W925E240 Datasheet, PDF (56/73 Pages) Winbond – 8-BIT CID MICROCONTROLLER

W925E/C240
DTMF Decoder
The DTMF decoder shares the same input pre-processor with FSK decoder. The dual tone is
separated into low group and high group by two SCFs (switched capacitor filter. The method of DTMF
detection is the same as alert tone detection. The present/absent guard time is adjusted by registers
DTMFPT/DTMFAT. As the DTMF signal is recognized and decoded, the bit DTMFD will be set and the
decoded DTMF data is stored in bit0 to bit3 of register DTMFDR. The rising edge of DTMFD produces
the flag DTMFDF. The bit DTMFD is controlled by hardware only. The flag DTMFDF is set by rising
edge of DTMFD and cleared by software.
Vin (Tip/ring)
ESt _*
DTMFD
DTMFDF
TONE #n
t DP
t GP
t REC
+
t DA
t GA
t _ABS
DTMFDR _{Tone #n-1}
13
Tone #n
23
1: Guard time timer is reset and starts to up count from 00H.
2: Guard time timer is reset and starts to up count from 00H.
3: The content of the up counting timer reaches the register DTMFPT/DTMFAT.
* ESt is an internal signal in the circuit.
+ Clear by software.
Figure 6-19 The Waveform of DTMF Detection
Tone Detector
In off-hook state, said type II system, detecting tone alert signal(CAS) is easily interfered by human’s
voice or other noise in voice band. Sometimes the interference makes falsely recognizing a noise as a
CAS(talk-off), or lost detecting a real CAS(talk-down). The DTMF can be programmed as a tone
detector by setting bit 4 of DTMFR2. The frequency band of the tone detector is DTMF frequency from
697Hz to 1633Hz. Once the tone detector gets signals in the band, the bit of DTMFH or DTMFL in
register DTMFDR will become high immediately. User can poll these 2 bits to check if the tone exists
on the tip/ring. The input gain of tone detector is the same as DTMF receiver.
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