Description: Adjust the brightness of the screen backlight.
Parameter Description
brightness TFT backlight brightness ( value: 7 - 15 )

https://github.com/m5stack/m5-docs/blob/master/docs/en/api/axp192_m5stickc.md says:

Description: Change the LDO3 output voltage of the AXP192 chip.
parameter description
brightness TFT backlight brightness (range: 7~12)

In https://github.com/m5stack/M5Core2/blob/master/src/AXP192.cpp it's 0 to 100:

void AXP192::ScreenBreath(int brightness) {
int vol = map(brightness, 0, 100, 2400, 3300);

In https://github.com/m5stack/M5StickC-Plus/blob/master/src/AXP192.cpp it's 0 to 100:

void AXP192::ScreenBreath(int brightness) {
if (brightness > 100 || brightness < 0) return;
int vol = map(brightness, 0, 100, 2500, 3200);
vol = (vol < 1800) ? 0 : (vol - 1800) / 100;

Currently, with the Arduino M5StickCPlus library 0.0.8, the range seems to be 7 to 12. Also, 0 to 7 are not quite all the way off (very dim).

[code]
#include <M5StickCPlus.h>

void setup() {
M5.begin();
M5.Lcd.setTextSize(5);
M5.Lcd.setRotation(3);
M5.Lcd.setTextColor(TFT_BLUE);
}

void loop() {
int bright = 0;
while (bright < 15) {
M5.Lcd.fillScreen(WHITE);
M5.Lcd.setCursor(20,20);
M5.Axp.ScreenBreath(bright);
M5.Lcd.println(bright);
bright = bright + 1;
M5.update();
delay(2000);
}
while (bright < 100) {
M5.Lcd.fillScreen(WHITE);
M5.Lcd.setCursor(20,20);
M5.Axp.ScreenBreath(bright);
M5.Lcd.println(bright);
bright = bright + 10;
M5.update();
delay(500);
}
}
[/code]