diff --git a/cpu/sam3/periph/timer.c b/cpu/sam3/periph/timer.c
index d4485f171434..721b6f879f44 100644
--- a/cpu/sam3/periph/timer.c
+++ b/cpu/sam3/periph/timer.c
@@ -111,7 +111,12 @@ int timer_init(tim_t tim, uint32_t freq, timer_cb_t cb, void *arg)
      * channel 1 toggles this line on each timer tick, the actual frequency
      * driving channel 0 is f_ch2 / 2 --> f_ch0/1 = (MCK / 2) / 2 / freq.
      */
-    dev(tim)->TC_CHANNEL[1].TC_RC = (CLOCK_CORECLOCK / 4) / freq;
+    uint32_t tc_rc = (CLOCK_CORECLOCK / 4) / freq;
+    /* the API expects apps to know in advance which frequencies are possible
+     * and only configure with supported frequencies. So aid debugging with
+     * an assert */
+    assert(tc_rc * freq == CLOCK_CORECLOCK / 4);
+    dev(tim)->TC_CHANNEL[1].TC_RC = tc_rc;
 
     /* start channel 1 */
     dev(tim)->TC_CHANNEL[1].TC_CCR = (TC_CCR_CLKEN | TC_CCR_SWTRG);
diff --git a/drivers/include/pcf857x.h b/drivers/include/pcf857x.h
index 4b270be1e649..c2ce3ba888b2 100644
--- a/drivers/include/pcf857x.h
+++ b/drivers/include/pcf857x.h
@@ -260,10 +260,6 @@ extern "C"
 #include "event.h"
 #endif /* MODULE_PCF857X_IRQ */
 
-#if !IS_USED(MODULE_PCF8574) && !IS_USED(MODULE_PCF8574A) && !IS_USED(MODULE_PCF8575)
-#error "Please provide a list of pcf857x variants used by the application (pcf8574, pcf8574a or pcf8575)"
-#endif
-
 /**
  * @name    PCF857X I2C slave addresses
  *
diff --git a/drivers/pcf857x/pcf857x.c b/drivers/pcf857x/pcf857x.c
index d74c4a8a18e4..a37d21b394b0 100644
--- a/drivers/pcf857x/pcf857x.c
+++ b/drivers/pcf857x/pcf857x.c
@@ -42,6 +42,10 @@
 
 #endif /* ENABLE_DEBUG */
 
+#if !IS_USED(MODULE_PCF8574) && !IS_USED(MODULE_PCF8574A) && !IS_USED(MODULE_PCF8575)
+#error "Please provide a list of pcf857x variants used by the application (pcf8574, pcf8574a or pcf8575)"
+#endif
+
 #if IS_USED(MODULE_PCF857X_IRQ_LOW)
 #define PCF857X_EVENT_PRIO EVENT_PRIO_LOWEST
 #elif IS_USED(MODULE_PCF857X_IRQ_MEDIUM)