In SX Microcontrollers, SX/B Compiler and SX-Key Tool, g_daubach wrote:

Hi electronic and the group,

to my knowledge, Ubicom never produced special SX versions for 100 MHz. As Bean mentioned, I also think they "cherry-picked" those that passed the 100 MHz test.

Devices like the SX get hot because each logical level change causes a logical cell on the chip to be charged or discharged, i.e. a current flows into, or out from such cells. Although the interconnections between the cells are very short, they have a certain resistance. So, there is a voltage drop across these interconnections. This voltage, multiplied by the current means dissipated power, or heat. Thus, the faster you clock an SX more heat will occur on the chip.

I agree with Peter (pjv) that the generated heat does vary with the executed code. For example, when the SX runs a simple, end-less loop (JMP $), only the state of the PC is changed, besides other states in the control unit, and on the program memory address and data bus. On the other hand, when it reads or writes a file register, additional state changes of the W register, or a file register will occur, as well as state changes on the register address and data bus. The same is true for all other controllers, or processors. When you have a PC with a temperature-controlled fan, you my notice that the fan sometimes turns on more often (or runs faster), depending on the program you are running.

A while ago, I had posted some graphs in the forum showing the SX supply current vs. various clock frequencies. These graphs show that the supply current is remarkably lower when you reduce the supply voltage from 5 to 3.3 Volts, or even lower. As lower supply current automatically means less power dissipation, or less heat, you should consider running the SX at 3.3 Volts or lower when driving it to the clock limit.

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