“Safe overclocking of the Raspberry Pi 4 to 2.1 GHz.
Introduction.
If one subject is surrounded by myths, it is overclocking. It would be dangerous and destroy your Raspberry Pi. This article provides the essential background, after which you can safely overclock the Raspberry Pi to 2.1 GHz.
Theory.
The BCM2711 chip on the Raspberry Pi 4 consists of millions of transistors. They are all MOSFET (metal-oxide-semiconductor field-effect transistor).
When a positive voltage is applied to the gate, negatively charged electrons inside the body are drawn just under the gate insulator, thereby forming a conductive tunnel between the source and the drain. The state is ‘on’. Removing the voltage on the gate and the MOSFET switches off again. The insulator between the gate and the body ensures that almost no current is needed to control the transistor. That explains why, for instance, pocket calculators can work years on a single battery.
Overvoltage.
To switch on, a positive electrical charge is routed to the gate through the previous MOSFET A output. The same applies to switch off when this charge is to be drained by MOSFET B. This sourcing and the draining of electrical charge generates a small current peak. It is clear that the faster the clock runs, the more energy is transferred and then the more heat the chip will generate.
This switching takes time, the so-called rising and falling time. During this periode the output is neither ‘0’ or ‘1’, the state is undefined. In the graph shown above, it’s the yellow band.
When the rise plus fall time is larger than the clock intervals, the output never hits the stable 1 or 0 bands but remains swinging in the undefined region. That’s the moment your Raspberry Pi becomes unstable. It is no longer able to run programs flawless and chances are that it will crash.
Now overvoltage comes into play. When the power supply is increased, there is more energy available for charging and draining gates. Subsequently, the rise and fall times decrease. Although the increase in power also increases the undefined area proportionally, the overall result is nevertheless faster. The second graph above illustrates this behavior. Mostly, overclocking goes hand in hand with overvoltage.
Heat.
The insulator inside the MOSFET, an extremely thin layer of metal-oxide, becomes your limiting factor when it comes to overvoltage. In modern 28 nm chips, this layer is only a few dozen atoms thick.
Such a thin layer can not withstand large potential differentials. The tension inside the material will become too large. The layer will burst, a current will flood from the gate to the body and destroying your transistor forever along with your Rpi. Overvoltage up to 6 is safe, above that number, you have your adventure without any warranty.
Throttling.
The Raspberry Pi monitor the temperature continuously. Above 82 ° C (180 ° F), the clock frequency is automatically lowered. This action will reduce heat development. Once cooled down, the clock is restored to its original frequency, the so-called throttling. During throttling, an icon is shown on your display.
In a normal situation, it may be sufficient to rely on the throttling to prevent overheating of the chip. However, this is not wise in the case of overclocking.Due to the overvoltage, stress on the insulators inside the transistors is already above normal, heat will only enlarge it even more. The upper-temperature limit must, therefore, decrease depending on your overvoltage. That is the most important lesson when overclocking!”