Alert Tone Circuits; Sub-Audible Data (Pl/Dpl) And High Speed Data Decoder - Motorola MCX600E Service Manual

Receive Signalling Circuits
7.1

Sub-audible Data (PL/DPL) and High Speed Data Decoder

The ASFIC (U0201) is used to filter and limit all received data. The data enters the ASFIC at U0201-
J7. Inside U0201 the data is filtered according to data type (HS or LS), then it is limited to a 0-5V
digital level. The MDC and trunking high speed data appear at U0201-G4, where it connects to the
µP U0101-11
The low speed limited data output (PL, DPL, and trunking LS) appears at U0201-A4, where it
connects to the µP U0101-10. While receiving low speed data, the µP may output a sampling
waveform, depending on the sampling technique, to U0201-C3 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 C0226, C0225,
and C0223 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 U0201-G1.
7.2

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 U0201 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 µ P and the ASFIC.
The allowable internal alert tones are 304, 608, 911, and 1823Hz. 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 voice volume setting).
For external alert tones, the µP can generate any tone within the 100-3000Hz audio band. This is
accomplished by the µP generating a square wave which enters the ASFIC at U0201-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 U0201 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. The tone exits at U0201-J4 and is routed to the
audio PA like receive audio.
4-16 Theory of Operation