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MC14584BCP-MC14584BDR2-MC14584BF-MC14584BFEL
Hex Schmitt Trigger
- -
Prepared by: Dennis Morgan
Motorola Analog Marketing
INTRODUCTIONThis circuit is intended to provide an auxiliary circuit for the
connection of a POTS phone (or answering machine, fax,
etc.) to a digital communication line (ISDN, e.g.). The first two
circuits assume there is a 5.0 V supply, a 12 V supply, and a
microprocessor or other controlling circuit, although the
circuits can also work from 5.0 V only. A single line, and a
dual line system are discussed.
The final two circuits described herein provide variations
of the Ring Generation portion of the circuits for the purpose
of lower cost, but with some compromises.
The MC33121 SLIC provides most of the central office
functions to the external POTS phone line – dc loop current,
off–hook detection, proper ac and dc impedances, 2–to–4
wire conversion, transhybrid rejection, and high longitudinal
balance (>58 dB).
The only functions not provided by the MC33121 are the
supplying of the ringing voltage, and off–hook detection
during ringing. Those are supplied by the Ring Generator
circuit, and the associated Off–Hook Detector. The Telecom
Relay switches the phone line between the SLIC and the
Ring Generator. In the normal (de–energized) state, the
phone line is connected to the MC33121 SLIC.
The MC145484 CODEC provides the A/D and D/A
conversions of the voiceband signals between the digital
system and the SLIC.
The Single Line System is discussed first.
BLOCK DIAGRAM – SINGLE LINE SYSTEMReferring to the block diagram (Figure 1), in the standby
state (phone on–hook, not ringing), the relay connects the
phone line to the SLIC.
When the phone is taken off–hook, the SLIC provides the
loop current, and the speech paths between the phone and
the CODEC. The HOOK STATUS output (from the SLIC) is
provided to the MCU via the Ring Control Logic.
The dc–to–dc converter provides the –24 V (from the
5.0 V supply) necessary to power the SLIC and provide the
loop current.
When the phone is on–hook, and ringing is to be applied,
the Ring Control Logic (controlled by the MCU) powers the
Ring Generator, and switches the relay so as to provide the
ringing voltage to the phone line. The Off–Hook Detector
informs the logic if the phone is taken off–hook during ringing.
The Ring Control Logic then switches the relay to the SLIC.
CIRCUIT DETAILS (Refer to Figure 2)
MC33121 SLICThe MC33121 is powered by –24 V at VEE (to provide the
loop current), and 5.0 V at VDD for the logic interface. The
circuit consumes ≈ 1.0 mA at VEE when the phone is
on–hook, and ≈ 9.0 mA more than the loop current when
off–hook. The following functions are provided by this circuit: DC Loop Current: The loop current limit is set by the
resistor at Pin 8 (RFO). A 6.98 kΩ resistor sets a
current limit of ≈ 30 mA. Using Figure 7 in the data
sheet, different resistor values can be picked to set
other current limit values in the range of 20 to 40 mA.
NOTE: It is recommended that the loop current not be lessthan 30 mA as there are many line powered speakerphones
on the market which do not work well at lower currents. Hook Status: ST1 (Pin 13) is a logic level output which
is high when the phone is on–hook, and low when
off–hook. Internally there is an NPN pulldown
transistor, and a 15 kΩ pullup resistor to 5.0 V. This
output also provides pulse dialing and flash
information.
c) AC Terminating Impedance: The 20.3 kΩ resistor
between Pins 10 and 11 sets the ac terminating (and
source) impedance at 600 Ω to the phone line. See the
data sheet (Equation 8) to set different impedances. Receive Gain: The 30.9 k resistor at Pin 10 sets a 0 dB
receive gain from the CODEC’s output (at its Pin 4) to
the phone line, based on a 600 Ω impedance at the
phone line. The 0.47 μF capacitor provides rolloff
below 300 Hz. See Equation 12 in the data sheet if the
gain is to be changed or different ac impedances are
used for the phone line and the MC33121. Transmit Gain: The transmit gain is fixed within the
MC33121 from the phone line to TXO (Pin 11) at 0.318
V/V (–10 dB). However, the overall transmit path
includes an operational amplifier external to the
MC33121, which (in this circuit) is located within the
MC145484 CODEC. The 31.6 kΩ resistor at the
CODEC’s Pins 17 and 18 is the feedback resistor for
the operational amplifier. In conjunction with the 10 k
resistor at the SLIC’s Pin 11, the overall transmit gain is
set to 0 dB from the phone line to Pin 17 of the
CODEC. If the transmit gain is to be varied, this should
be done with the resistor at Pins 17 and 18 of the
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