W925E240 Datasheet, PDF (55/73 Pages) Winbond – 8-BIT CID MICROCONTROLLER

W925E/C240
CAS/DTAS Detection
In off-hook services (type II), the detection of CAS/DTAS will affect the quality of the call waiting
service. When the CAS/DTAS is sent from far end, sometimes the near end user maybe still talking.
The CPE must be able to detect the CAS/DTAS successfully in the presence of near end speech. To
detect CAS/DTAS from telephone hybrid receiver pair improves the detection. However, in BT’s on-
hook CID system the CAS/DTAS detection is from Tip/Ring pair.
The dual tone alert signal is separated into high and low tones and detected by a high/low tone
detector. When the alert tone is recognized by the detector, the bit ALGO will go high and the rising
signal will set the bit ALGOF in CIDFG to produce the CID flag(CIDF). Figure 6-18 shows the guard
time waveform of detecting alert tone. The total recognition time is t_REC=t_DP+t_GP, where t_DP is the tone
present detect time and t_GP is the tone present guard time. The total absent guard time is t_ABS=t_DA+t_GA
where t_DA is the tone absent detect time and t_GA is the tone absent guard time. The tone
present/absent guard time is determined by guard-time timer, which the input clock period is 0.858mS.
When the alert tone is detected, the internal signal ALGR will be set and the rising edge of ALGR
resets the guard-time timer and the timer starts up counting from 00H. As the content of the timer is
the same as the register CASPT, the timer stops counting and the bit ALGO will be set and the rising
edge of ALGO triggers the flag ALGOF to become high. The counting of tone absent time is similar to
the counting of tone present time but the falling edge of ALGR/ ALGO replaces the rising edge and the
CASAT replaces the CASPT. The bit ALGO is controlled by hardware only. The flag ALGOF is set by
rising edge of ALGO and cleared by software.
Vin
ALGR_*
ALGO
Dual Alert Tone Signal
t DP
t GP
t DA
t DP
t DA
t GA
ALGOF
+
t REC
13
t ABS
23 1
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 guard-time timer reaches the content of ASPT/ASAT.
*ALGR is an internal signal in the uC.
+ Clear by software.
Figure 6-18 Guard Time Waveform of Alert Tone Signal Detection
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Publication Release Date: July 12, 2005
Revision A10