Arduino紅外發射的調頻定時器設置
原博客格式更友好：http://www.straka.cn/blog/arduino-uno-infrared-emission-timer-setup/
網上了解了下ARDUINO的定時器、中斷、PWM、舵機控制，紅外收發等相關知識。尤其是仔細閱讀了AVR atmega328p，也就是ARDUINO UNO的芯片手冊的定時器部分，其中有兩點：
- AT mega328p的定時器有3個，對應Arduino UNO板子，
- Timer0 對應 5、6引腳pwm, 8bit
- Timer1 對應 9、10引腳pwm, 16bit
- Timer2 對應 11、3引腳pwm, 8bit
- 舵機的pwm頻率爲50Hz / 20ms, 但是控制舵機需要的佔空比比較小，爲20ms中的5 ~ 2.5ms。
先講講紅外發射的調頻原理吧
那麼根據1，由於紅外發射需要38khz左右的載波頻率，不過不用特別精確的38khz，所以紅外收發的庫裏，
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#define TIMER_RESET #define TIMER_ENABLE_PWM (TCCR2A |= _BV(COM2B1)) #define TIMER_DISABLE_PWM (TCCR2A &= ~(_BV(COM2B1))) #define TIMER_ENABLE_INTR (TIMSK2 = _BV(OCIE2A)) #define TIMER_DISABLE_INTR (TIMSK2 = 0) #define TIMER_INTR_NAME TIMER2_COMPA_vect
#define TIMER_CONFIG_KHZ(val) ({ \ const uint8_t pwmval = SYSCLOCK / 2000 / (val); \ TCCR2A = _BV(WGM20); \ TCCR2B = _BV(WGM22) | _BV(CS20); \ OCR2A = pwmval; \ OCR2B = pwmval / 3; \ })
#define TIMER_COUNT_TOP (SYSCLOCK * USECPERTICK / 1000000) |
假若是16M的晶振，根據上述代碼和手冊，WGM20和WGM22置1，所以Mode爲5
那麼就是pwm，phase correct模式，
主要意思就是，每週期在ocrA處達到計數頂部，觸發中斷，然後從0開始計數，由於不是到達頂部255溢出才從0開始重新計數，所以能實現相位的調整，而不再是fclk / prescale / 256的頻率。因爲CS20置位，所以預除數是1，即沒有預除數prescale。
那麼頻率就是fout= fclk / ocrA / 2，除2是因爲兩個中斷才完成電平翻轉，形成載波的一個週期。
那我們很容易得到，如果我們想要38khz的頻率，用16M晶振獲取，需要設置OCRA爲
16M/38k/2=210，那麼實際頻率 16m / 210 / 2 = 38095.238hz。
中斷向量TIMER2_COMPA_vect的處理函數
中要做的就是根據要收發的消息，對載波進行調製，根據需要調製上載波或者去掉載波。
此處見代碼，不詳述。
C++
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//+============================================================================= // Interrupt Service Routine - Fires every 50uS // TIMER2 interrupt code to collect raw data. // Widths of alternating SPACE, MARK are recorded in rawbuf. // Recorded in ticks of 50uS [microseconds, 0.000050 seconds] // 'rawlen' counts the number of entries recorded so far. // First entry is the SPACE between transmissions. // As soon as a the first [SPACE] entry gets long: // Ready is set; State switches to IDLE; Timing of SPACE continues. // As soon as first MARK arrives: // Gap width is recorded; Ready is cleared; New logging starts // ISR (TIMER_INTR_NAME) { TIMER_RESET;
// Read if IR Receiver -> SPACE [xmt LED off] or a MARK [xmt LED on] // digitalRead() is very slow. Optimisation is possible, but makes the code unportable uint8_t irdata = (uint8_t)digitalRead(irparams.recvpin);
irparams.timer++; // One more 50uS tick if (irparams.rawlen >= RAWBUF) irparams.rcvstate = STATE_OVERFLOW ; // Buffer overflow
switch(irparams.rcvstate) { //...................................................................... case STATE_IDLE: // In the middle of a gap if (irdata == MARK) { if (irparams.timer < GAP_TICKS) { // Not big enough to be a gap. irparams.timer = 0;
} else { // Gap just ended; Record duration; Start recording transmission irparams.overflow = false; irparams.rawlen = 0; irparams.rawbuf[irparams.rawlen++] = irparams.timer; irparams.timer = 0; irparams.rcvstate = STATE_MARK; } } break; //...................................................................... case STATE_MARK: // Timing Mark if (irdata == SPACE) { // Mark ended; Record time irparams.rawbuf[irparams.rawlen++] = irparams.timer; irparams.timer = 0; irparams.rcvstate = STATE_SPACE; } break; //...................................................................... case STATE_SPACE: // Timing Space if (irdata == MARK) { // Space just ended; Record time irparams.rawbuf[irparams.rawlen++] = irparams.timer; irparams.timer = 0; irparams.rcvstate = STATE_MARK;
} else if (irparams.timer > GAP_TICKS) { // Space // A long Space, indicates gap between codes // Flag the current code as ready for processing // Switch to STOP // Don't reset timer; keep counting Space width irparams.rcvstate = STATE_STOP; } break; //...................................................................... case STATE_STOP: // Waiting; Measuring Gap if (irdata == MARK) irparams.timer = 0 ; // Reset gap timer break; //...................................................................... case STATE_OVERFLOW: // Flag up a read overflow; Stop the State Machine irparams.overflow = true; irparams.rcvstate = STATE_STOP; break; }
#ifdef BLINKLED // If requested, flash LED while receiving IR data if (irparams.blinkflag) { if (irdata == MARK) if (irparams.blinkpin) digitalWrite(irparams.blinkpin, HIGH); // Turn user defined pin LED on else BLINKLED_ON() ; // if no user defined LED pin, turn default LED pin for the hardware on else if (irparams.blinkpin) digitalWrite(irparams.blinkpin, LOW); // Turn user defined pin LED on else BLINKLED_OFF() ; // if no user defined LED pin, turn default LED pin for the hardware on } #endif // BLINKLED } |
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