diff --git a/config/board.gen7-arm.h b/config/board.gen7-arm.h
index 7c1239b..fe9fc26 100644
--- a/config/board.gen7-arm.h
+++ b/config/board.gen7-arm.h
@@ -211,17 +211,26 @@ DEFINE_TEMP_SENSOR(extruder, TT_THERMISTOR, PIO1_0,THERMISTOR_EXTRUDER)
   device has the index 0 (zero).
 
   Set 'pwm' to ...
-    1  for using PWM on a PWM-able pin and on/off on other pins.
-    0  for using on/off on a PWM-able pin, too.
+    frequency  in Hertz (Hz) on ARM based controllers to set PWM frequency of
+               this pin's output. Frequency isn't always accurate, Teacup
+               will choose the closest possible one. FAST_PWM is ignored
+               on such controllers. Valid range is 2 to 200'000 Hz.
+    1          on AVR based controllers for using Pulse Width Modulation (PWM)
+               on a pin supporting it. PWM frequency can be influenced only
+               somewhat and only globally with FAST_PWM.
+    0          for using a PWM-able pin in on/off mode.
 
   Using PWM usually gives smoother temperature control but can conflict
-  with slow switches, like solid state relays. PWM frequency can be
-  influenced globally with FAST_PWM, see below.
+  with slow switches, like solid state relays. A too high frequency can
+  overheat MOSFETs; a too low frequency can make your heater to emit audible
+  noise; so choose wisely.
+
+  Pins which don't allow PWM are always operated in on/off mode.
 */
 //DEFINE_HEATERS_START
 //            name      port   pwm
-DEFINE_HEATER(extruder, PIO0_10, 1)
-//DEFINE_HEATER(bed,      PIO0_11, 1)
+DEFINE_HEATER(extruder, PIO0_10, 20000)
+//DEFINE_HEATER(bed,      PIO0_11, 10)
 
 #define HEATER_EXTRUDER HEATER_extruder
 //#define HEATER_BED HEATER_bed
diff --git a/heater-arm.c b/heater-arm.c
index ffc8942..5dcdd47 100644
--- a/heater-arm.c
+++ b/heater-arm.c
@@ -115,6 +115,7 @@ void heater_init() {
       PIO1_9   CT16B1_MAT0     0x1            ---
   */
   if (NUM_HEATERS) {                            // At least one channel in use.
+    uint32_t freq;
 
     // Auto-generate pin setup.
     #undef DEFINE_HEATER
@@ -132,10 +133,12 @@ void heater_init() {
       LPC_IOCON->pin ## _CMSIS = pin ## _PWM;     /* Connect to timer.   */ \
       /*pin ## _TIMER->IR  = 0; ( = reset value)     No interrupts.      */ \
       pin ## _TIMER->TCR   = (1 << 0);            /* Enable counter.     */ \
-      /* TODO: set PWM frequency here according to configuration. */        \
-      /* TODO: check wether configured freq. is within range (1..65536). */ \
-      pin ## _TIMER->PR    =                      /* Prescaler to        */ \
-          F_CPU / PWM_SCALE / 1000 - 1;           /*   1000 Hz.          */ \
+      freq = F_CPU / PWM_SCALE / pwm;             /* Figure PWM freq.    */ \
+      if (freq > 65535)                                                     \
+        freq = 65535;                                                       \
+      if (freq < 1)                                                         \
+        freq = 1;                                                           \
+      pin ## _TIMER->PR    = freq - 1;            /* Prescaler to freq.  */ \
       pin ## _TIMER->MCR   = (1 << 10);           /* Reset on Match 3.   */ \
       /* PWM_SCALE - 1, so match = 255 is full off. */                      \
       pin ## _TIMER->MR[3] = PWM_SCALE - 1;       /* Match 3 at 254.     */ \