# How to configure STM32 with two different clock speeds

I'm using STM32F103RE for implementing a time-consuming operation. This device is connected to the PC via a USB port. I'm not expert in configuring these MCUs, but it seems when it configures to communicate via USB, it can't use maximum possible processing speed which is 72 MHZ.

Is there a way to configure my STM32F103 with two different frequencies (one for USB communication & the other for internal proccessing at 72MHZ)?

This is part of the USB configuration code:

RCC->APB1ENR |= (1 << 23);                /* Enable clock for USB */

/* Enable USB interrupts */
NVIC->IPR [5] |= 0x00000010;              /* Set priority lower than SVC */

NVIC->ISER[0] |= (1 << (USB_LP_CAN_RX0_IRQChannel & 0x1F));

/* Control USB connecting via SW */
RCC->APB2ENR |= (1 << 5);                 /* Enable clock for GPIOD */
GPIOD->CRL &= ~0x00000F00;                /* Clear port PD2 */
GPIOD->CRL |=  0x00000700;                /* PD2 General purpose output open-drain, max speed 50 MHz */
GPIOD->BRR  = 0x0004;                     /* Reset PD2 (set to low) */


And this is my smt32_init clock configuration:

#define __CLOCK_SETUP              1
#define __RCC_CR_VAL               0x01010082
#define __RCC_CFGR_VAL             0x001D8402
#define __HSE                      8000000


I guess some change in RCC_CR or RCC_CFGR should solve the problem, but I think there are also some other configurations for flash to work with high frequency.

How can I do that?

• I think the question is a bit different. The question is why can't you run at 72MHz when connected to the USB. USB usually runs from a PLL that is clocked from an external crystal. There is likely a firmware issue where the USB configuration modifies the clock for all. Can you post your clock and USB initialization routine code? – Gustavo Litovsky Feb 22 '13 at 19:46
• Please show the actual code, not just the values. Also, what code do you try when you try to run it at 72MHz? – Gustavo Litovsky Feb 22 '13 at 20:44
• @PeterMotsenen - what really is to be gained by making trivial edits to a nearly four year old question? Do you really believe that balances the cost of bumping it to the top of the main page when it has no need of anyone's attention? – Chris Stratton Jan 4 '17 at 19:58

When it comes to programming the STM32, the RM0008 Reference Manual should be your constant companion.

Section 7.2.3 PLL:

If the USB interface is used in the application, the PLL must be programmed to output 48 or 72 MHz. This is needed to provide a 48 MHz USBCLK.

So it's not only possible, it's one of only two possible frequencies you can choose when utilizing the USB.

You should be able to set the system clock to 72 MHz using the defines and functions in the STM supplies libraries. But your RCC_CFGR looks okay. You've got HSE selected as your PLLSRC. And you're setting the PLLMUL to x9. So as long as your HSE is supplied by an 8 MHz oscillator (which it looks like it is), you'll get 72 MHz to the SYSCLK. And you have USBPRE set to 0 so that will divide the PLL by 1.5 to get your 48 MHz.

Looking at the clock tree on page 90, that looks like all that is required.

As for accessing flash, you need to make sure you have two wait states. That is done by writing 010 to the FLASH_ACR register. See page 60 for more detail on that register.