EECS20: Introduction to Real-Time Digital Systems
Lab08: Multiple Tones/Tones in Real Time
EECS20: Introduction to Real-Time Digital Systems
©1996 Regents of the University of California.
By K. H. Chiang, William T. Huang, Brian L. Evans.
URL: http://www-inst.eecs.berkeley.edu/~ee20
News: ucb.class.ee20
In this lab, we will study the generation of Touchtone (Dual-Tone Multiple Frequency or DTMF) on the C50. The following sections will guide you through a complete, working C50 code that generates a sequence of dial tones with proper pulse duration and pause intervals. In the writeup, you will write codes that generate sequences of musical notes.
"Welcome to TeleBear. Please enter your eight digit student ID number...." Well, if your phone does not support touchtone, you will have to wait for the assistance of an operator. Each touchtone pulse represents a button on the keypad of your telephone. Each touchtone pulse is consisted of two tones. The low frequency indicates the row the button is located on the keypad; the high frequency indicates the column. So, the touchtone pulse is the sum of a pair of tones whose frequencies are selected from a low frequency group of four and a high frequency group of three, respectively, hence the name DTMF.
| 1209 Hz | 1336 Hz | 1477 Hz | ||||||||||||||
| Low- Frequency Group | 697 Hz |
| ||||||||||||||
| 770 Hz | ||||||||||||||||
| 852 Hz | ||||||||||||||||
| 941 Hz | ||||||||||||||||
Frequency specs aren't the only ones DTMF tones have to meet; duration specs apply also. According to AT&T specs, 10 digits/sec (or 100 msec/digit) is the maximum data rate for touchtone signals. Within its 100 msec interval, a tone must be present for at least 45 msec and no more than 55. You will learn the rest of the story about touchtone in Laboratory 10.
In this section, we will look into Texas Instruments'
DTMF generation code dtmf.asm.
You should read the comment at the beginning of the assembly file. It explains the internal representation of the phone button, silence, end of sequence, etc. in terms of number. When you modify the code to play musical notes, you should write a new comment about your design of the internal representation of music notes.
Now, let's look at the assembly directives. The .ds directive tell the assembler to begin assembling source code into data memory. Its counter part for program memory is .ps. For this lab, you should pay attention to the address you use with these directives and make sure that you do not overlap your source codes or write into non-user spaces. Refer to page 1-4 in the TMS320C5x DSP Starter Kit User's Guide for the C5x DSK memory map.
We will skip the initialization of the Analog Interface Circuit (AIC) here, but notice that the "Conversion frequency" is an half of the sampling frequency. Also, we will ignore the part on C50 initialization.
Let's look at the main loop of this program now. The new
instructions here are CALL, CLRC, BCND, LACL.
Please do read the TMS320C5x User's Guide on this
instructions. Also, review the introduction on the indirect
addressing mode to learn about the use of INDX
in the addressing mode.
The main loop read the digit to be transmitted, call UNPACK to select the coefficients for sinusoidal waveform generation, call PAUSESH to run a counter loop for 50 msec in silence before enabling the receive interrupt (which is our sampling timer). This guarantees pause between pulses. After the interrupt is enabled, the main loop runs into a polling loop for the duration of the pulse except in the case of silence. You should figure out how this code handle the case of silence in subroutine UNPACK. The polling loop keeps on checking the content of address TIMER which is decremented by the RECEIVE interrupt service routine everytime an interrupt occurs.
We have seen interrupts before this lab. In the previous labs, the DSP become idle until the timer drivened interrupt occur. In those cases, the DSP returns to the operations in the main loop directly. In this program, the output value is computed and transmitted during the interrupt before the DSP resume its original activities which are tracking the pulse duration, select coefficients for a new pulse, etc. Silence can be achieved through disabling the occurance of interrupts.
The output value is computed mainly by subroutine GENR8TOR.
This subroutine illustrate a very efficient way to compute
sinusoidal waveforms through difference equations. Please read
about the instruction MADD and be ready to explain
how the subrouting works. Also pay attention to how instructions
EXAR and ADDB are used here.
The instruction RET--Return From Subroutine return the
program to where the subroutine was called. The instruction
RETE--Enable Interrupts and Return From Interrupt allows
the DSP to resume its original execution at the point of interrupt.
The key instruction in UNPACK is BCND--
Branch Conditionally. You should be able to find a more
efficient way to do the coefficient selection.
For the pause generation subroutines, notice that the cycle time is 50ns but some instruction takes more than one instruction to be completed. Also, the subroutine returns to different places if called from different places.
In your writeup, concisely record your modification to the code and comment the code you write carefully.
dtmf.asm to generate
a sequence of tones on the music scale. You should use no less
then seven notes. Change the duration of the note to 1 sec.
ACC Accumulator register that stores intermediate results ADD Add to ACC, a C50 instruction ADDB Add ACCB to ACC, a C50 instruction AIC Analog Interface Circuit that allows analog devices to be connected to the C50 DSK board ALU Arithmetic Logic Unit that adds, shifts, and performs logical operations. APAC Add PREG to ACC, a C50 instruction AR Auxiliary register that stores indirect pointer to addresses ARn Auxiliary register number n(0-7) ARP Auxiliary register pointer that points to currently used AR B Branch Unconditionally, a C50 instruction BCND Branch Conditionally, a C50 instruction C50 Texas Instruments TMS 320C50 DSP processor (16-bit, 28.6/20 MIPS) CALL Call Unconditionally, a C50 instruction CBCR Circular buffer control register that selects ARn and enable/disable the buffers n(1,2) CBERn Circular buffer end register that stores the last address in the circular buffer n(1,2) CBSRn Circular buffer start register that stores the first address in the circular buffer CLRC Clear Control Bit, a C50 instruction DMOV Data Move in Data Memory, a C50 instruction DP Data memory page pointer that points to the current 128-byte page of data memory DSK DSP Starter Kit, a C50 development package DSP Digital Signal Processor, a special-purpose microprocessor DXR Data transmit register that stores samples to be transmitted to AIC EXAR Exchange ACCB with ACC, a C50 instruction INDX Index register for indirect addressing LACC Load ACC, a C50 instruction LACL Load Low ACC and Clear High ACC, a C50 instruction LAMM Load ACC with Memory-Mapped Register, a C50 instruction LAR Load ARn, a C50 instruction MACD Multiply and Accumulate with Data Move, a C50 instruction MADD Multiply and ACC with Data Move and Dynamic Addressing, a C50 instruction MAR Modify ARn and ARP, a C50 instruction NEG Negate ACC, a C50 instruction PLU Parallel logic unit that performs logical operations in parallel to ALU PREG Product register that stores multiplication results RET Return From Subroutine, a C50 instruction RETE Return From Interrupt, a C50 instruction RCA A connector standard SACH Shift Accumulator and Store High Word Result, a C50 instruction SACL Shift Accumulator and Store Low Word Result, a C50 instruction SAMM Store ACC to memory-mapped register, a C50 instruction SPLK Store long immediate to data memory location, a C50 instruction ZAC Zero ACC, a C50 instruction ZAP Zero ACC and PREG, a C50 instruction