The DTMF Decoder The DTMF decoder, IC U3, a Holtek HT9170B, has the task of monitoring all the input audio signals. When a valid DTMF signal is received, it sets a 4 bit digital code on output pins 11, 12, 13, and 14. Why a 4 bit code you may ask? Think back to your high school math class for a moment when you learned about different base numbering systems. Humans like to work in a base 10 system (decimal system, deci = 10) using the digits 1 through 10 (or 0 through 9 to be more precise). This is probably due to the 10 fingers and 10 toes we all learned to count on as kids.
Computers on the other hand (pardon the pun) like to work with a base 2 numbering system (binary system, bi = 2) using digits 0 and 1; 0 for off / low and 1 for on / high. In order to represent the 16 different possible tones (base 10) you need to have 4 bits in a binary system 2 4 = 1111 (binary) = 16 (decimal). Wow… kind of got off on a tangent there! Internally, U3 has a set of counters that latch the 4 outputs depending on the detected tones. The counters are referenced to a television color-burst crystal operating at 3.579 MHz (X1) for rock solid measurements. Notice how C1 couples a bit of the X1 crystal frequency over to U1 (the micro-controller) to set its timing as well.
The Micro-controller U1 is a pre-programmed Freescale (Motorola) MC68HC908JK3 micro- controller. This IC contains the software required to take the DTMF tone data from U3 and process it while reading and writing to FLASH memory, generating the feedback tones, monitoring the programming switch, and performing all the output functions of the DC2. It does all this according to how you have configured the DCI2. Don’t forget that it also handles all the configuration set- up you do when you first start programming your DCI2. We won’t go into all the functions of U1 because each of the configurations have different signal input and output requirements.
You can think of it as the traffic controller for the DCI2. Quite a bit for one little part! One other important signal line between U3 pin 15 and U1 pin 7 is the data valid signal. Whenever U3 detects a valid DTMF signal it sends a high level signal on this line to tell the microprocessor it needs to look at the 4 data lines and make a decision what to do with the data. This signal line is also used by U2 which will be discussed next. The 4 to 16 Line Decoder As you can see the DCI2 actually has two major “brains”. The DTMF decoder U1 and the microprocessor U3. There is one more smart device we need to talk about and that is U2, a 74HC154, a 4 line to 16 line decoder. If that sounds DCI2 5