Figure 7-5. Receive Signalling Paths - Motorola MCS 2000 Service Instructions Manual

Controller Section Theory of Operation: Audio and Signalling Circuits
The low speed limited data output (PL, DPL, and trunking LS) appears at U0200-A4, where it
connects to the SLIC. While receiving low speed data, the µP may output a sampling waveform,
depending on the sampling technique, to U0200-C3 of between 1 and 2 kHz.
The low speed data is read by the µP at twice the frequency of the sampling waveform; a latch
configuration in the ASFIC stores one bit every clock cycle. The external capacitors C0211, C0212,
and C0203 set the low frequency pole for a zero crossings detector in the limiters for PL and HS
data. The hysteresis of these limiters is programmed based on the type of received data. Note that
during HS data the µP may generate a sampling waveform seen at U0200-G1.
DISC
(DISCRIMINATOR AUDIO
FROM RF SECTION)
Alert Tone Circuits
When the software determines that it needs to give the operator an audible feedback (for a good key
press, or for a bad key press), or radio status (trunked system busy, phone call, circuit failures), it
sends an alert tone to the speaker.
It does so by sending SPI BUS data to U0200 which sets up the audio path to the speaker for alert
tones. The alert tone itself can be generated in one of two ways: internally by the ASFIC, or
externally using the µP and the ASFIC.
The allowable internal alert tones are 304, 608, 911, and 1823 Hz. In this case a code contained
within the SPI BUS load to the ASFIC sets up the path and determines the tone frequency, and at
what volume level to generate the tone. (It does not have to be related to the setting of the volume
knob).
For external alert tones, the µP can generate any tone within the 100-3000 Hz audio band. This is
accomplished by the µP generating a square wave which enters the ASFIC at U0200-C3.
Inside the ASFIC, this signal is routed to the alert tone generator; the output of the generator is
summed into the audio chain just after the RX audio de-emphasis block. Inside U0200 the tone is
amplified and filtered, then passed through the 8-bit digital volume attenuator, which is typically
loaded with a special value for alert tone audio. Note that the Hear Clear expander is bypassed even
if U0250 is present. The tone exits at U0200-J4, then is routed to the audio PA like receive audio.
68P81083C20-D
G1
HIGH SPEED
CLOCK
DATA FILTER
LIMITER
AND DEEMPHASIS
J7
PL
ASFIC
IN
U0200
FILTER
LOW SPEED
LIM CAP
C5

Figure 7-5. Receive Signalling Paths

C3
LOW SPEED
CLOCK
RX G4
LIM OUT
A4
PL
LIMITER
LIM
RX LIM
CAP
J3
7-25
MICRO
CONTROLLER
December 6, 2004