EE 345S Real-Time Digital Signal Processing Lab Fall 2007 Lab #2 Generating a Sine Wave Using the Hardware & Software Tools for the TI TMS320C6713 DSP Akshaya Srivatsa

2 Outline DSK initialization TLV320AIC23 Audio Interface Sine Wave Generation  Function Call  Lookup Table  Difference Equation Outputting Methods  Polling  Interrupts  Direct Memory Access (DMA)

3 Profiling To find the number of cycles for execution of code. Requires you to set software breakpoints ONLY.

4 Linker File: dsk6713.cmd Defines the following  Relocation of program code  Autoinitialize all C variables  Chip support libraries  Allocation of FLASH(256kB), SDRAM(16MB) and IRAM(256kB).  Allocation of stack, C I/O functions, global and static variables, interrupts Loading GEL (general extension language) extensions.  Used to initialize CPU in debug mode.

5 AIC23 Control Registers Gives control to modify the inputs and outputs DSK This is for real-time implementation purely between DSK and external measurement devices. Source\shared_codec.c will contain configurations for these resgisters.

6 AIC23 Data Word and Operations Sample on each channel is 16 bit 2’s compliment. Data between DSP and AIC23 interface is sent as 32 bit data.  Upper 16 bits = Left channel  Lower 16 bits = Right Channel  Present in Data Receive Register of McBSP 2 important functions used:  DSK6713_AIC23_read  DSK6713_AIC23_write Present in dsk6713.bsl

7 Sine Wave Generation One-sided discrete-time signal of frequency ω 0 cos(ω 0 n)u[n] One-sided continuous-time signal of frequency ω 0 cos(2πf 0 t)u(t) Using a sample frequency f s such that f s > 2.f 0 Substitute t=nT s =n/f s cos(2π(f 0 /f s )n)u[n] ω 0 =2π(f 0 /f s ) radians/sample

8 Sine Wave Generation Function Call: Use the C library function sin(x) whenever a sine value is needed, approximated as the ratio of 2 10 th order polynomials in x  Computation: 21 multiplications, 21 additions and 1 division  Memory Usage: 22 coefficients and 1 intermediate variable (x) and one constant (2  ) Lookup Table: Pre-compute and store one period of samples, can even store one-half or one-quarter period and use symmetry.  Frequency is ω 0 =2πN/L  L is the period  Interpolate stored values to get result at all frequencies  No computation needed, just memory reads.

9 Sine Wave Generation Difference Equation: Input x[n] and output y[n] (zero IC’s) y[n] = (2 cos  0 ) y[n-1] - y[n-2] + x[n] - (cos  0 ) x[n-1]  Results from z-transform of cos(ω 0 n)u[n]  Computation: 2 multiplications and 3 additions  Memory Usage: 2 coefficients, 2 previous values of y[n] and 1 previous value of x[n]  Drawback is the build-up of error due to feedback

10 Outputting Methods Polling: means constantly monitoring a device, a flag or a register until its value changes.  Poll XRDY (transmit ready) bit of the McBSP0 (Multi- Channel Buffered Serial Port) serial port until TRUE then write to DXR (data transmit register) of the McBSP1. If XRDY is TRUE, function returns the sample and the value 1, If XRDY is FALSE, function returns immediately without sending a sample and return the value 0.

11 Serial Port Transmitter CPU writes a 32-bit word to the DXR. XRDY flag is cleared whenever data is written to DXR. After a word is shifted out of XSR, a transfer of the DXR to the XSR is performed. XRDY flag is set. When XRDY is set, serial port transmitter sends an interrupt request (XINT) to CPU.

12 Serial Port Receiver Bits received serially into RSR. When word is complete it is transferred to RBR. RBR copied to DRR if it is empty. RRDY bit in SPCR is set when RSR is moved to DRR and is cleared when DRR is read. When RRDY is set, McBSP generates a CPU interrupt request (RINT).

13 Outputting Methods Polling: for ( ; ; )// Infinite Loop { x_left = scale*sin(angle_left); x_right = scale*sin(angle_right); // Increment phase angles of sine waves angle_left += delta_left; angle_right += delta_right; // Reduce angles modulo 2 pi so no overflow if (angle_left > 2.0*PI) angle_left -= 2.0*PI; if (angle_right > 2.0*PI) angle_right -= 2.0*PI; WriteSample( x_left, x_right ); } Multiply the sin() by a scale so that it doesn’t become 0 when it’s converted to integer since |sin()|<1 and floats less 1 are converted to 0

14 Outputting Methods Polling: void WriteSample( float left, float right) { Uint32 ileft = (Uint32) left; Uint32 iright = (Uint32)right; Uint32 output; /* Combine ileft and iright into a 32-bit word */ output = (iright << 16) | (ileft & 0x0000ffff); /* DSK6713_AIC23_write polls the Ready flag of the serial port and returns true/false to indicate success. */ while(!DSK6713_AIC23_write(hCodec, output)); } Poll XRDY bit and write output to McBSP0 DXR Combine both samples into a 32-bit word to transmit via McBSP1 Convert float to integer (IMPORTANT)

15 Important Points Know your sampling frequency.  Its in shared_codec.c Do not put a huge value for scaling factor (Keep it less than 15000) Signal frequency must be kept below one half of sampling frequency.

16 Outputting Methods Polling:  Most of the time in the polling method is spent in the infinite loop waiting for the XRDY flag to get set,  DSP is doing nothing,  A more efficient approach is to let DSP run tasks in background (modulating/demodulating, coding/decoding…),  So, serial port will interrupt background tasks when sample is received and needs to be transmitted.

17 Outputting Methods Interrupts: are signals from hardware/software indicating the need for attention or change of execution. C6713 DSP has a vectored priority interrupt controller that can handle 16 different interrupts:  RESET interrupt has highest priority, cannot be masked,  NMI (Non-Maskable Interrupt) has the second highest priority (used to notify DSP of serious hardware errors),  2 reserved maskable interrupts come next,  12 additional maskable interrupts (INT4-INT15) have the lowest priority.

18 Interrupts Interrupt Control Registers Interrupt Source Default Mappings

19 Outputting Methods Conditions for an interrupt to occur:  Global Interrupt Enable bit (bit 0 of Control Status Register) must be set to 1. If GIE=0, no maskable interrupt can occur,  Non-Maskable Interrupt Enable bit (in the Interrupt Enable Register) must be set to 1. If NMIE=0, no maskable interrupt can occur,  Bit corresponding to desired interrupt must be set to 1 in Interrupt Enable Register. When interrupt occurs, corresponding bit in IFR (interrupt flag register) is set to 1. No higher priority interrupt flag should be set to 1 in IER.

20 Outputting Methods When CPU interrupt ‘n’ occurs, program execution jumps to byte offset 32×n in an Interrupt Service Table (IST) which contains 16 Interrupt Service Fetch Packets (ISFP) each containing 8 32-bit instructions. ISFP may contain an entire Interrupt Service Routine (ISR) or may branch to a larger one. IST is relocated during initialization  Base address is held by Interrupt Service Table Pointer(ISTP)

21 Outputting Methods When an interrupt occurs:  DSP sets corresponding interrupt flag in IFR to 1,  If GIE=NMIE=1 and no higher priority interrupts are pending, interrupt is serviced: GIE copied to PGIE (previous global interrupt enable bit) and GIE is cleared, Flag bit in IFR is cleared indicating interrupt is serviced, Return address is put in the Interrupt Return Pointer (IRP), Execution jumps to corresponding ISFP entry, Service routine must save the CPU state and restore it on exit. User defined ISRs syntax  interrupt void your_isr_name(){body}

22 Outputting Methods /* Select McBSP1 transmit int for INT15 */ intr_map(CPU_INT15, ISN_XINT1); /* Hook our ISR to INT15 intr_hook(tx_isr, CPU_INT15); /* Clear old interrupts INTR_CLR_FLAG(CPU_INT15); /* Enable interrupts INTR_ENABLE(CPU_INT_NMI); /* Set INT15 bit in IER INTR_ENABLE(CPU_INT15); /* Turn on enabled ints INTR_GLOBAL_ENABLE(); /*To write out MCBSP_write(DSK6713_AIC23_DATAHANDLE, output);

23 Templates for programs are available online ab2/index.html Lets get started then!!