DTMF transmiter with Pic16F84
 
This is modified Aplication Note AN655 from Microchip (www.microchip.com).
The original program generate DTMF with too big frequency difference and MT8870 (DTMF decoder) can't decode it.
In fact the program is same but have difference in sinus tables.
The program with modifications generate better DTMF and MT8870 decode it without any problem.
 
Original Frequency (Hz) AN655 DTMF (Hz) Modified Program (Hz)
697 709 709
770 798 774
852 880 852
941 982 946
1209 1278 1217
1336 1419 no meazurement :-(
1447 1503 no meazurement :-(
 
Components:
 
1. Pic16F84 microcontroller
2. IC K572PA1 10bit DAC (used only 8bits)
3. LM741
4. 3.579545Mhz crystal
5. a NPN transistor
6. few resistors and capacitors

Download Schematic in GIF format : dtmf_tx.gif (32,330 bytes)
Download ASM file: 84_dtmf.asm (11,367 bytes)
Download both files in ZIP archive: dtmf_all.zip (34,224 bytes)