Datasheet

ADSP-21061/ADSP-21061L
Rev. D | Page 19 of 52 | May 2013
EXTERNAL POWER DISSIPATION (3.3 V)
Total power dissipation has two components, one due to inter-
nal circuitry and one due to the switching of external output
drivers. Internal power dissipation is dependent on the instruc-
tion execution sequence and the data operands involved.
Internal power dissipation is calculated in the following way:
P
INT
= I
DDIN
 V
DD
The external component of total power dissipation is caused by
the switching of output pins. Its magnitude depends on:
—the number of output pins that switch during each cycle
(O)
—the maximum frequency at which they can switch (f)
—their load capacitance (C)
—their voltage swing (V
DD
)
and is calculated by:
PEXT = O
 C  V
DD
2
 f
The load capacitance should include the processor’s package
capacitance (CIN). The switching frequency includes driving
the load high and then back low. Address and data pins can
drive high and low at a maximum rate of 1/(2t
CK
). The write
strobe can switch every cycle at a frequency of 1/t
CK
. Select pins
switch at 1/(2t
CK
), but selects can switch on each cycle.
Example: Estimate P
EXT
with the following assumptions:
• A system with one bank of external data memory RAM
(32-bit)
• Four 128k  8 RAM chips are used, each with a load of
10 pF
• External data memory writes occur every other cycle, a rate
of 1/(4t
CK
), with 50% of the pins switching
• The instruction cycle rate is 40 MHz (t
CK
= 25 ns)
The P
EXT
equation is calculated for each class of pins that can
drive:
A typical power consumption can now be calculated for these
conditions by adding a typical internal power dissipation:
P
TOTAL
= P
EXT
+ (I
DDIN2
 3.3 V)
Note that the conditions causing a worst-case P
EXT
are different
from those causing a worst-case P
INT
. Maximum P
INT
cannot
occur while 100% of the output pins are switching from all ones
to all zeros. Note also that it is not common for an application to
have 100% or even 50% of the outputs switching
simultaneously.
Table 5. External Power Calculations
Pin Type No. of Pins % Switching  C  f  V
DD
2
= P
EXT
Address 15 50  44.7 pF  10 MHz  10.9 V = 0.037 W
MS0
10  44.7 pF  10 MHz  10.9 V = 0.000 W
WR
1—  44.7 pF  20 MHz  10.9 V = 0.010 W
Data 32 50  14.7 pF  10 MHz  10.9 V = 0.026 W
ADDRCLK 1 —  4.7 pF  20 MHz  10.9 V = 0.001 W
P
EXT
= 0.074 W