Merge pull request #6989 from photonthunder/samd21_waitstates

samd21/cpu:waitstates for low voltage

cpu/samd21/cpu.c

@@ -22,6 +22,24 @@
 #include "periph_conf.h"
 #include "periph/init.h"
 
+#ifndef VDD
+/**
+ * @brief   Set system voltage level in mV (determines flash wait states)
+ *
+ * @note    Override this value in your boards periph_conf.h file
+ *          if a different system voltage is used.
+ */
+#define VDD                 (3300U)
+#endif
+
+/* determine the needed flash wait states based on the system voltage (Vdd)
+ * see SAMD21 datasheet Rev A (2017) table 37-40 , page 816 */
+#if (VDD > 2700)
+#define WAITSTATES          ((CLOCK_CORECLOCK - 1) / 24000000)
+#else
+#define WAITSTATES          ((CLOCK_CORECLOCK - 1) / 14000000)
+#endif
+
 /**
  * @brief   Configure clock sources and the cpu frequency
  */
@@ -31,12 +49,10 @@ static void clk_init(void)
     PM->APBAMASK.reg = (PM_APBAMASK_PM | PM_APBAMASK_SYSCTRL |
                         PM_APBAMASK_GCLK);
 
-    /* adjust NVM wait states, see table 42.30 (p. 1070) in the datasheet */
-#if (CLOCK_CORECLOCK > 24000000)
+    /* adjust NVM wait states */
     PM->APBBMASK.reg |= PM_APBBMASK_NVMCTRL;
-    NVMCTRL->CTRLB.reg |= NVMCTRL_CTRLB_RWS(1);
+    NVMCTRL->CTRLB.reg |= NVMCTRL_CTRLB_RWS(WAITSTATES);
     PM->APBBMASK.reg &= ~PM_APBBMASK_NVMCTRL;
-#endif
 
     /* configure internal 8MHz oscillator to run without prescaler */
     SYSCTRL->OSC8M.bit.PRESC = 0;