Crystal DSP Board Operation and Debug
" AINL+ -- Left Channel + Differential buffer input
2.6 EPROM MEMORY
This memory is the DSP external program memory. Included are the
EPROM, address latches, and an output data buffer.
2.6.1 EPROM memory
" An AMD 27C020 is used for the DSP micro code boot memory. The
AM27C020 provides 512KB organized as follows:
" 8 - 32KB pages for DSP microcode storage
" 4 - 64KB pages for external DSP tables, such as DTS tables.
" It would be possible to use these pages for microcode, although
half of each 64KB page used in this way would be unused
(maximum of 32KB microcode RAM inside the DSP chip).
" NOTE!!!!!!! YOU MUST USE A ONE-TIME PROGRAM MEMORY, NOT A FLASH
PROM (e.g., AT29C020) in this location (U27).
" This is due to the function of the \PGM pin in FLASH parts and
that pin?s use in this application.
2.6.2 Address Latches
The DSP chip uses a multiplexed addr/data bus.
" The Addr Latches latch a 16-bit address off an 8-bit bus by
following the DSP chip timing protocol.
" First the MS addr bits are latched, then the LS addr bits are
latched. The 16 address bits are presented to the PROM as a
parallel input to allow the DSP chip to address up to 64KB (for DTS
table access).
" In this case the upper address bit (A15) is used.
" The Micro Code Program boot sequence accesses only 32KB of this
address space.
" In this case the upper address bit (A15) is unused.
" The DSP board CPU uses this to dynamically assign usage of A15 to
either a 32KB page size or a 64KB page size for optimum utilization
of the PROM memory.
2.6.3 Memory Data Output Buffer
This serves to enable the data onto the multiplexed Addr/Data bus only
when the DSP drives both the EXTMEM and EMOE signals.
" A 5V / 3.3V buffer chip is used to interface the 5V PROM to the 3.3V
DSP chip.
" 5V PROMs are much cheaper than 3.3V PROMs so this tradeoff was made
in favor of cost over I.C. count.
2.7 Analog I/O
This function provides the analog buffering between the System Analog
circuits and the DACs and ADCs of the CODEC. This function consists of
Input and Output buffers.
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ACEC
