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EE 345S Real-Time Digital Signal Processing Lab Fall 2007

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 Wael Barakat. Outline. Sine Wave Generation Function Call Lookup Table Difference Equation Outputting Methods Polling Interrupts

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EE 345S Real-Time Digital Signal Processing Lab Fall 2007

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  1. EE 345S Real-Time Digital Signal Processing LabFall 2007 Lab #2Generating a Sine Wave Using the Hardware & Software Tools for the TI TMS320C6713 DSPWael Barakat

  2. Outline • Sine Wave Generation • Function Call • Lookup Table • Difference Equation • Outputting Methods • Polling • Interrupts • Direct Memory Access (DMA)

  3. Sine Wave Generation • One-sided discrete-time signal of frequency ω0 cos(ω0n)u[n] • One-sided continuous-time signal of frequency ω0 cos(2πf0t)u(t) Using a sample frequency fssuch that fs > 2.f0 Substitute t=nTs=n/fs cos(2π(f0/fs)n)u[n] ω0=2π(f0/fs) radians/sample

  4. Sine Wave Generation • Function Call: Use the C library function sin(x) whenever a sine value is needed, approximated as the ratio of 2 10th 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 ω0=2πN/L • L is the period • Interpolate stored values to get result at all frequencies • No computation needed, just memory reads.

  5. 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(ω0n)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

  6. 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.

  7. 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.

  8. 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).

  9. 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

  10. 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)); } Convert float to integer Combine both samples into a 32-bit word to transmit via McBSP1 Poll XRDY bit and write output to McBSP0 DXR

  11. 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.

  12. 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.

  13. Interrupts Interrupt Control Registers Interrupt Source Default Mappings

  14. 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.

  15. 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.

  16. 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 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.

  17. 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();

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