ADSP-21990 Datasheet, PDF (40/50 Pages) Analog Devices – Mixed-Signal DSP Controller

ADSP-21990
POWER DISSIPATION
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.
The external component of total power dissipation is caused by
the switching of output pins. Its magnitude depends on:
• 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 the formula below.
PEXT = O × C × VDD2 × f
The load capacitance includes the processor package capaci-
tance (C_IN). 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. For example, esti-
mate P_EXT with the following assumptions:
• A system with one bank of external data memory—asyn-
chronous RAM (16-bit)
• One 64K ؋ 16 RAM chip is used with a load of 10 pF
• Maximum peripheral speed CCLK = 80 MHz, HCLK =
80 MHz
• External data memory writes occur every other cycle, a rate
of 1/(4t_HCLK), with 50% of the pins switching
• The bus cycle time is 80 MHz (t_HCLK = 12.5 ns)
The P_EXT equation is calculated for each class of pins that can
drive as shown in Table 16.
A typical power consumption can now be calculated for these
conditions by adding a typical internal power dissipation with
the following formula.
PTOTAL=
P
EXT
+
PINT
Where:
• P_EXT is from Table 16.
• P_INT is I_DDINT ϫ 2.5 V, using the calculation I_DDINT listed in
Power Dissipation.
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 16. P_EXT Calculation Example
Pin Type
Address
MSx
WR
Data
CLKOUT
Number of Pins
15
1
1
16
1
% Switching
50
0
50
؋C
10 pF
10 pF
10 pF
10 pF
10 pF
؋f
20 MHz
20 MHz
40 MHz
20 MHz
80 MHz
؋ V_DD2
10.9 V
10.9 V
10.9 V
10.9 V
10.9 V
= P_EXT
= 0.01635 W
= 0.0 W
= 0.00436 W
= 0.01744 W
= 0.00872 W
= 0.04687 W
Rev. A | Page 40 of 50 | August 2007