Preliminary Technical Data
ADSP-21375
ments. These computation units support IEEE 32-bit single-
precision floating-point, 40-bit extended precision floating-
point, and 32-bit fixed-point data formats.
On-Chip Memory
The ADSP-21375 contains 0.5 megabits of internal RAM and
two megabits of internal mask-programmable ROM. Each block
can be configured for different combinations of code and data
storage (see Table 2 on page 6). Each memory block supports
single-cycle, independent accesses by the core processor and I/O
processor. The ADSP-21375 memory architecture, in combina-
tion with its separate on-chip buses, allow two data transfers
from the core and one from the I/O processor, in a single cycle.
Data Register File
A general-purpose data register file is contained in each pro-
cessing element. The register files transfer data between the
computation units and the data buses, and store intermediate
results. These 10-port, 32-register (16 primary, 16 secondary)
register files, combined with the ADSP-2136x enhanced Har-
vard architecture, allow unconstrained data flow between
computation units and internal memory. The registers in PEX
are referred to as R0-R15 and in PEY as S0-S15.
The ADSP-21375’s, SRAM can be configured as a maximum of
16K words of 32-bit data, 32K words of 16-bit data, 10.9K words
of 48-bit instructions (or 40-bit data), or combinations of differ-
ent word sizes up to 0.5 megabits. All of the memory can be
accessed as 16-bit, 32-bit, 48-bit, or 64-bit words. A 16-bit float-
ing-point storage format is supported that effectively doubles
the amount of data that may be stored on-chip. Conversion
between the 32-bit floating-point and 16-bit floating-point for-
mats is performed in a single instruction. While each memory
block can store combinations of code and data, accesses are
most efficient when one block stores data using the DM bus for
transfers, and the other block stores instructions and data using
the PM bus for transfers.
Single-Cycle Fetch of Instruction and Four Operands
The ADSP-21375 features an enhanced Harvard architecture in
which the data memory (DM) bus transfers data and the pro-
gram memory (PM) bus transfers both instructions and data
(see Figure 1 on page 1). With the ADSP-21375’s separate pro-
gram and data memory buses and on-chip instruction cache,
the processor can simultaneously fetch four operands (two over
each data bus) and one instruction (from the cache), all in a sin-
gle cycle.
Using the DM bus and PM buses, with one bus dedicated to
each memory block, assures single-cycle execution with two
data transfers. In this case, the instruction must be available in
the cache.
Instruction Cache
The ADSP-21375 includes an on-chip instruction cache that
enables three-bus operation for fetching an instruction and four
data values. The cache is selective—only the instructions whose
fetches conflict with PM bus data accesses are cached. This
cache allows full-speed execution of core, looped operations
such as digital filter multiply-accumulates, and FFT butterfly
processing.
EXTERNAL MEMORY
The external port on the ADSP-21375 SHARC provides a high
performance, glueless interface to a wide variety of industry-
standard memory devices. The 16-bit wide bus may be used to
interface to synchronous and/or asynchronous memory devices
through the use of it's separate internal memory controllers: the
first is an SDRAM controller for connection of industry-stan-
dard synchronous DRAM devices and DIMMs (Dual Inline
Memory Module), while the second is an asynchronous mem-
ory controller intended to interface to a variety of memory
devices. Four memory select pins enable up to four separate
devices to coexist, supporting any desired combination of syn-
chronous and asynchronous device types. Non SDRAM
external memory address space is shown in Table 3.
Data Address Generators With Zero-Overhead Hardware
Circular Buffer Support
The ADSP-21375’s two data address generators (DAGs) are
used for indirect addressing and implementing circular data
buffers in hardware. Circular buffers allow efficient program-
ming of delay lines and other data structures required in digital
signal processing, and are commonly used in digital filters and
Fourier transforms. The two DAGs of the ADSP-21375 contain
sufficient registers to allow the creation of up to 32 circular buff-
ers (16 primary register sets, 16 secondary). The DAGs
External Memory Execution
automatically handle address pointer wraparound, reduce over-
head, increase performance, and simplify implementation.
Circular buffers can start and end at any memory location.
In the ADSP-21375, the program sequencer can execute code
directly from external memory (SRAM, SDRAM). This allows a
reduction in internal memory size, thereby reducing the die
area. It also allows for faster code development. With external
execution, programs run at slower speeds since 48-bit instruc-
tions are fetched in parts from a 16-bit external bus coupled
with the inherent latency of fetching instructions from SDRAM.
Fetching instructions from external memory generally take
three core clock cycles per instruction.
Flexible Instruction Set
The 48-bit instruction word accommodates a variety of parallel
operations, for concise programming. For example, the
ADSP-21375 can conditionally execute a multiply, an add, and a
subtract in both processing elements while branching and fetch-
ing up to four 32-bit values from memory—all in a single
instruction.
ADSP-21375 MEMORY
The ADSP-21375 adds the following architectural features to
the SIMD SHARC family core.
Rev. PrB
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Page 5 of 42
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December 2005
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