| 摘要 |
<p>A dual-tone multi-frequency (DTMF) tone generator circuit (10) produces selected frequency row and column tones which are combined to generate a DTMF signal. Key board scan circuits (42, 44) scan a conventional push-button telephone key board to produce row and column input signals. Row and column fundamental rate signals are generated by fundamental counters (48, 76) from a reference signal derived from an external crystal (12). Row and column integration rate signals are generated by integrator counters (50, 78) also derived from the reference signal. Specialized row and column clock control signals (SLOPE RATE, SLOPE SIGN, AUTO ZERO) are produced by clock generators (58, 82). Row and column integrators (64, 92) integrate reference signals to produce discrete voltage steps at the rate of the row and column integration rate signals to produce row and column signals made up of a plurality of segments for each cycle of the signal. Each signal segment comprises a plurality of similar voltage steps having amplitude and polarity determined by the specialized row and column clock signals (SLOPE RATE, SLOPE SIGN). The row and column signals are combined in a summer (68) to produce the DTMF signal. </p> |
