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I would like to know what frequency the LPC2138 works at after reset. The Xtal is 12 Mhz.
After start it runs with XTAL frequency (12MHz). You may simply use the Simulator and open the dialog Peripheral - System Control Block - Phase Locked Loop (PLL). Then you can see how the CPU frequency changes when you setup the PLL. Reinhard
With Startup.s file not open in uVision3, click on Starup.s (in project workspace). This will bring up a window showing, Stack Configuration, PLL Setup, etc. Expand PLL Setup. The default shows MSEL at 5 and PSEL at 2. Doing the math with a 12MHz crystal, will give you a frequency of 60 Mhz. The default Pclk divide is 4, (reset value of Pclk divide register) yielding 15 Mhz Pclk. Changing these values and recompling should change Startup.s assembly code.