sam: Add convenience function for clock setup using internal 4MHz.

include/libopencm3/sam/pmc.h

@@ -140,6 +140,7 @@ void pmc_plla_config(u8 mul, u8 div);
 void pmc_peripheral_clock_enable(u8 pid);
 void pmc_peripheral_clock_disable(u8 pid);
 void pmc_clock_setup_in_xtal_12mhz_out_84mhz(void);
+void pmc_clock_setup_in_rc_4mhz_out_84mhz(void);
 
 #endif
 

lib/sam/common/pmc.c

@@ -79,3 +79,17 @@ void pmc_clock_setup_in_xtal_12mhz_out_84mhz(void)
 	pmc_mck_frequency = 84000000;
 }
 
+void pmc_clock_setup_in_rc_4mhz_out_84mhz(void)
+{
+	eefc_set_latency(4);
+
+	/* Select as main oscillator */
+	CKGR_MOR = CKGR_MOR_KEY |
+		(CKGR_MOR & ~(CKGR_MOR_MOSCSEL | CKGR_MOR_MOSCRCF_MASK));
+	/* Multiply by 21 for 84MHz */
+	pmc_plla_config(21, 1);
+	pmc_mck_set_source(MCK_SRC_PLLA);
+
+	pmc_mck_frequency = 84000000;
+}
+