Design Ideas: October 13, 1994
A telephone DTMF IC produces, by design, two tones if you connect a single intersection of the IC's 4[x]4 keypad matrix (Fig 1). (Telephones commonly have 3×4 keypads but, unknown to many people, the applicable standard allows for a fourth row of four keys.) But, if you connect two key inputs at once, the IC produces a single-frequency, low-distortion sine wave. Table 1 lists values for an HM9187 (Winbond Electronics, Santa Clara, CA, (408) 943-6666. (DI #1592]
| Frequency Values | ||
|---|---|---|
| Keys | Frequency (Hz) | fCLK/fOUT |
| 1 and 2 | 679 | 5270.98 |
| 4 and 5 | 770 | 4648.05 |
| 7 and 8 | 852 | 4200.70 |
| * and 0 | 941 | 3803.42 |
| 1 and 4 | 1209 | 2960.29 |
| 2 and 5 | 1336 | 2678.89 |
| 3 and 6 | 1477 | 2423.15 |
| A and B | 1633 | 2191.67 |