CYG23XX Datasheet, PDF (4/5 Pages) Clare, Inc. – CYBERGATE

CYG23XX
Off-Hook Transient Requirement
In order to meet Section 4.6.1 of the CTR-21 requirement, it is necessary to assert the MUTE pin of the
CYG23XX) for a duration of 80mS after the OH pin is driven low as shown in Figure 1. This can be
accomplished via the host firmware or external hardware as shown in Figures 2 and 3 respectively.
OH
MUTE
80mS
MUTE and OH Timing
Figure 1
Output Port
µC
Output Port
OH
CYG23XX
MUTE
Figure 2
µC
Output Port
One-shot*
80ms
OH
Output Port
(Pulse Dial)
Shunt
CYG23XX
MUTE
* Micrel MIC1555 or Equivalent
Figure 3
Asserting the MUTE pin causes the internal gyrator circuit in the CYG23XX to be bypassed, allowing
low impedance pulse dialing to be performed by pulsing the OH pin. In Figure 2, the microcontroller
output port going to the MUTE pin is used as a shunt for low impedance dialing and is asserted for
80mS when the OH signal is asserted. This method is preferred when the user has control of the host
firmware and can easily write a subroutine to accomplish this function.
For users that do not have easy access to the modem firmware, some external hardware can be
added to accomplish the same function. Figure 3 shows a monostable (one-shot) such as a 555 timer
which is designed to generate an 80mS low going pulse upon the assertion of the OH signal. This
80mS pulse is ANDed with the low impedance pulse dial shunt signal which overrides the 80mS sig-
nal when pulse dialing is enabled. The pulse dial shunt signal is included as a standard output pin in
most modem chipsets. This pin is activated when an ATDP command is issued to the modem.
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