AN97 Datasheet PDF

SIGNAL GENERATION CIRCUITS Application Note 97: Jul 09, 1998 Telephone Tone Generator Requires No Trimming Many products that connect to phone lines (modems, for instance) incorporate a "callprogress monitoring" function known as CPM. CPM circuits "listen" to the lines as a human would, and respond according to what they "hear." You shouldn't dial a number unless you first hear a dial tone, for example. Neither should your computer. Tone accuracy is not very important when people monitor the call-progress tones, but the use of computers for this purpose has produced a need for tone-accuracy specifications to prevent errors in interpretation. Accordingly, CCITT has introduced the North American Precise Audible Tone Plan (the following data is from the CCITT Green Book, Volume VI-4): Figure 1 illustrates a simple circuit for generating single or dual tones. They must be ±0.5% accurate in frequency, and they must be gated as shown in the "Cadence" column Page 1 of 5 (a µP can control the cadence).