Electrical Specifications (Continued)
6.6 I/O CURRENT DE-RATING CURVE
As mentioned Section 6.5.2.3 “Definition of System Con-
ditions for Measuring "On" Parameters” on page 191, the
I/O current of the processor is affected by two system
parameters, DCLK and SDRAM frequency. A de-rating
curve (see Figure 6-3) is provided so that the system
designer can determine the absolute maximum I/O cur-
rent used by the processor for a particular design. Core
current is not significantly affected by these two parame-
ters, so a core current de-rating curve is not provided.
6.6.2 Memory Speed
Each device in the GXLV processor series is defined by a
particular core voltage and core frequency. The SDRAM
frequency is derived internally by a programmable divisor
of the core frequency. Typically, there are three SDRAM
frequencies between 55 and 100 MHz that can be derived
from a single core frequency. These three frequencies are
provided in the following de-rating curve so that their
effect on current can be seen. Just as with the display res-
olution, current de-rating due to memory speed is linear.
SDRAM frequencies between 79 and 100 MHz are only
supported for certain types of closed systems and strict
design rules must be adhered to. For further details,
please contact your local National Semiconductor techni-
cal support representative.
6.6.1 Display Resolution
The change in current of five common display resolutions
is used to extrapolate the de-rating curve. DCLK is
derived from the display resolution, color depth, and
refresh rate. The relationship between DCLK and I/O cur-
rent is linear. The system designer must determine the
maximum DCLK frequency required in the system based
on the maximum display that will be supported.
6.6.3 I/O Current De-rating Curve
The I/O current de-rating curve, shown in Figure 6-3, is
the same for all devices in the GXLV series of processors.
While the memory speeds for the various core frequen-
cies are different, the three memory speeds for each
device produce the same de-rating effect.
1280x1024x75 Hz,
Mem 1
Absolute Maximum
0
(DCLK = 135 MHz)
Mem 2
Mem 3
1280x1024x60 Hz,
Mem 1
-50
(DCLK = 108 MHz)
Mem 2
Mem 3
1024x768x75 Hz,
(DCLK = 79 MHz)
-100
Mem 1
Mem 2
Mem 3
800x600x72 Hz,
(DCLK = 50 MHz)
-150
-200
-250
Mem 1
Mem 2
640x480x72 Hz,
Mem 1
Mem 2
Mem 3
Mem 3
(DCLK = 32 MHz)
MHz
Mem 1 MHz
÷2.0
Mem 2 MHz
÷2.5
Mem 3 MHz
÷3.0
166
180
200
233
266
83.3
90
66.7
72
55.6
60
÷2.0
÷2.0
÷2.5
÷3.0
÷2.5
÷2.5
÷3.0
÷3.5
÷3.0
÷3.0
÷3.5
÷4.0
100
93.3
88.9
80
66.7
66.7
66.7
77.8
76.2
Note: Pixel color depth does not affect power consumption or DCLK frequency.
Figure 6-3. Absolute Max I/O Current De-rating Curve
(All Speeds and Core Voltages)
Revision 1.1
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