在官方例程中看到了这个绘制时钟的例程，实在是巧妙，在这篇随笔中总结一下：
1.初始时间从编译时间__TIME__获得,详细资料参考 C语言获取编译时间
2.通过millis()函数计时，参考 millis()函数作用；大概思路就是初始有一个targetTime比millis()返回值大1000(即大1秒)，在loop中如果targetTime
3.绘制指针时充分使用正余弦，秒针的移动通过不断地用背景色覆盖旧指针，绘制新指针实现，分针和时针类似，不过是在秒数等于0时做相应操作
#include
#define TFT_GREY 0x5AEB
//uint16_t 就是无符号的2字节单元
float sx = 0, sy = 1, mx = 1, my = 0, hx = -1, hy = 0; // Saved H, M, S x & y multipliers
float sdeg=0, mdeg=0, hdeg=0;
uint16_t osx=120, osy=120, omx=120, omy=120, ohx=120, ohy=120; // Saved H, M, S x & y coords
uint16_t x0=0, x1=0, yy0=0, yy1=0;
uint32_t targetTime = 0; // for next 1 second timeout
static uint8_t conv2d(const char* p); // Forward declaration needed for IDE 1.6.x
uint8_t hh=conv2d(__TIME__), mm=conv2d(__TIME__+3), ss=conv2d(__TIME__+6); //通过编译结束获得时间Get H, M, S from compile time
boolean initial = 1;
void setup(void) {
M5.begin();
M5.Power.begin();
// M5.Lcd.setRotation(0);
//M5.Lcd.fillScreen(TFT_BLACK);
//M5.Lcd.fillScreen(TFT_RED);
//M5.Lcd.fillScreen(TFT_GREEN);
//M5.Lcd.fillScreen(TFT_BLUE);
//M5.Lcd.fillScreen(TFT_BLACK);
M5.Lcd.fillScreen(TFT_GREY);
M5.Lcd.setTextColor(TFT_WHITE, TFT_GREY); // Adding a background colour erases previous text automatically
// 表盘
M5.Lcd.fillCircle(160, 120, 118, TFT_GREEN);
M5.Lcd.fillCircle(160, 120, 110, TFT_BLACK);
// Draw 12 lines
for(int i = 0; i<360; i+= 30) {
sx = cos((i-90)*0.0174532925);
sy = sin((i-90)*0.0174532925);
x0 = sx*114+160;
yy0 = sy*114+120;
x1 = sx*100+160;
yy1 = sy*100+120;
M5.Lcd.drawLine(x0, yy0, x1, yy1, TFT_GREEN);
}
// 画60个点
for(int i = 0; i<360; i+= 6) {
sx = cos((i-90)*0.0174532925);
sy = sin((i-90)*0.0174532925);
x0 = sx*102+160;
yy0 = sy*102+120;
// Draw minute markers
M5.Lcd.drawPixel(x0, yy0, TFT_WHITE);
// Draw main quadrant dots
if(i==0 || i==180) M5.Lcd.fillCircle(x0, yy0, 2, TFT_WHITE);
if(i==90 || i==270) M5.Lcd.fillCircle(x0, yy0, 2, TFT_WHITE);
}
M5.Lcd.fillCircle(160, 121, 3, TFT_WHITE);
// Only font numbers 2,4,6,7 are valid. Font 6 only contains characters [space] 0 1 2 3 4 5 6 7 8 9 : . - a p m
// Font 7 is a 7 segment font and only contains characters [space] 0 1 2 3 4 5 6 7 8 9 : .
targetTime = millis() + 1000; //millis()返回上电启动后的时间
}
void loop() {
if (targetTime < millis()) {
targetTime += 1000;
ss++; // Advance second
if (ss==60) {
ss=0;
mm++; // Advance minute
if(mm>59) {
mm=0;
hh++; // Advance hour
if (hh>23) {
hh=0;
}
}
}
// Pre-compute hand degrees, x & y coords for a fast screen update
sdeg = ss*6; // 0-59 -> 0-354
mdeg = mm*6+sdeg*0.01666667; // 0-59 -> 0-360 - includes seconds
hdeg = hh*30+mdeg*0.0833333; // 0-11 -> 0-360 - includes minutes and seconds
hx = cos((hdeg-90)*0.0174532925);
hy = sin((hdeg-90)*0.0174532925);
mx = cos((mdeg-90)*0.0174532925);
my = sin((mdeg-90)*0.0174532925);
sx = cos((sdeg-90)*0.0174532925);
sy = sin((sdeg-90)*0.0174532925);
if (ss==0 || initial) {
initial = 0;
// Erase hour and minute hand positions every minute
//擦除表上的旧时针和分针
M5.Lcd.drawLine(ohx, ohy, 160, 121, TFT_BLACK);
ohx = hx*62+161;
ohy = hy*62+121;
M5.Lcd.drawLine(omx, omy, 160, 121, TFT_BLACK);
omx = mx*84+160;
omy = my*84+121;
}
// Redraw new hand positions, hour and minute hands not erased here to avoid flicker
M5.Lcd.drawLine(osx, osy, 160, 121, TFT_BLACK);//擦除旧秒针
osx = sx*90+161;
osy = sy*90+121;
M5.Lcd.drawLine(osx, osy, 160, 121, TFT_RED);
M5.Lcd.drawLine(ohx, ohy, 160, 121, TFT_WHITE);
M5.Lcd.drawLine(omx, omy, 160, 121, TFT_WHITE);
M5.Lcd.drawLine(osx, osy, 160, 121, TFT_RED);//画了分针可能把秒针给盖住了
M5.Lcd.fillCircle(160, 121, 3, TFT_RED);
}
}
static uint8_t conv2d(const char* p) {//将字符型的10-99转为无符号数
uint8_t v = 0;
if ('0' <= *p && *p <= '9')
v = *p - '0';
return 10 * v + *++p - '0';
}