Design Notes ============ The microprocessor runs at 4 MHz. It is powered directly from three 1.5 V mercury button cells in series. Current draw at nominal volume is only 12 ma (idle) to 17 ma (tone being generated). One of the 8-bit 68HC705C8 ports is wired to an R-2R resistive network made with precision 10K/20K resistors. The output of the network (input to the LM386N audio amp) is the sum of the binary-weighted output of the port (0xFF = 5 volts, 0x00 = 0 volts). It is very linear. Each button drives an input port on the microcontroller. No keyboard scanning is used. To generate tones, an interrupt service routine driven by an on-chip timer samples a sign wave table at 12KHz. The index amount into the table determines the frequency. Two indices are run. The retrieved values are then summed and divided by 2. The result is sent to the 8-bit output port, which generates the dual-tone waveform. Tone duration is accurate to a single sampling interval. Frequency acuracy is typically within 1 Hz. The microcontroller automatically shuts off the amplifier and goes into low-power mode (microamps) after 5 seconds of inactivity. Three modes of operation are supported. In MF burst mode (power-up default) the digit buttons each generate an MF pulse of the maximum recommended duration (75 ms.). An additional button can be wired to its own pin on the micro for 2600, or * and # may be depressed together in the MF modes. 2600 plays as long as the button is depressed. The * key alone is KP. The # key alone is ST. The KP burst is timed to 120 ms. The ST burst is 75 ms. Pressing keys 1 and 3 together play the STP tone. 4 and 6 play the ST2P tone. 7 and 9 play the ST3P tone. Depressing buttons 7 and 8 together toggle in and out of MF continuous mode. A low to high or high to low beep confirms the mode change. In continuous mode, the MF digits play as long as the key is depressed. KEYPRESS DIGIT OR MF FREQ CCITT CODE IN Hz SYS NO. 5 1 1 700 +900 2 2 700 +1100 3 3 900 +1100 4 4 700 +1300 5 5 900 +1300 6 6 1100 +1300 7 7 700 +1500 8 8 900 +1500 9 9 1100 +1500 0 0 1300 +1500 * KP 1100+ 1700 KPl # ST 1500+ 1700 1+3 STP 900+ 1700 Code 12 4+6 ST2P 1300+ 1700 KP2 7+9 ST3P 700+ 1700 Code 11 KP = Preparatory for digits ST = End of pulsing STP, ST2P, ST3P = Control codes Depressing buttons 8 and 0 together toggle in and out of DTMF mode. Beeps confirm the mode change. In DTMF mode, the 2600 button is not active. The digit buttons play their corresponding tones in continuous mode (burst mode is not supported in DTMF mode). Depressing the first and third buttons in each row play the DTMF A, B, C, and D tones. DIGIT OR DTMF FREQ KEYPRESS CODE IN Hz 1 1 697 + 12092 2 2 697 + 1336 3 3 697 + 1477 4 4 770 + 1209 5 5 770 + 1336 6 6 770 + 1477 7 7 852 + 1209 8 8 852 + 1336 9 9 852 + 1477 0 0 941 + 1336 * * 941 + 1209 # # 941 + 1477 1+3 A 697 + 1633 4+6 B 770 + 1633 7+9 C 852 + 1633 *+# D 941 + 1633 Depressing buttons 8 and 9 together toggle in and out of calibration mode. Beeps confirm the mode change. In this mode buttons 1-6 play the MF master tones alone; 700, 900, 1100, 1300, 1500, and 1700. Button 8 plays a 1004 Hz test tone. *, 0, and # play nickel, dime and quarter ACTS red box tone sequences. All other buttons are not active. A power down resets the box out of calibration mode, but the other mode settings are retained. In any mode: Pressing buttons 1 and 2 together generates a standard dial tone. Pressing buttons 2 and 3 together generates a standard 2-second/4-second ringback tone. Pressing buttons 4 and 5 together generates a standard busy signal (60 ppm). Pressing buttons 5 and 6 together generates a standard reorder (120 ppm). Pressing buttons 4 and 7 together generates an intercept SIT tone sequence (subscriber disconnected). Pressing buttons 5 and 8 together generates an inter-LATA all circuits busy SIT tone sequence. Pressing buttons 6 and 9 together generates an inter-LATA reorder SIT tone sequence (mangled MF addressing indication).