Addressing issue #501 with start_pwm

Use register.modify instead of register.write method to start timer
(i.e. do same like calling timer.enable_counter()).
This avoids touching bits like ARPE (or DIR,..) which may have been set before
and should stay intact.

Author: TorstenStrunk

CHANGELOG.md

@@ -20,6 +20,9 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 ### Removed
  - `i2s-audio-out-dma.rs` example, too difficult to fix.
 
+### Fixed
+ - use register.modify instead of register.write to start PWM [#501]
+
 ### Added
 
 - Implementation of From trait for Pin-to-PartiallyErasedPin [#507]
@@ -31,6 +34,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 [#489]: https://github.com/stm32-rs/stm32f4xx-hal/pull/489
 [#490]: https://github.com/stm32-rs/stm32f4xx-hal/pull/490
 [#497]: https://github.com/stm32-rs/stm32f4xx-hal/pull/497
+[#501]: https://github.com/stm32-rs/stm32f4xx-hal/issues/501
 [#507]: https://github.com/stm32-rs/stm32f4xx-hal/pull/507
 [#508]: https://github.com/stm32-rs/stm32f4xx-hal/pull/508
 [#510]: https://github.com/stm32-rs/stm32f4xx-hal/pull/510

src/timer.rs

@@ -365,11 +365,11 @@ macro_rules! hal {
                 }
                 #[inline(always)]
                 fn start_one_pulse(&mut self) {
-                    self.cr1.write(|w| unsafe { w.bits(1 << 3) }.cen().set_bit());
+                    self.cr1.modify(|_, w| unsafe { w.bits(1 << 3) }.cen().set_bit());
                 }
                 #[inline(always)]
                 fn start_no_update(&mut self) {
-                    self.cr1.write(|w| w.cen().set_bit().udis().set_bit());
+                    self.cr1.modify(|_, w| w.cen().set_bit().udis().set_bit());
                 }
                 #[inline(always)]
                 fn cr1_reset(&mut self) {
@@ -445,7 +445,7 @@ macro_rules! with_pwm {
 
             #[inline(always)]
             fn start_pwm(&mut self) {
-                self.cr1.write(|w| w.cen().set_bit());
+                self.cr1.modify(|_, w| w.cen().set_bit());
             }
 
             #[inline(always)]
@@ -495,7 +495,7 @@ macro_rules! with_pwm {
 
             #[inline(always)]
             fn start_pwm(&mut self) {
-                self.cr1.write(|w| w.cen().set_bit());
+                self.cr1.modify(|_, w| w.cen().set_bit());
             }
 
             #[inline(always)]
@@ -553,7 +553,7 @@ macro_rules! with_pwm {
             #[inline(always)]
             fn start_pwm(&mut self) {
                 $(let $aoe = self.bdtr.modify(|_, w| w.aoe().set_bit());)?
-                self.cr1.write(|w| w.cen().set_bit());
+                self.cr1.modify(|_, w| w.cen().set_bit());
             }
 
             #[inline(always)]