ATI Radeon HD 3870 Voltmods

ATI Radeon HD 3870 Extreme Overclocking: Methodology

It has been a while since we last experimented with extreme overclocking. Many things have changed. Power circuitry of graphics cards has become far more complicated and now uses smart multi-channel programmable PWM-controllers. Among other things these controllers help implement power saving technologies by reducing the frequency and voltage of the core when the graphics card is idle.

A modern GPU has a few zones clocked at different frequencies. The GeForce 8/9 series is an obvious example but this also refers to modern Radeon HD chips which come with only one frequency declared but actually have a number of zones clocked at different frequencies. Moreover, the frequencies can change dynamically within the framework of PowerPlay technology.

You have to account for these peculiarities when you want to overclock a modern graphics card. For example, the original BIOS of the Radeon HD 3870 doesn’t contain divisors for frequencies above 862MHz. Therefore, your attempt to set the core frequency above this value will hang the system irrespective of the GPU voltage. Fortunately, this limitation is not fundamental and can be easily bypassed by means of a modified BIOS you can download here. You can use the ATIFlash tool (we used version 3.59) to reflash the BIOS. Then you can try to volt-mod your Radeon HD 3870.

The GPU power circuit of this card is based on the uPI uP6201 controller. It is difficult to find any documentation on this chip and we couldn’t find it, either. However, the volt-mod is most simple: you should connect a trimming resistor between the uP6201’s Pin 13 and the ground. We used a multiturn 100kOhm resistor. The points of connection are marked with blue in the photo below:

You can use one of the Vgpu control points marked in red to keep track of the GPU voltage. By default, the GPU voltage should be within 1.3V in 3D mode.

It is just as simple to volt-mod the memory circuit: another 100kOhm trimming resistor is connected to Pins 3 and 7 of the uPI uP6101BSA chip. These points are marked with green in the photo:

You can monitor the memory voltage in the point marked as Vgddr control. The memory voltage is 1.89V by default.

ATI Radeon HD 3870 Extreme Overclocking: Results

After the BIOS update to solve the 862MHz problem, the described hardware modification enabled us to adjust the GPU and memory voltages in a wide range. The cooler’s fan was set at its maximum speed by means of RivaTuner to keep the GPU temperature within reasonable limits. Our attempt to overclock the GPU produced the following results:

• 900MHz (891MHz according to RivaTuner) at 1.44V
• 950MHz (945MHz according to RivaTuner) at 1.51V
• 975MHz (972MHz according to RivaTuner) at 1.55V
• 1000MHz (999MHz according to RivaTuner) at 1.71V

Obviously, we’ve got not the best sample of the ATI Radeon HD 3870. We only achieved a frequency of 1GHz at a voltage of 1.71V although other cards were reported to be stable at that frequency with 1.55-1.6V voltage. We could not lift the frequency to 1.1GHz at 1.75-1.77V and we didn’t increase the voltage further because 1.7V was already dangerous for the 55nm chip with a default voltage of 1.33V. The temperature showings were indicative of that: the fan working at the max speed, the GPU was as hot as 69-72°C when idle and over 90°C hot under load. We wouldn’t recommend you to repeat our experiment because there is a risk of damaging the card even if you provide enough cooling. Well, ordinary users do not use this kind of overclocking whereas true enthusiasts can’t be stopped by any warnings.

We managed to overclock the memory chips to 1242 (2484) MHz at a voltage of 2.05V. This increased the memory bandwidth from 72 to 79.5GBps. That’s not much. Anyway, that’s not the memory bandwidth that is going to be the main factor in games.

The card successfully passed a couple of test cycles in 3DMark06 and 3DMark Vantage at 999/2484MHz but would hang up in games after a while. We achieved perfect stability at 972MHz GPU and 1224 (2448) MHz memory frequencies. The card passed all of our tests successfully at these clock rates.