Senior Design II paper
Headphone to Television Identification System
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- Infrared Receiver Headphone Identification Program
Headphone to Television Identification System
This program allows the user to lock in a televisions audio through a button press. Essentially one program was created for all the headphone IR emissions with the acceptation of changing one line of code for the desired 8 bit headphone ID. Before the main loop, the variables and function prototypes are declared. The main program first disables the Watch Dog Timer, and initializes the Digitally Controlled Oscillator (DCO) to ~300KHz by setting RSEL=3, DCOx=3 and MODx=3. Once the DCO is initialized to ~300KHz it can be scaled down to 37.5KHz by dividing the clock by 8. Now the SMCLK is running on 37.5KHz. This frequency falls within the acceptable range of carrier frequency of the carrier. By setting the frequency of the SMCLK to 37.5KHz, P1.4 on the MSP430 can output an SMCLK signal. This pin is the same pin connected to the LED array. The PSEL feature of P1.4 allows for the LEDs to blink at 37.5KHz when the PSEL1 line for bit 4 goes high. Next the program initializes the pins used on the MSP430. For the infrared LED headphone tag program, two pins are initialized. P1.2 (the button pin) is set as an input with a high to low interrupt and interrupt enable. Then P1.4 is set as an output with a value initialized as zero. The infinite while loop within the main program waits for port1flag to go high otherwise it remains stuck in its own while loop. The variable port1flag is only set high only within the interrupt Port1 function. Then the program continues to the transmit function. This program consists of seven dependent functions described below:
The delay functions utilize TimerA that sets the clock source to the sub-main clock , stops it, and clears it. The register CCR0 set the number of SMCLK cycles to count up to. Only the Sub-Main clock can be used on P1.4 operating on 37.5KHz. The CCR0 value must be 1 cycle less than the desired clock cycle. TimerA is then set to count up and then stopped and cleared once the register counts up.
This program is responsible for translating the infrared detector output back into an 8 bit ID. This ID is then compared to the IDs of headphone1 and headphone2 to identify which headphone sent the PWM to the television. Once a match is recognized the program communicates with the master board. The MSP430 is connected to the Stellaris through four lines, RTS, Line0, Line1 and ground. The IR emitted signal has a Start bit followed by 8 bits used to for identification. The emitted signal has a Start bit of "high" 50 ms "low" 25 ms. A One bit is sent as "high" 40 ms and "low" 25 ms. A Zero bit is sent as "high" 25 ms and "low" 25 ms. Whenever the emitted signal is "high" an IR LED pulsates at 38 kHz in the direction the headphone user is viewing. An IR detector module (TSOP32338) located on the top of each television can detect the high signal sent by the LED array. The IR detector outputs VCC when the signal detected is "low" meaning it did not detect an IR signal pulsation at 38KHz. The detector outputs ground when the signal detected is "high". The MSP430g2231 connected to the detector Vout pin will use capture feature on P1.2 to reform an 8 bit ID. The 2 IDs in the system are headphone 1 and headphone 2 which are directly related to the emitted IR signal sent out by each headphone. In this program the variables are 16 bit integers where only the 8 least significant bits are used. These IDs will be compared to the received infrared data from the detector. This program identifies which headphone is viewing the detector. For this application an SIRC protocol in most remote controls will be used to create an 8 bit variable (rxirdata) from detected IR LED Pulse Width Modulation (PWM). The detector Vout will be directly connected to P1.2 to the TimerA capture mode. The clock source used in this program will be Sub-Main Clock which uses the internal clock to keep time. TimerA is set to the SMCLK. In the subroutine IR_Ready the SMCLK is set by the CCTL1 (Timer A Control 1) to a capture, falling edge and interrupt enabled. The SMCLK is initally set to falling edge because the Start bit will begin on a falling edge. At this point, a TimerA interrupt will occur. Within the TimerA interrupt the CCTL1 is set to capture on a rising edge from here on out. The time the negative edge took place will be stored in CCR1. This value will be saved in newtime then transfers the value to oldtime at the end of the interrupt. When the next interrupt occurs the current "time" will be saved in newtime. Therefore subtracting newtime from oldtime will give the period of the Start bit. To confirm this Start bit is for our application, a comparison will be done to confirm it within the range of 45 ms (IR_Start_min) and 55 ms (IR_Start_max). If this is true, each time an interrupt occurs (only on positive edge), the length of the bit will be determined (timedif = newtime-oldtime) and compared to IR_Mid. IR_Mid is (Bit1 time period + Bit0 time period)/2. For example: (65ms + 50ms)/2 = 57.5 ms. If the period of the bit value captured is greater than IR_Mid, then bit=1, if the period of the bit value is less than IR_Mid, then bit=0. The bits are stored into rxirdata and shifted each time to complete the 8 bit value. For the MSP430 to send data to the Stellar, 4 lines were established. The ReadytoSend line goes high when a match is found and data needs to be sent to the Stellaris. Line0 and Line1 represent bit0 and bit1 respectively for the variable match. The RTS flags the Stellaris that the data lines are new IDs. Right after the RTS line is sent, Line0 and Line1 are set high or low to the Stellaris. Once the data is sent out, the program awaits for another IR signal to decode. To confirm the correct headphone IR signal is being matched with the correct decoded signal and sent to the Stellaris correctly. LED1 will turn ON when headphone 1 is matched and LED2 will turn ON when headphone 2 is matched. The LEDs will turn off once the program sends out data on Line0 and Line1. The main functions in this program are:
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