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Max Vcore for Tbred B and Barton
There are some slight difference between the various Vcore rated CPU, due to the internal threshold voltage and leakage current characteristic of the transistors from manufacturing process variation (I think). As a result, if going into technicality and hair splitting, DLT3C (1.5V rated Vcore) is 100 mV lower than DUT3C, and DUT3C (1.6V rated Vcore) is 50 mV lower than DKT3C (1.65V rated Vcore), in Vcore absolute rating.
According to the AMD datasheet for model 8 and 10:
Data sheet for Model 10 (Barton)
Data sheet for Model 8 (Tbred)
In Chapter 8,
Quoted:
"The AMD Athlon XP processor model 8 should not be subjected to conditions exceeding the absolute ratings, as such conditions can adversely affect long-term reliability or result in functional damage."
The absolute rating for Vcore = Vcc_core_dc_max + 0.5 V
Vcc_core_dc_max = Vcc_core_nominal + 0.05 V
Vcc_core_ac_max = Vcc_core_nominal + 0.15 V
Vcc_core_nominal = 1.50 V for DLT3C
Vcc_core_nominal = 1.60 V for DUT3C
Vcc_core_nominal = 1.65 V for DKT3C
For DLT3C, e.g. 1700+ DLT3C
Vcc_core_dc_max = 1.5 + 0.05 = 1.55 V
The
absolute rating for Vcore = 1.55 + 0.5 = 2.05 V
For DUT3C, e.g. 1700+ DUT3C, 2100+
Vcc_core_dc_max = 1.6 + 0.05 = 1.65 V
The
absolute rating for Vcore = 1.65 + 0.5 = 2.15 V
For DKT3C, e.g. 2600+
Vcc_core_dc_max = 1.65 + 0.05 = 1.70 V
The
absolute rating for Vcore = 1.70 + 0.5 = 2.20 V
The numbers are the same for model 10 (Barton).
For Barton (DKT3C), e.g. 2500+, 2800+
Vcc_core_nominal = 1.65 V
Vcc_core_dc_max = 1.65 + 0.05 = 1.70 V
The
absolute rating for Vcore = 1.70 + 0.5 = 2.20 V
The above is based on my interpretation. Pls read the datasheet, and make your judgement.
How much voltage can be applied to a CPU
There are two components of power going into a CPU at a given Vcore:
1. The "good" CV^2f active power which powers the CPU to run at frequency f.
2. The "wasted" V^2/R static power which biases the CPU as leakage current, but also heats up the chip.
Putting in higher Vcore would generally let you clock faster (CVcore^2f). But unfortunately, it also heats up the chip (Vcore^2/R).
So on air, at lower temperature 10-20-30 C, the CPU can be clocked faster and faster at a rate about 130-140 MHz/100mV (for Tbred B, Barton). So far so good. But at the same time, the chip will begin to
heat up due to Vcore increases (the V^2/R compoent and also from the active CV^2f component). As a result of heat, the electrons move slower inside the chip and the CPU begin to run slower, the above rate delf/delVcore begins to drop to 120 then 110 then 100 then 50 MHz /100 mV when die temperature reaches beyond 30 C, 40 C, 50 C, ... correspondingly for Vcore above 1.7, 1.8, 1.9 V, ... The heat increases at a rate faster than the Vcore increase to slow down the chip.
This is what we call the diminishing return on CPU frequency. And eventually, around 1.95 - 2 V for Tbred B, 1700+ DLT3C, it will come to "stop" (due to heat, high current and system instability) even when more Vcore is put in, since the heat slows the chip down. There is no more reason to increase Vcore anymore (even you don't kill the chip). For higher Vcore rated ones such as 2100+, Barton 2500+, that Vcore wall is around 2.05 - 2.2 V on air.
The above numbers are mainly for illustration, and they are roughly correct. But don't quote and use them for exact calculation.
If you use thermoelectric, phase change, .... exterme cooling, due to the
lower die temperature, as mentioned above, the chips can run much
faster and reach much higher frequency (e.g. 3+ GHz)
at the same Vcore (compared to air/water) before the die reaching the higher temperature as cooled by air. E.g. at 1.95-2V 1700+ will run at 2.5-2.6 GHz on air at 50 C, but it will run at 3 - 3.2 GHz at -10 C.
It does not mean you can put much higher Vcore onto the chips at lower die temperature. Vcore is subjected to transistor leakage increase, gate breakdown constraints.
They run faster is a combination of higher active power to substain the computation (both logical and electrical) and lower die temperature, not higher Vcore alone.
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