
ADSP-21261
Rev. 0
|
Page 17  of 44
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March 2006
TIMING SPECIFICATIONS
The ADSP-21261’s internal clock (a multiple of CLKIN) pro-
vides the clock signal for timing internal memory, processor 
core, serial ports, and parallel port (as required for read/write 
strobes in asynchronous access mode). During reset, program 
the ratio between the DSP’s internal clock frequency and exter-
nal (CLKIN) clock frequency with the CLKCFG1–0 pins. To 
determine switching frequencies for the serial ports, divide 
down the internal clock, using the programmable divider con-
trol of each port (DIVx for the serial ports). 
The ADSP-21261’s internal clock switches at higher frequencies 
than the system input clock (CLKIN). To generate the internal 
clock, the DSP uses an internal phase-locked loop (PLL). This 
PLL-based clocking minimizes the skew between the system 
clock (CLKIN) signal and the DSP’s internal clock (the clock 
source for the parallel port logic and I/O pads).
Note the definitions of various clock periods that are a function 
of CLKIN and the appropriate ratio control (
Table 7
 and 
Table 8
).
Figure 5
 shows core to CLKIN ratios of 3:1, 8:1, and 16:1 with 
external oscillator or crystal. Note that more ratios are possible 
and can be set through software using the power management 
control register (PMCTL). For more information, see the 
ADSP-2126x SHARC DSP Core Manual
.
Use the exact timing information given. Do not attempt to 
derive parameters from the addition or subtraction of others. 
While addition or subtraction would yield meaningful results 
for an individual device, the values given in this data sheet 
reflect statistical variations and worst cases. Consequently, it is 
not meaningful to add parameters to derive longer times.
See 
Figure 30 on Page 37
 under Test Conditions for voltage 
reference levels.
Switching characteristics specify how the processor changes its 
signals. Circuitry external to the processor must be designed for 
compatibility with these signal characteristics. Switching char-
acteristics describe what the processor will do in a given 
circumstance. Use switching characteristics to ensure that any 
timing requirement of a device connected to the processor (such 
as memory) is satisfied.
Timing requirements apply to signals that are controlled by cir-
cuitry external to the processor, such as the data input for a read 
operation. Timing requirements guarantee that the processor 
operates correctly with other devices.
Table 7. ADSP-21261 CLKOUT and CCLK Clock 
Generation Operation
Timing Requirements
CLKIN 
CCLK
Description
Input Clock
Core Clock
Calculation
1/t
CK
1/t
CCLK
Table 8. Clock Periods
Timing 
Requirements
t
CK
t
CCLK
t
SCLK
t
SPICLK
Description
1
CLKIN Clock Period 
(Processor) Core Clock Period 
Serial Port Clock Period = (t
CCLK
) × SR
SPI Clock Period = (t
CCLK
) × SPIR
1
where:
 SR = serial port-to-core clock ratio (wide range, determined by SPORT 
CLKDIV)
SPIR = SPI-to-core clock ratio (wide range, determined by SPIBAUD 
register)
DAI_Px = serial port clock
SPICLK = SPI clock
Figure 5. Core Clock and System Clock Relationship to CLKIN
CLKIN
CCLK
(CORE CLOCK)
PLLICLK
XTAL
XTAL
OSC
PLL
3:1, 8:1,
16:1
CLKOUT
CLKCFG1–0