CPU Frequency

You use the IO-registers as usual to set the CPU. The setting bit below was more or less taken from sdk2x.

void hw_set_cpu(int speed)
{
	int m = 0;
	// Calculating the FPLL value from frequency is a bit complex
	// so we just use some known-to-work values instead.
	switch(speed)
	{
	case 250: m = 0x5D04; break;
	case 225: m =  0x5304; break;
	case 200: m = 0x4904; break;
	case 175: m = 0x3F04; break;
	case 150: m = 0x4901; break;
	case 125: m = 0x3c01; break;
	case 100: m = 0x6502; break;
	case 75: m = 0x4902; break;
	case 50: m = 0x6503; break;
	}

	if(m)
	{
		// Get interupt flags
		unsigned int l = memregs32[0x808>>2];

		// Turn off interrupts
		memregs32[0x808>>2] = 0xFF8FFFE7;

		// Set new clock frequency
		memregs16[0x910>>1]=m;

		// Wait for it to take
		while(memregs16[0x0902>>1] & 1);

		// Turn on interrupts again
		memregs32[0x808>>2] = l;
	}
}

This routine seems to work for every clockspeed.

#define SYS_CLK_FREQ 7372800

void SetClock(unsigned int MHZ)
{
#ifdef GP2X
  unsigned int v;
  unsigned int mdiv,pdiv=3,scale=0;
  MHZ*=1000000;
  mdiv=(MHZ*pdiv)/SYS_CLK_FREQ;

  mdiv=((mdiv-8)<<8) & 0xff00;
  pdiv=((pdiv-2)<<2) & 0xfc;
  scale&=3;
  v = mdiv | pdiv | scale;
  
  unsigned int l = memregs32[0x808>>2];// Get interupt flags
  memregs32[0x808>>2] = 0xFF8FFFE7;   //Turn off interrupts
  memregs16[0x910>>1]=v;              //Set frequentie
  while(memregs16[0x0902>>1] & 1);    //Wait for the frequentie to be ajused
  memregs32[0x808>>2] = l;            //Turn on interrupts
#endif
}

To appropriately set memregs16 and memregs32, see GPIO Programming Example.