stm32: Fix typo in adc startup delay

Fix (harmless) typo and add comments on adc timing.

Signed-off-by: Kevin O'Connor <[email protected]>

src/stm32/adc.c

@@ -34,6 +34,11 @@ static const uint8_t adc_pins[] = {
 #define CR2_FLAGS ADC_CR2_ADON
 #endif
 
+// ADC timing:
+// stm32f103: ADC clock=9Mhz, Tconv=12.5, Tsamp=28.5, total=4.556us
+// stm32f407: ADC clock=21Mhz, Tconv=12, Tsamp=56, total=3.238us
+// stm32f446: ADC clock=22.5Mhz, Tconv=12, Tsamp=56, total=3.022us
+
 struct gpio_adc
 gpio_adc_setup(uint32_t pin)
 {
@@ -72,7 +77,7 @@ gpio_adc_setup(uint32_t pin)
 
 #if CONFIG_MACH_STM32F1
         // Perform calibration
-        udelay(timer_from_us(1));
+        udelay(10);
         adc->CR2 = ADC_CR2_CAL | CR2_FLAGS;
         while (adc->CR2 & ADC_CR2_CAL)
             ;