
ADSP-21261
Rev. 0
|
Page 5  of 44
|
March 2006
element are arranged in parallel, maximizing computational 
throughput. Single multifunction instructions execute parallel 
ALU and multiplier operations. In SIMD mode, the parallel 
ALU and multiplier operations occur in both processing ele-
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.
Data Register File
A general-purpose data register file is contained in each 
processing 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-2126x 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.
Single-Cycle Fetch of Instruction and Four Operands
The ADSP-21261 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-21261’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 
single cycle.
Instruction Cache
The ADSP-21261 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.
Data Address Generators with Zero-Overhead Hardware 
Circular Buffer Support
The ADSP-21261’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-21261 contain 
sufficient registers to allow the creation of up to 32 circular buff-
ers (16 primary register sets, 16 secondary). The DAGs 
automatically handle address pointer wraparound, reduce over-
head, increase performance, and simplify implementation. 
Circular buffers can start and end at any memory location.
Figure 2. ADSP-21261 System Sample Configuration
DAI
SPORT3
SPORT2
SPORT1
SPORT0
SCLK0
SFS0
SD0A
SD0B
SRU
DAI_P1
DAI_P2
DAI_P3
DAI_P18
DAI_P19
DAI_P20
DAC
(OPTIONAL)
ADC
(OPTIONAL)
FS
CLK
SDAT
FS
CLK
SDAT
3
CLOCK
2
2
CLKIN
XTAL
CLK_CFG1–0
BOOTCFG1–0
FLAG3–1
ADDR
PARALLEL
PORT
RAM, ROM
BOOT ROM
I/O DEVICE
OE
WE
CS
DATA
RD
WR
CLKOUT
ALE
AD15–0
LATCH
RESET
JTAG
6
ADSP-21261
A
D
C
FLAG0
PCGB
PCGA
CLK
FS