Fix issue #5119, changed pwmout_api.

The period of pwm we could get was limited to 69.9 ms, because prescaler value was set once only during initialization. base->mod is a 16 bit register, to get longer period we have to slow down the clk.

Author: MateuszM

targets/TARGET_Freescale/TARGET_MCUXpresso_MCUS/TARGET_MCU_K64F/pwmout_api.c

@@ -124,8 +124,25 @@ void pwmout_period_us(pwmout_t* obj, int us)
     FTM_Type *base = ftm_addrs[obj->pwm_name >> TPM_SHIFT];
     float dc = pwmout_read(obj);
 
-    // Stop FTM clock to ensure instant update of MOD register
-    base->MOD = FTM_MOD_MOD((pwm_clock_mhz * (float)us) - 1);
+    uint32_t pwm_base_clock;
+    uint32_t clkdiv = 0;
+    pwm_base_clock = CLOCK_GetFreq(kCLOCK_BusClk);
+    pwm_clock_mhz = (float)pwm_base_clock / 1000000.0f;
+    uint32_t mod = (pwm_clock_mhz*(float)us) - 1;
+    while(mod > 0xFFFF){
+    	++clkdiv;
+    	pwm_clock_mhz /= 2.0f;
+    	mod = (pwm_clock_mhz*(float)us) - 1;
+    	if(clkdiv==7){
+    		break;
+    	}
+    }
+    uint32_t SC = base->SC & ~FTM_SC_PS_MASK;
+    SC |= FTM_SC_PS((ftm_clock_prescale_t)clkdiv);
+    base->SC = SC;
+
+    //Stop FTM clock to ensure instant update of MOD register
+    base->MOD = FTM_MOD_MOD(mod);
     pwmout_write(obj, dc);
 }