Memory speed: Difference between revisions

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[[File:Aes_cxk5814.jpg|thumb|[[VRAM]] chip, note the "-35" marking. 35ns is the highest speed requirement in the NeoGeo.]]
[[File:Aes_cxk5814.jpg|thumb|[[VRAM]] chip, note the "-35" marking. 35ns is the highest speed requirement in the NeoGeo.]]


Memory speed (or speed grade) is a general concept in electronics, and a value representing the time it takes for a memory chip to output valid data after the address is changed.
Memory speed (or speed grade) is a value representing the maximum time it takes for a memory chip to output valid data after the address is changed.


It is essential for memory chips to be fast enough to avoid crashes or glitches. When replacing RAMs or making cartmods, care must be taken to use chips with appropriate speed grades.
It is essential for memory chips to be fast enough to avoid crashes or glitches. When replacing RAMs or making cartmods, care must be taken to use chips with appropriate speed grades.


Speed is usually part of the chip's markings. It is actually a time value (speed = 1/t) specified in tens or hundreds of nanoseconds.
Speed is usually indicated on the chip. It is actually a time value (speed = 1/t) specified in nanoseconds or tens of nanoseconds.


During the specified time, the memory data output isn't guaranteed to be valid. So if the system expects data to be valid too soon, it might catch wrong bits resulting in unstable operation.
During the specified time, the memory data output isn't guaranteed to be valid. So if the system expects data to be valid faster, it might catch wrong bits resulting in unstable operation.


Regarding the NeoGeo, faster (lower value) isn't a problem, same speed is fine, slower (higher value) is bad. A slower chip can work in some cases, but you should never expect stable operation in all conditions. For example, a 120ns chip could work in place of a 100ns one at 5.1V, but cause an immediate crash if voltage goes below 5.05V. Trust the manufacturer's specifications.
Regarding the NeoGeo, faster (lower value) isn't a problem, same speed is fine, slower (higher value) is bad. A slower chip can work in some cases, but you should never expect stable operation in all conditions. For example, some 120ns chips could work in place of a 100ns one at 5.1V, but cause an immediate crash if voltage goes below 5.05V. Trust the manufacturer's specifications.


=Standard speed grades=
=Standard speed grades=
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-250 (250ns), -200 '''or -20''' (200ns), -150 '''or -15''' (150ns), -120 '''or -12''' (120ns), -90 (90ns), -70 (70ns), -55 (55ns), -35 (35ns).
-250 (250ns), -200 '''or -20''' (200ns), -150 '''or -15''' (150ns), -120 '''or -12''' (120ns), -90 (90ns), -70 (70ns), -55 (55ns), -35 (35ns).


Note the possible confusion. If the value is less than 30 you certainly have to add a 0, even if SRAM is now available at 25ns (not 250) speed grades.
[[Category:Chips]]
[[Category:Base system]]
[[Category:Base system]]
[[Category:Repairs]]
[[Category:Repairs]]

Revision as of 22:59, 4 May 2016

File:Aes cxk5814.jpg
VRAM chip, note the "-35" marking. 35ns is the highest speed requirement in the NeoGeo.

Memory speed (or speed grade) is a value representing the maximum time it takes for a memory chip to output valid data after the address is changed.

It is essential for memory chips to be fast enough to avoid crashes or glitches. When replacing RAMs or making cartmods, care must be taken to use chips with appropriate speed grades.

Speed is usually indicated on the chip. It is actually a time value (speed = 1/t) specified in nanoseconds or tens of nanoseconds.

During the specified time, the memory data output isn't guaranteed to be valid. So if the system expects data to be valid faster, it might catch wrong bits resulting in unstable operation.

Regarding the NeoGeo, faster (lower value) isn't a problem, same speed is fine, slower (higher value) is bad. A slower chip can work in some cases, but you should never expect stable operation in all conditions. For example, some 120ns chips could work in place of a 100ns one at 5.1V, but cause an immediate crash if voltage goes below 5.05V. Trust the manufacturer's specifications.

Standard speed grades

-250 (250ns), -200 or -20 (200ns), -150 or -15 (150ns), -120 or -12 (120ns), -90 (90ns), -70 (70ns), -55 (55ns), -35 (35ns).

Note the possible confusion. If the value is less than 30 you certainly have to add a 0, even if SRAM is now available at 25ns (not 250) speed grades.