diff --git a/heater-avr.c b/heater-avr.c
index 309195c..1f7049c 100644
--- a/heater-avr.c
+++ b/heater-avr.c
@@ -26,14 +26,16 @@ typedef struct {
   uint16_t max_value;
 } heater_definition_t;
 
+#define PWM_TYPE(pwm, pin) (((pwm) >= HARDWARE_PWM) ? ((pin ## _PWM) ? (pin ## _PWM) : (U8_HEATER_PWM)SOFTWARE_PWM) : (U8_HEATER_PWM)(pwm))
+
 #undef DEFINE_HEATER_ACTUAL
 /// \brief helper macro to fill heater definition struct from config.h
 #define DEFINE_HEATER_ACTUAL(name, pin, invert, pwm, max_value) { \
   &(pin ## _WPORT), \
   pin ## _PIN, \
   invert ? 1 : 0, \
-  (pwm >= HARDWARE_PWM) ? ((pin ## _PWM) ? (pin ## _PWM) : (U8_HEATER_PWM)SOFTWARE_PWM) : (U8_HEATER_PWM)pwm, \
-  ((max_value * 64 + 12) / 25), \
+  PWM_TYPE(pwm, pin), \
+  (PWM_TYPE(pwm, pin) != (U8_HEATER_PWM)SOFTWARE_PWM) ? (((max_value) * 64 + 12) / 25) : (uint16_t)(255UL * 100 / (max_value)), \
   },
 static const heater_definition_t heaters[NUM_HEATERS] =
 {
@@ -43,7 +45,7 @@ static const heater_definition_t heaters[NUM_HEATERS] =
 
 // We test any heater if we need software-pwm
 #define DEFINE_HEATER_ACTUAL(name, pin, invert, pwm, ...) \
-  | (((pwm >= HARDWARE_PWM) ? ((pin ## _PWM) ? HARDWARE_PWM : SOFTWARE_PWM) : pwm) == SOFTWARE_PWM)
+  | (PWM_TYPE(pwm, pin) == (U8_HEATER_PWM)SOFTWARE_PWM)
 static const uint8_t software_pwm_needed = 0
   #include "config_wrapper.h"
 ;
diff --git a/heater-lpc.c b/heater-lpc.c
index 02472c1..c775919 100644
--- a/heater-lpc.c
+++ b/heater-lpc.c
@@ -73,14 +73,16 @@ typedef struct {
   uint8_t invert;
 } heater_definition_t;
 
+#define PWM_TYPE(pwm, pin) (((pwm) >= HARDWARE_PWM) ? ((pin ## _TIMER) ? HARDWARE_PWM : SOFTWARE_PWM) : (pwm))
+
 #undef DEFINE_HEATER_ACTUAL
 #define DEFINE_HEATER_ACTUAL(name, pin, invert, pwm, max_value) \
   { \
-    { pwm && pin ## _TIMER ? \
+    { (PWM_TYPE(pwm, pin) == HARDWARE_PWM ? \
       &(pin ## _TIMER->MR[pin ## _MATCH]) : \
       &(pin ## _PORT->MASKED_ACCESS[MASK(pin ## _PIN)]) }, \
-    ((max_value * 64 + 12) / 25), \
-    ((pwm >= HARDWARE_PWM) ? ((pin ## _TIMER) ? HARDWARE_PWM : SOFTWARE_PWM) : pwm), \
+    (PWM_TYPE(pwm, pin) != SOFTWARE_PWM) ? ((max_value * 64 + 12) / 25) : (uint16_t)(255UL * 100 / max_value), \
+    PWM_TYPE(pwm, pin), \
     invert ? 1 : 0 \
   },
 static const heater_definition_t heaters[NUM_HEATERS] = {
@@ -90,7 +92,7 @@ static const heater_definition_t heaters[NUM_HEATERS] = {
 
 // We test any heater if we need software-pwm
 #define DEFINE_HEATER_ACTUAL(name, pin, invert, pwm, ...) \
-  | (((pwm >= HARDWARE_PWM) ? ((pin ## _TIMER) ? HARDWARE_PWM : SOFTWARE_PWM) : pwm) == SOFTWARE_PWM)
+  | (PWM_TYPE(pwm, pin) == SOFTWARE_PWM)
 static const uint8_t software_pwm_needed = 0
   #include "config_wrapper.h"
 ;
@@ -143,7 +145,7 @@ void heater_init() {
   // Auto-generate pin setup.
   #undef DEFINE_HEATER_ACTUAL
   #define DEFINE_HEATER_ACTUAL(name, pin, invert, pwm, ...) \
-    if ((pwm >= HARDWARE_PWM) && pin ## _TIMER) {                                             \
+    if (PWM_TYPE(pwm, pin) == HARDWARE_PWM) {                               \
       uint32_t freq;                                                        \
                                                                             \
       if (pin ## _TIMER == LPC_TMR16B0) {                                   \
diff --git a/heater.c b/heater.c
index 02fd4b2..cd328e8 100644
--- a/heater.c
+++ b/heater.c
@@ -252,7 +252,6 @@ void heater_tick(heater_t h, temp_type_t type, uint16_t current_temp, uint16_t t
 
 /** \brief software PWM routine
 */
-#define PWM_MAX 255
 
 void heater_soft_pwm(heater_t index) {
   if (software_pwm_needed) {
@@ -266,15 +265,16 @@ void heater_soft_pwm(heater_t index) {
     }
 
     // here we are doing a small trick. normally this is
-    // sd_accu += pwm - (sd_dir * PWM_MAX)
-    // here we scale the (sd_dir * PWM_MAX) -part with the max_value.
+    // sd_accu += pwm - sd_dir, where sd_dir is 255 or 0.
+    // here we scale the (sd_dir) -part with the max_value.
+    // max_value is precalculated with 255 * 100 / max_value(%).
     // so we get a smooth PWM also for downscaled heaters. the "pwm"-value
     // is from 0 to 255 in any case, but the sd_accu becomes bigger with
     // smaller max_values.
     soft_pwm_runtime[index].sd_accu += pwm - soft_pwm_runtime[index].sd_dir;
 
     if (soft_pwm_runtime[index].sd_accu > 0) {
-      soft_pwm_runtime[index].sd_dir = PWM_MAX * 256 / heaters[index].max_value;
+      soft_pwm_runtime[index].sd_dir = heaters[index].max_value;
       do_heater(index, 255);
     }
     else {