed_putc1);
  Serial1.printf("\nuart_buff_switch(0)           putc1 = %p\n", installed_putc1);
  print_test();

}

void loop() {}

Debug Messages

From Serial

Original value at setup()     putc1 = 0x401049e0

After Serial.begin(115200)    putc1 = 0x40203d40
  ets_uart_printf()
  ets_putc()

Serial.setDebugOutput(true)   putc1 = 0x40203024
  os_print()
  os_print_plus()
  ets_printf()
  installed_putc1()
  ets_write_char()
  ets_uart_printf()
  ets_putc()

Serial1.setDebugOutput(true)  putc1 = 0x40202fe8
  ets_uart_printf()
  ets_putc()

Serial1.setDebugOutput(false) putc1 = 0x40203d40
  ets_uart_printf()
  ets_putc()

uart_buff_switch(0)           putc1 = 0x40203d40
  ets_uart_printf()
  ets_putc()

uart_buff_switch(1)           putc1 = 0x40203d40

uart_buff_switch(0)           putc1 = 0x40203d40
  ets_uart_printf()
  ets_putc()

From Serial1

Original value at setup()     putc1 = 0x401049e0

After Serial.begin(115200)    putc1 = 0x40203d40

Serial.setDebugOutput(true)   putc1 = 0x40203024

Serial1.setDebugOutput(true)  putc1 = 0x40202fe8
  os_print()
  os_print_plus()
  ets_printf()
  installed_putc1()
  ets_write_char()

Serial1.setDebugOutput(false) putc1 = 0x40203d40

uart_buff_switch(0)           putc1 = 0x40203d40

uart_buff_switch(1)           putc1 = 0x40203d40
  ets_uart_printf()
  ets_putc()

uart_buff_switch(0)           putc1 = 0x40203d40