13: Internal Options
13-4
PRC1099A-MS
Dual Port RAM
Dual port RAM (DPRAM) chip U9 provides an easy interface between the
main ALE processor and the DSP processor. Each time the DSP processor
receives ALE data, it stores the data in DPRAM. The DPRAM sends an
interrupt to the main ALE processor to tell it there is data waiting. The main
ALE processor then retrieves and processes the received data. Conversely,
when the main ALE processor wants to transmit, it stores the data in the
DPRAM. The DPRAM then sends an interrupt to the DSP processor. The DSP
processor then retrieves the data and transmits it.
Control Logic
PAL chip U20 is a programmable logic-array chip that contains all the
interface logic between the main ALE processor and its memories, and the
DSP processor and its memories. It also provides the control interface
between the main ALE and DSP processors.
Bus multiplex chip U21 multiplexes the address and data lines coming from
the main ALE processor. The address is first presented to the lower address
bus and is then latched with the ALE signal from the main ALE processor.
The bus is then able to read or write data on AC0-AC8.
Digital Signal
Processor (DSP)
Digital Signal Processor (DSP) U13 is a special purpose processor optimized
for rapidly processing math functions. The digital processing enables the DSP
processor to distinguish ALE tones from normal background noise in noisy
environments. It also generates a transmit tone with superb quality, enhancing
the likelihood of its detection at the receiving end.
The DSP processor operates at a clock frequency of 20.736 MHz derived from
high-stability oscillator U12. This high-stability is necessary for generating
and decoding ALE tones with precision.
The DSP processor interfaces to audio interface chip U11 that processes all
the incoming received audio and digitizes it for the DSP processor. It also
generates the ALE transmit tones from the digital data that the DSP processor
sends. The DS, PS, IS, STAB, and RW are used in conjunction with U20 to
provide the switching logic for the external EPROM, RAM and DPRAM
chips. Address lines A0-A15 and D0-D15 are for communications with the
external EPROM, RAM and DPRAM chips.
Audio Interface
Audio interface chip U11 is responsible for the decoding and generating ALE
tones. The chip is divided into two parts: receive and transmit circuitry. The
receive circuits consist of a 14-bit analog-to-digital converter (ADC) that
converts the incoming analog audio signal into a digital sequence that it sends
to the DSP processor. The transmitting circuits use a 14-bit digital-to-analog
converter (DAC) to convert the incoming digital sequence from the DSP
processor into a precise transmit audio tone.
-5V Generator
Voltage converter U10 uses a monolithic charge pump inverter to invert the
+5VDC supply to the -5 VDC used by audio interface chip U11 This uses
capacitors C3 and C4 at 45 kHz to generate the necessary voltage and current.
