EM78815
8-Bit Microcontroller
In many situations not requiring independent selection of received and paused, the
simple steering circuit is applicable. Component values are chosen according to the
following formula:
t REC = t DP + t GTP t ID = t DA + t GTA
The value of t DP is a parameter of the device and t REC is the minimum signal
duration to be recognized by the receiver. A 0.1 µF value for C is recommended for
most applications, leaving R to be selected by the designer. For example, a suitable
value of R for a t REC of 30mS would be 300k.
Different steering arrangements may be used to select independently the guard-times
for tone-present (t GTP) and tone-absent (t GTA). This may be necessary to meet
system specifications which place both accept and reject limits on both tone duration
and inter digital pause.
Guard-time adjustment also allows the designer to tailor system parameters such as
talk off and noise immunity. Increasing t REC improves talk-off performance, since it
reduces the probability that tones simulated by speech will maintain signal condition
long enough to be registered. On the other hand, a relatively short t REC with a long
t DO would be appropriate for extremely noisy environments where fast acquisition
time and immunity to drop-outs would be required.
VDD
VDD
C
ST/GT
EST
R
Fig. 15 DTMF Receiver Delay Time Control
30 •
Product Specification (V2.4) 02.17.2006
(This specification is subject to change without further notice)
ELAN [ ELAN MICROELECTRONICS CORP ]
