The example shows a method of generation of DTMF signal. The following assumptions have been adopted:

• the level of the signal of the higher frequency is 0 to 3dB higher compared to the signal of the lower frequency,
• transmission of one character lasts 90ms, the pause between two characters is 200ms,
• the sampling frequency is 20kHz,
• input data are sent to the microcontroller via the UART module at the rate of 9600 bps.

The algorithm of generating the signal is the following. On the basis of the calculated frequencies the square signal is sent first. Then, this signal is filtered by a low pass IIR filter of the third order (4 coefficients). The output signal specifications are:

• frequency deviation is less than 1.5%,
• the level of all harmonics is 20dB below the useful signal.

The algorithm is shown in Fig. 14-1. Digital filter is lowpass Chebyshev type 1 filter. Wp is 1700 Hz, Ap is 3dB, sampling frequency is 20kHz. Third order fiter can meet the requirements: a) level of all harmonics is 20dB below the useful signal: Harmonic nearest to the useful signal is 3xf1. f1 is lowest frequency of useful signal, f1=697 Hz. Fourier series of square wave signal implies that amplitude of 3xf1=2091 Hz harmonic is 1/3 of f1signal amplitude. So we already have -9.5dB. In IIR frequency window we can see that 3xf1 frequency is 12dB weaker than useful signal. The most critical harmonic 3xf1 is -9.5dB -12dB = -21.5dB weaker than useful signal – so this requirement is fullfilled. b) the level of the signal of the higher frequency is 0 to 3dB higher compared to the signal of the lower frequency: IIR frequency window also shows that low frequencies 697-941Hz filter attenuation is in -3dB to -2.8dB range and low frequencies 1209-1633Hz filter attenuation is in –1.8dB to 0dB range. So any higher frequency signal is 1 to 3dB stronger than any lower frequency signal. Fig. 14-2 shows the generated signal. The signals of the lower and higher frequencies are added, with the higher frequency signal 3dB below the lower frequency signal. For generation of the filter coefficients use the Filter Designer Tool which is a constituent part of mikroBasic for dsPIC. The program for generation of the described signal is as follows:

' This project is designed to work with PIC P30F6014A. It has been tested
' on dsPICPRO3 board with 10.0 MHz crystal and 8xPLL. It should work with any
' other crystal. Note: the maximum operating frequency for dsPIC is 120MHz.
' With minor adjustments, this example should work with any other dsPIC MCU
'
' On-board DAC module
' Enable SPI connection to DAC on SW4 and DAC's Load(LD) and Chip Select(CS) pins on SW3.

program DTMFout

' *** Filter Designer Tool outputs *** '
const
  BUFFER_SIZE  = 8
  FILTER_ORDER = 3
  COEFF_B as integer[FILTER_ORDER+1] = (0x21F3, 0x65DA, 0x65DA, 0x21F3)
  COEFF_A as integer[FILTER_ORDER+1] = (0x2000, 0xB06D, 0x47EC, 0xE8B6)
  SCALE_B =  6
  SCALE_A = -2

' *** DAC pinout *** '
const  LOAD_PIN = 2                         ' DAC load pin
const  CS_PIN   = 1                         ' DAC CS pin

dim
  THalf_Low, THalf_High as word   ' half-periods of low and high-frequency square signals
  char2send as byte               ' char recived from UART
  sample, sending_ch_cnt as word  ' digital signal sample, sending char counter
  us_cntL, us_cntH as word        ' low and high-frequency square signal microseconds counters
  input_ as integer[BUFFER_SIZE]  ' filter input signal (two square signals)
  output as integer[BUFFER_SIZE]  ' filtered signal
  sample_index as word            ' index of current sample
  voltageL, voltageH as integer   ' square signals amplitudes

sub procedure InitMain()

  LATC.CS_PIN    = 1                        ' set DAC CS to inactive
  LATC.LOAD_PIN  = 0                        ' set DAC LOAD to inactive
  TRISC.LOAD_PIN = 0                        ' configure DAC LOAD pin as output
  TRISC.CS_PIN   = 0                        ' configure DAC CS pin as output

  ' Initialize SPI2 module
  Spi2_Init_Advanced(_SPI_MASTER, _SPI_16_BIT, _SPI_PRESCALE_SEC_1, _SPI_PRESCALE_PRI_1,
                     _SPI_SS_DISABLE, _SPI_DATA_SAMPLE_MIDDLE, _SPI_CLK_IDLE_HIGH,
                     _SPI_ACTIVE_2_IDLE)

  Uart1_Init(9600)                          ' Initialize UART1 module
end sub

sub procedure DAC_Output(dim valueDAC as word)

  LATC.CS_PIN = 0                           ' CS enable for DAC
  ' filter output range is 16-bit number DAC input range is 12-bit number
  valueDAC = valueDAC >> 4
  ' now both numbers are 12-bit but filter output is signed and DAC input is unsigned.
  '  Half of DAC range 4096/2=2048 is added to correct this
  valueDAC = valueDAC + 2048
  SPI2BUF = 0x3000 or valueDAC              ' write valueDAC to DAC (0x3 is required by DAC)
  while (SPI2STAT.1 = 1)                    ' wait for SPI module to finish sending
    nop
  wend
  LATC.CS_PIN = 1                           ' CS disable for DAC
end sub

sub procedure SetPeriods(dim ch as word)

' DTMF frequencies:
'
'            1209 Hz     1336 Hz     1477 Hz     1633 Hz
' 697 Hz       1           2           3           A
' 770 Hz       4           5           6           B
' 852 Hz       7           8           9           C
' 941 Hz       *           0           #           D

  ' Calculate half-periods in microseconds
  '   example: 1/697Hz = 0.001435 seconds = 1435 microseconds
  '            1435/2  = 717
  select case ch
    case 49
      THalf_Low=717 THalf_High=414 '1'
    case 50
      THalf_Low=717 THalf_High=374 '2'
    case 51
      THalf_Low=717 THalf_High=339 '3'
    case 65
      THalf_Low=717 THalf_High=306 'A'
    
    case 52
      THalf_Low=649 THalf_High=414 '4'
    case 53
      THalf_Low=649 THalf_High=374 '5'
    case 54
      THalf_Low=649 THalf_High=339 '6'
    case 66
      THalf_Low=649 THalf_High=306 'B'
    
    case 55
      THalf_Low=587 THalf_High=414 '7'
    case 56
      THalf_Low=587 THalf_High=374 '8'
    case 57
      THalf_Low=587 THalf_High=339 '9'
    case 67
      THalf_Low=587 THalf_High=306 'C'
    
    case 42
      THalf_Low=531 THalf_High=414 '*'
    case 48
      THalf_Low=531 THalf_High=374 '0'
    case 35
      THalf_Low=531 THalf_High=339 '#'
    case 68
      THalf_Low=531 THalf_High=306 'D'
  end select
  
end sub

sub procedure ClearBufs()
  'Clear buffers
  Vector_Set(input_, BUFFER_SIZE, 0)
  Vector_Set(output, BUFFER_SIZE, 0)
end sub

sub procedure Timer1Int org $1A      ' interrupt frequency is 20kHz

  ' calculate sample
  sample = voltageL + voltageH       ' add voltages
  input_[sample_index] = sample      ' write sample to input buffer
  
  ' update low-frequency square signal microseconds counter
  us_cntL = us_cntL + 50             ' since us_cntL and THalf_Low are in microseconds
                                     ' and Timer1 interrupt occures every 50us
                                     ' increment us_cntL by 50
  if us_cntL > THalf_Low then        ' half-period exceeded, change sign
    voltageL = -voltageL
    us_cntL  = us_cntL - THalf_Low   ' subtract half-period
  end if
    
  ' update high-frequency square signal microseconds counter
  us_cntH = us_cntH + 50
  if us_cntH > THalf_High then
    voltageH = -voltageH
    us_cntH  = us_cntH - THalf_High
  end if
    
  'IIR(amp), filtering new sample
  sample = IIR_Radix(SCALE_B, SCALE_A, @COEFF_B, @COEFF_A, FILTER_ORDER+1, 
  @input_, BUFFER_SIZE, @output, sample_index)
  
  DAC_Output(sample)                 ' send sample to digital-to-analog converter

  output[sample_index] = sample      ' write filtered sample in output buffer
  
  Inc(sample_index)                  ' increment sample index, prepare for next sample
  if sample_index = BUFFER_SIZE then
    sample_index = 0
  end if

  Dec(sending_ch_cnt)                ' decrement char sending counter
                                     '   (character transmition lasts 90ms = 1800 samples)
  if sending_ch_cnt = 0 then         ' if character transmition is over
    T1CON=0                          ' turn off Timer1
    Delay_ms(200)                    ' pause between two characters is 200ms
  end if
    
  IFS0.3 = 0                         ' clear Timer1 interrupt flag
end sub


' --- main --- '
main:

  InitMain()                         ' perform initializations

  sending_ch_cnt = 0                 ' reset counter
  sample_index   = 0                 ' initialize sample index

  ' Clear interrupt flags
  IFS0 = 0
  IFS1 = 0
  IFS2 = 0
  
  INTCON1 = $8000                    ' disable nested interrupts
  IEC0    = $0008                    ' enable Timer1 interrupt
  
  ' Timer1 input clock is Fosc/4. Sampling frequency is 20kHz. Timer should
  ' raise interrupt every 50 microseconds. PR1 = (Fosc[Hz]/4) / 20000Hz = Fosc[kHz]/(4*20)
  PR1 = Clock_kHz() div 80
  ' Note: interrupt routine execution takes ~10us

  while true
    if (sending_ch_cnt = 0) and      ' check if sending of previous character is over
       (Uart1_Data_Ready() = 1) then ' check if character arrived via UART1

      char2send = Uart1_Read_Char()  ' read data from UART and store it
      SetPeriods(char2send)          ' set periods for low and high-frequency square signals
      ClearBufs()                    ' clear input and output buffers
      ' digital filter computing error is smaller for signals of higher amplitudes
      '   so signal amplitude should as high as possible. The highest value for
      '   signed integer type is 0x7FFF but since we are adding 2 signals we must
      '   divide it by 2.
      voltageH = $7FFF div 2         ' high-frequency square signal amplitude
      voltageL = $7FFF div 2         ' low-frequency square signal amplitude
      us_cntL = 0                    ' low-frequency square signal microseconds counter
      us_cntH = 0                    ' high-frequency square signal microseconds counter
      
      ' start Timer T1
      sending_ch_cnt =  1800         ' character tansmition lasts 90ms = 1800 samples * 50us
      T1CON          = $8000         ' enable Timer1 (TimerOn, prescaler 1:1)
    end if
  wend
    
end.