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Review: Giga Designs G4 1.25GHz CPU Upgrade
By Mike
Published: 3/14/2003
(Revised 3/15/2003 5AM for Vreg chart)
Details of the Giga Designs Module
Intro | Benchmarks  | Apps Tests | Game Tests | Installation | Specs/Design

Giga Designs G4/1.25GHz w/2MB DDR L3 Cache: (from sample provided)

  • CPU: Motorola 1.25GHz rated 7455A revison 3.3
  • 256KB on-chip L2 cache (@cpu clock speed)
  • 2MB DDR L3 Cache
    (Samsung K7D803671B-HC33 x2, BGA package)

Giga Top side

Giga Bottom side

Bus Multiplier Jumper Settings Info (J1-J5):
The Giga Designs AGP upgrades are the only ones to date that have jumper settings to allow changing both the bus multiplier (determines CPU speed/allows for 100 or 133MHz bus use) as well as the CPU core voltage. (The Sonnet AGP upgrades autodetect the system bus speed, but are not manually adjustable, the original Powerlogix AGP upgrades were sold by 100MHz (Series 100) or 133MHz (Series 133) bus models and are not adjustable although their later Series233 models are adjustable.)
Giga Designs told me recently that they have changed their warranty policy on Overclocking; no longer is it considered "abuse" - as long as the CPU core voltage settings are not changed.
Here's a (large unfortunately) image capture of the settings from their PDF file. (Of course the higher settings are not going to be reliable with current CPUs.)

Notes on L3 Cache Speeds: Just a FYI that the specs on the current 7455A used in this upgrade list a max relible L3 cache bus speed of 266MHz. With the upgrade set to 10x ratio in a 133MHz bus Mac (1.33GHz CPU speed), that means the cache is running at 266.6MHz (with the default 5:1 L3 ratio). Therefore when overclocking the CPU problems that occur may be L3 cache (not just CPU) related. Also those that see wake-from-sleep problems at 1.3GHz or above should try either lowering the CPU speed, or see if some 3rd party cache control software can reduce the L3 cache speed to see if that helps. I verified that my problem with waking from sleep were not seen at 1.26GHz, but were when set to 1.33GHz. (Tested in both OS X 10.1.5 and 10.2.4)

giga designs CPU speed settings

Note: The revison 3.3 7455A CPUs have additional multiplier settings not available on the earlier rev 2.1 7455 CPUs used on most G4 800-1GHz upgrades. (This was noted in the Oct. 31st, 2002 news page here from a post by Michiro Isobe on the 3.x rev 7455's which also have a lower core voltage - however the spec doc linked in that news page has been revised.) The 7455A has more bus/cpu ratio (multiplier) settings at the mid and high end (at the expense of lower ratio settings that were re-used for the upper ones due to a limited number of pins for that function). As I said on the installation page, some of the new ratios (like 9.5x - see table above) are re-used settings that were for much lower ratios with the previous CPU versions. Because of this, if set to one of these re-used settings, your Mac may not report the correct CPU speed. For instance 9.5x (1.266GHz for this Digital Audio 133MHz bus system) results in Apple System Profiler reporting a 600MHz CPU speed, even though the CPU is actually running at the higher speed. (Benchmarks like Xbench will also report the incorrect speed.) When set to a 10x ratio, the correct speed of 1.33GHz was reported by ASP however. (10x is not one of the 're-used' settings from the low end ratios of the previous 7455 revisions.) A list of these "re-used" settings (which will not be correctly reported) is included in the image above.

CPU Voltage Settings:
NOTE: At first I was confused as the Voltage settings for his sample indicated a 1.55V core setting (per the Vreg chip docs/table I had made for review of the previous model), but the 7455A CPU spec I had listed a 1.30V core for the 7455A. I asked Tim of GigaDesigns about this and he said the sample 1.25GHz CPU version on this upgrade was a 1.55V part. (I asked if all 1.25GHz future models would be using that same part.)

voltage settings

References: The Voltage Regulator chip used is the LTC1709 2-Phase, Adjustable, High Efficiency switching regulator (PDF doc link, here's a google based HTML version.) Page 14 of that spec doc lists all the options/voltage settings. To clarify things, I again created a table showing open pins and jumpers populated for each of the positions. VID4 in the Vreg specs doc table is not used for this upgrade design, since it's only for >2.05 Volt settings. VID3-VID0 in the specs doc corresponds to J7-J10 on the Giga board. As most of you know, 0 in the specs docs means TTL low (jumper present), 1 means TTL high (open/no jumper.) For clarity I translated the specs doc table of 1/0 (TTL hi/lo) to a matrix (image above) with open pins/Jumpers on.
To repeat my earlier comments, this sample ran fine at 1.33GHz with the default voltage settings and the warranty is void if the voltage is changed from the default.

The Giga Design upgrades docs listed a 30 day Money Back Guarantee and a 3 year limited warranty as of the review date. And again, overclocking does not void the warranty I'm told, unless you boost the voltage. (30 day MBG subject to the returns policy of the dealer.) See the included docs or their online warranty page for complete details.

For owner reports on CPU upgrades (any brand/mac model), search my Rate Your CPU Upgrade database.
I stress that owners of any upgrade should use it for several weeks at least before posting an entry in my database. If there are problems - double check the installation/settings and contact the vendor immediately if you need help. They often can provide a solution or identify the source of the problem.

Index of Giga Designs G4 800/1GHz Upgrade Review Pages

Intro | Benchmarks  | Apps Tests | Game Tests | Installation | Specs/Design

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