TM4C1230D5PM: How do you set system clock to 80 MHz

Jeffrey Tuttle

Part Number: TM4C1230D5PM

When setting up the PLL and clocking, I expected to get 80 MHz. My test loop has 7 instructions in it and it produces 5.7MHz square wave

Assuming 1 clock per instruction (that's an assumption), the system clock is 40 MHz.

If each instruction takes 2 clocks, then everything is working correctly.

I use the following call from the SDK

SysCtlClockSet(SYSCTL_SYSDIV_2_5 | SYSCTL_USE_PLL | SYSCTL_XTAL_25MHZ |
SYSCTL_OSC_MAIN);

over 2 years ago

+1 Ralph Jacobi over 2 years ago

TI__Guru*** 128095 points

Hello Jeffrey,

One clock cycle per instruction is definitely an assumption, there are multiple common instructions that take up two cycles and it is possible to take up even more, so without seeing your code I can't really assess if one cycle per instruction is the correct assumption. Here is a summary of Arm M4 Cycle Count per instruction: https://developer.arm.com/documentation/100166/0001/Programmers-Model/Instruction-set-summary/Table-of-processor-instructions?lang=en

You can also see some more advanced details here: https://developer.arm.com/documentation/100166/0001/Programmers-Model/Instruction-set-summary/Load-store-timings?lang=en

Regarding your clock configuration, that should definitely have it running at 80MHz so I suspect it has to do with your assumption about the instructions.

Best Regards,

Ralph Jacobi

Arm-based microcontrollers

Arm-based microcontrollers forum

TM4C1230D5PM: How do you set system clock to 80 MHz