diff --git a/src/lpc176x/adc.c b/src/lpc176x/adc.c
index 1f5fbbde..26965fb2 100644
--- a/src/lpc176x/adc.c
+++ b/src/lpc176x/adc.c
@@ -51,7 +51,7 @@ gpio_adc_setup(uint8_t pin)
     if (!is_enabled_pclock(PCLK_ADC)) {
         // Power up ADC
         enable_pclock(PCLK_ADC);
-        uint32_t prescal = DIV_ROUND_UP(CONFIG_CLOCK_FREQ*4, ADC_FREQ_MAX) - 1;
+        uint32_t prescal = DIV_ROUND_UP(CONFIG_CLOCK_FREQ, ADC_FREQ_MAX) - 1;
         LPC_ADC->ADCR = adc_status.adcr = (1<<21) | ((prescal & 0xff) << 8);
         LPC_ADC->ADINTEN = 0xff;
         adc_status.chan = ADC_DONE;
@@ -104,7 +104,7 @@ gpio_adc_sample(struct gpio_adc g)
     LPC_ADC->ADCR = adc_status.adcr | (1 << g.chan) | (1<<16);
 
 need_delay:
-    return ((64 * DIV_ROUND_UP(CONFIG_CLOCK_FREQ*4, ADC_FREQ_MAX)
+    return ((64 * DIV_ROUND_UP(CONFIG_CLOCK_FREQ, ADC_FREQ_MAX)
              * ARRAY_SIZE(adc_status.samples)) / 4 + timer_from_us(10));
 }