Service manual

68P81076C25-C July 1, 2002
Theory of Operation: ASTRO Spectra Plus VOCON Board 3-43
The analog microphone signal from the command board is passed to the ASTRO Spectra Plus
VOCON on MAI (Mic Audio In). This signal passes through gain and attenuation stages so that the
correct amplitude level of the audio is presented to the CODEC input. The CODEC contains a
microphone A/D. The microphone A/D converts the analog signal to a digital data stream and
transmits them to the SAP of the Patriot IC. The DSP accesses this data through this port. As with
the speaker data samples, the DSP reads the microphone samples from registers mapped into its
memory space.
As with the received trunking low-speed data, low speed transmit data is processed by the MCU and
returned to the DSP. For secure messages, the digital signal data may be passed to the secure
module prior to DSP processing before the ADDAG IC. The DSP transfers the data to and from the
secure module through it's SSI port consisting of TXD and RXD. The secure module communicates
with the DSP through its SPI bus, therefore a SSI to SPI conversion circuit is on the ASTRO Spectra
Plus VOCON board to ensure communication between the DSP and the secure module.
Configuration and mode control of the secure module is performed by the MCU through the SSI / SPI
bus.
The DSP processes these microphone samples, generates and mixes the appropriate signaling, and
filters the resultant data. This data is then transferred to the ADDAG IC on the DSP BBP (Baseband
Port) - SSI port. The transmit side of the SSI port consists of SC2B, SCKB, and STDB. The DSP
BBP-SSI port is a synchronous serial port. SCKB is the 2.4 MHz clock input derived from the
ADDAG, which makes it synchronous. The data is clocked over to the ADDAG on STDB at a
2.4 MHz rate. The ADDAG generates a 48 kHz interrupt on SC2B so that a new sample data packet
is transferred at a 48 kHz rate, which sets the transmit data sampling rate at 48Ksp.
Within the ADDAG IC, these samples are then input to a transmit D/A, which converts the data to an
analog waveform. This waveform is the modulation out signal from the ADDAG ports, FMOUT,
OUTQ, and OUTQB. FMOUT is single-ended, while OUTQ and OUTQB form a differential pair. This
pair is then sent to an Op-Amp (U202), which outputs a single-ended waveform. FMOUT is passed
through an Op-Amp (U202) for attenuation. These signals are both sent to the command board,
where they go through a gain stage and then to the VCO and Synthesizer. MODOUT is used
primarily for audio frequency modulation; REFMOD is used to compensate for low-frequency
response to pass subaudible modulated signals (such as PL).
3.4.6 ASTRO Spectra Plus Controller Bootstrap and Asynchronous Busses
The SB9600 bus (see Figure 3-18) is an asynchronous serial communication bus, utilizing a
Motorola proprietary protocol. It provides a means for the MCU to communicate with other hardware
devices. In the ASTRO Digital Spectra Plus radio, it communicates with hardware accessories
connected to the accessory connector and the remote interface board.
The SB9600 bus utilizes the UART internal to the MCU, operating at 9600 baud. The SB9600 bus
consists of LH / TX_Data (J501-18), LH / RX_Data (J501-17), and BUSY_RTS (J501-20) signals.
LH / TX_Data and LH / RX_Data are the UTXD1 (K11) and URXD1 (K12) ports of the Patriot IC
(U300), respectively. BUSY_RTS (U300-URTS1- L16) is an active-low signal, which is pulled low
when a device wants control of the bus.
The same UART internal to the MCU is used in the controller bootstrap mode of operation. This
mode is used primarily in downloading new program code to the FLASH ROM (U301) on the
VOCON board. In this mode, the MCU accepts special code downloaded at 115k baud through the
UART instead of operating from program code resident in its ROMs.
A voltage greater than 11 Vdc applied to J501-31 (Vpp) will trip the circuit comprising VR304, Q300,
and U307. This circuit sets the MOD pin (J1) of the MCU to bootstrap mode (logic 1). A voltage
greater than 7 Vdc applied to J501-31 (Vpp) will trip the circuit comprising VR305 and Q302. This will
not put the MCU in Bootstrap mode, but the software will detect this using pin PA7 (G11), which will
allow the user to interface with the Customer Programming Software, Tuner, and Flashport.