cpu.c
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/*
* Copyright (C) 2014-2016 Freie Universität Berlin
*
* This file is subject to the terms and conditions of the GNU Lesser
* General Public License v2.1. See the file LICENSE in the top level
* directory for more details.
*/
/**
* @ingroup cpu_sam3
* @{
*
* @file
* @brief Implementation of the CPU initialization
*
* @author Hauke Petersen <hauke.petersen@fu-berlin.de>
* @}
*/
#include "cpu.h"
#include "periph_conf.h"
/**
* @brief Keys needed for editing certain PMC registers
* @{
*/
#define WPKEY (0x504D43)
#define MORKEY (0x37)
/** @} */
/**
* @brief Start-up time for external crystal (will be multiplied by 8)
*/
#define XTAL_STARTUP (8U)
/**
* @brief PLL is incremented to this value until considered stable
*/
#define PLL_CNT (64U)
/**
* @brief Initialize the CPU, set IRQ priorities
*/
void cpu_init(void)
{
/* disable the watchdog timer */
WDT->WDT_MR |= WDT_MR_WDDIS;
/* initialize the Cortex-M core */
cortexm_init();
/* setup the flash wait states */
EFC0->EEFC_FMR = EEFC_FMR_FWS(CLOCK_FWS);
EFC1->EEFC_FMR = EEFC_FMR_FWS(CLOCK_FWS);
/* unlock write protect register for PMC module */
PMC->PMC_WPMR = PMC_WPMR_WPKEY(WPKEY);
/* activate the external crystal */
PMC->CKGR_MOR = (CKGR_MOR_KEY(MORKEY) |
CKGR_MOR_MOSCXTST(XTAL_STARTUP) |
CKGR_MOR_MOSCXTEN |
CKGR_MOR_MOSCRCEN);
/* wait for crystal to be stable */
while (!(PMC->PMC_SR & PMC_SR_MOSCXTS));
/* select crystal to clock the main clock */
PMC->CKGR_MOR = (CKGR_MOR_KEY(MORKEY) |
CKGR_MOR_MOSCXTST(XTAL_STARTUP) |
CKGR_MOR_MOSCXTEN |
CKGR_MOR_MOSCRCEN |
CKGR_MOR_MOSCSEL);
/* wait for main oscillator selection to be complete */
while (!(PMC->PMC_SR & PMC_SR_MOSCSELS));
/* setup PLLA */
PMC->CKGR_PLLAR = (CKGR_PLLAR_ONE |
CKGR_PLLAR_PLLACOUNT(PLL_CNT) |
CKGR_PLLAR_MULA(CLOCK_PLL_MUL) |
CKGR_PLLAR_DIVA(CLOCK_PLL_DIV));
/* wait for PLL to lock */
while (!(PMC->PMC_SR & PMC_SR_LOCKA));
/* before switching to PLLA, we need to switch to main clock */
PMC->PMC_MCKR = PMC_MCKR_CSS_MAIN_CLK;
while (!(PMC->PMC_SR & PMC_SR_MCKRDY));
/* use PLLA as main clock source */
PMC->PMC_MCKR = PMC_MCKR_CSS_PLLA_CLK;
/* wait for master clock to be ready */
while (!(PMC->PMC_SR & PMC_SR_MCKRDY));
}