
Rev. 0
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Page 8  of 44
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March 2006
ADSP-21261
DMA transfers are used to move data to and from internal 
memory. Access to the core is also facilitated through the paral-
lel port register read/write functions. The RD, WR, and ALE 
(address latch enable) pins are the control pins for the 
parallel port.
Timers
The ADSP-21261 has a total of four timers: a core timer able to 
generate periodic software interrupts, and three general-
purpose timers that can generate periodic interrupts and be 
independently set to operate in one of three modes:
 Pulse waveform generation mode
 Pulse width count/capture mode
 External event watchdog mode
The core timer can be configured to use FLAG3 as a timer 
expired output signal, and each general-purpose timer has one 
bidirectional pin and four registers that implement its mode of 
operation: a 6-bit configuration register, a 32-bit count register, 
a 32-bit period register, and a 32-bit pulse width register. A sin-
gle control and status register enables or disables all three 
general-purpose timers independently.
Program Booting
The internal memory of the ADSP-21261 boots at system 
power-up from an 8-bit EPROM via the parallel port, an SPI 
master, an SPI slave, or an internal boot. Booting is determined 
by the boot configuration (BOOTCFG1–0) pins. Selection of the 
boot source is controlled via the SPI as either a master or slave 
device, or it can immediately begin executing from ROM. 
Phase-Locked Loop 
The ADSP-21261 uses an on-chip phase-locked loop (PLL) to 
generate the internal clock for the core. On power-up, the 
CLKCFG1
–
0 pins are used to select ratios of 16:1, 8:1, and 3:1. 
After booting, numerous other ratios can be selected via soft-
ware control. The ratios are made up of software configurable 
numerator values from 1 to 32 and software configurable divi-
sor values of 1, 2, 4, 8, and 16.
Power Supplies
The ADSP-21261 has separate power supply connections for the 
internal (V
DDINT
), external (V
DDEXT
), and analog (A
VDD
/A
VSS
) 
power supplies. The internal and analog supplies must meet the 
1.2 V requirement. The external supply must meet the 3.3 V 
requirement. All external supply pins must be connected to the 
same power supply.
Note that the analog supply pin (A
VDD
) powers the processor’s 
internal clock generator PLL. To produce a stable clock, it is rec-
ommended that PCB designs use an external filter circuit for the 
A
VDD
 pin.  Place the filter components as close as possible to the 
A
VDD
/A
VSS
 pins.  For an example circuit, see 
Figure 4
. (A recom-
mended ferrite chip is the muRata BLM18AG102SN1D).  To 
reduce noise coupling, the PCB should use a parallel pair of 
power and ground planes for V
DDINT
 and GND. Use wide traces 
to connect the bypass capacitors to the analog power (A
VDD
) and 
ground (A
VSS
) pins.  Note that the A
VDD
 and A
VSS
 pins specified 
in 
Figure 4
 are inputs to the processor and not the analog 
ground plane on the board—the A
VSS
 pin should connect 
directly to digital ground (GND) at the chip.
TARGET BOARD JTAG EMULATOR CONNECTOR
Analog Devices DSP Tools product line of JTAG emulators uses 
the IEEE 1149.1 JTAG test access port of the ADSP-21261 pro-
cessor to monitor and control the target board processor during 
emulation. Analog Devices DSP Tools product line of JTAG 
emulators provides emulation at full processor speed, allowing 
inspection and modification of memory, registers, and proces-
sor stacks. The processor’s JTAG interface ensures that the 
emulator will not affect target system loading or timing.
For complete information on Analog Devices’ SHARC DSP 
Tools product line of JTAG emulator operation, see the appro-
priate emulator hardware user’s guide.
DEVELOPMENT TOOLS
The ADSP-21261 is supported by a complete set of 
CROSSCORE
 software and hardware development tools, 
including Analog Devices emulators and VisualDSP++
development environment. The same emulator hardware that 
supports other SHARC processors also fully emulates the 
ADSP-21261.
The VisualDSP++ project management environment lets pro-
grammers develop and debug an application. This environment 
includes an easy to use assembler (which is based on an alge-
braic syntax), an archiver (librarian/library builder), a linker, a 
loader, a cycle-accurate instruction-level simulator, a C/C++ 
compiler, and a C/C++ runtime library that includes DSP and 
mathematical functions. A key point for these tools is C/C++ 
code efficiency. The compiler has been developed for efficient 
translation of C/C++ code to DSP assembly. The ADSP-21261 
SHARC DSP has architectural features that improve the 
efficiency of compiled C/C++ code.
The VisualDSP++ debugger has a number of important fea-
tures. Data visualization is enhanced by a plotting package that 
offers a significant level of flexibility. This graphical representa-
tion of user data enables the programmer to quickly determine 
the performance of an algorithm. As algorithms grow in      
Figure 4. Analog Power Filter Circuit
CROSSCORE is a registered trademark of Analog Devices, Inc.
VisualDSP++ is a registered trademark of Analog Devices, Inc.
HI Z FERRITE
BEAD CHIP
LOCATE ALL COMPONENTS
CLOSE TO AVDDAND AVSSPINS
A
VDD
A
VSS
100nF
10nF
1nF
ADSP-21261
VDDINT