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196

Revision 1.0

Electrical Specifications (Continued)

6.6

I/O CURRENT DE-RATING CURVE

As mentioned Section 6.5.2.3 “Definition of System Condi-

tions for Measuring “On” Parameters” on page 192, the I/O

current of the processor is affected by two system parame-

ters, 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 current used by the

processor for a particular design. Core current is not signif-

icantly affected by these two parameters, so a core current

de-rating curve is not provided.

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 current is linear.

The system designer must determine the maximum DCLK

frequency required in the system based on the maximum

display that will be supported.

6.6.2

Memory Speed

Each device in the GX1 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 resolution,

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 technical support represen-

tative.

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 GX1 series of processors. While

the memory speeds for the various core frequencies are

different, the three memory speeds for each device pro-

duce the same de-rating effect.

Figure 6-3. Absolute Max I/O Current De-rating Curve

(All Speeds and Core Voltages)

-120

-160

-40

-80

Mem 2

Mem 3

Mem 1

MHz

Mem 1 MHz

Mem 2 MHz

Mem 3 MHz

166

÷2.0

83.3

÷2.5

66.7

÷3.0

55.6

200

÷2.0

100

÷2.5

÷3.0

66.7

233

÷2.5

93.3

÷3.0

77.8

÷3.5

66.7

266

÷3.0

88.9

÷3.5

76.2

÷4.0

66.7

300

÷3.5

85.7

÷4.0

75.0

÷4.5

66.7

1024x768x75 Hz,

(DCLK = 79 MHz)

1280x1024x60 Hz,

(DCLK = 108 MHz)

Note:

Pixel color depth does not affect power consumption or DCLK frequency.

800x600x72 Hz,

(DCLK = 50 MHz)

640x480x72 Hz,

(DCLK = 32 MHz)

1024x768x75 Hz,

(DCLK = 79 MHz)

Absolute Maximum

-140

-100

-60

-20

Mem 2

Mem 3

Mem 1

1280x1024x75 Hz,

(DCLK = 135 MHz)

Mem 2

Mem 3

Mem 1

Mem 2

Mem 3

Mem 1

Mem 2

Mem 3

Mem 1

lay

lut

ion

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