E.F. Johnson Company 5100 ES, Service Manual

Page: 65 / 180

January 2007 5100 ES Series Portable Radio Service Manual 5-21
Circuit Description
5.5.2.1 Control Interface
The MPC870 processor on the UI Board communicates with the DSP via a multiplexed 16
bit host peripheral (HPI) bus. This interface is used by the MPC870 to load application
code into the DSP’s internal SRAM and for exchanging control/status with the DSP. The
DSP has three internal registers, accessible by the MPC870, for transferring of data.
Access to the internal registers is controlled by the DSP’s chip select (signal H_nCS), data
strobe (DSP_DS0), read/write (H_R/nW), and register select (HDS1, HDS2) input signals.
The DSP uses its HRDY line to signal the MPC870 when a bus transfer has completed.
5.5.2.2 Transmit Operation
In transmit mode the microphone audio signal is converted to a differential signal on the
UI board and then routed to the Codec on the Logic Board where it is digitized by an
internal A/D converter and forwarded to the DSP via a synchronous serial interface (SSI)
bus. The DSP processes the incoming data and sends the digitized modulation signals for
the RF Deck VCO and TCXO back to the Codec via the SSI bus. The Codec converts the
digitized modulation signals into analog signals which are level shifted by an op amp
(U26) to derive the TXMOD1 and TXMOD2 signals for the RF Deck.
5.5.2.3 Receive Operation
In receive mode, the DSP receives I/Q data from the Rx Backend IC on the RF Deck (J1)
via an SSI bus. The Rx Backend IC controls the timing on SSI bus with the clock set to 1.4
MHz and a frame time of 40 KHz. The SSI bus signals from the RF Deck are heavily
filtered and are buffered through U20 to improve the wave shape. The DSP filters and
demodulates the I/Q samples from the Rx Backend IC. The resulting demodulated signal
is sent to the Codec via the SSI bus. The Codec converts the processed signal from the
DSP to an audio signal which is passed to an op amp (U8) that converts it to a differential
form. This differential signal is amplified on the UI Board and applied to either the
internal on external speaker. Additionally, the DSP sends data to the Codec via the SSI bus
to adjust the TXMOD1 and TXMOD2 signals to set the RF Deck receiver frequency.