I recently modded my N64, first by converting it to NTSC, then by adding an Tim Worthington’s RGB kit. The motherboard was still out of its shell and the temptation was high to apply a last mod before remounting it : overclock ! A small and heap kit can be found on ebay or from https://www.delta-island.com/forum (french). The purpose is to be able to switch the original CPU frequency from 93.75Mhz to 125 Mhz. A video on youtube show how to install the mod.
Here is my experience about this mod…
Requirements
- A NUS-CPU-03 or NUS-CPU(P)01 N64 motherboard (NUS-CPU-01 can reach 125Mhz and NUS-CPU-05/08 seems not stable when overclocked)
- A small overlcock kit
- Some thin wires
- A cutter or very thin knife
- A thin soldering iron
The concept
The concept is pretty simple : the MIPS VR4300i reads its base frequency from pin 16 (masterclock) then, it applies a multiplier factor which can be 1, 1.5, 2 or 3. This multiplier is driven by pin 112 and 116 like the flollowing :
| pin 112 state | pin 116 state | multiplier |
|---|---|---|
| low | low | x1 |
| high | low | x1.5 |
| low | high | x2 |
| high | high | x3 |
So, changing the CPU frequency is just a matter of changing the pins’ states. The small board on the kit has a micro-controller that does just that : switching pins’ state to get x1.5 or x2 multiplier.
On the N64 motherboard, the masterclock is set at 62.5 Mhz and the default multiplier is x1.5, which gives 93.75 Mhz. by using a 2x multiplier, the CPU is then clocked at 125Mhz. The x3 multiplier has been reported to fail on the N64.
The mod
Here is where the CPU is on the motherboard (on the right, you can see the RGB mod)
The mod can be done quickly, but I have to admit that it is a bit tricky because the pin 112 and 116 of the CPU have to be lifted:
To lift these pins, I used my soldering iron to heat the end of the pin and used a cutter as a lever with the pin next to the one to lift. I started to lift the pin 112, then the 116
Before fixing the kit, I filled all the pads with some solder. Then I put the kit correctly aligned and I first soldered pin 112 to fix the kit. Then I soldered the remaining one : pin 116, gnd and vdd.
The kit comes with a 2 colors led to replace the orignal power led so we can have a different color depending on the CPU’s frequency. I chose to use blue for normal clock and red for overclocked mode. The new led has only its ground leg soldering onto the motherboard, as the two anode’s leg are driven by the micro-controller.
Finally, I used 3 thin wires : one for the reset signal to switch clock mode and one for each led’s anode. The reset signal is taken on the PIF chip (gray wire bellow)
Tests
The result can be frustrating. Unlike the overclock of the Megadrive which provides better framerate or make slowdowns disapear, this is not really the case with the N64. I can see at least two reasons for that:
- The N64’s CPU is rarely the bottleneck (except in 4 players mode). The problems are more often on RDAM and/or GPU
- Some games rely on the exact default clock speed, so when the CPU is overclocked they are too fast and/or have audio sync problems
