stm32_part1 Finished part 1 timer write-up

content/post/stm32-part1.md

@@ -1,8 +1,7 @@
 ---
 title: "STM32F0 with libopencm3 - Part 1: Simple Timer"
 date: 2020-02-12
-lastmod: 2020-02-12
+lastmod: 2020-02-17
-draft: true
 tags: ["libopencm3", "stm32", "tutorial"]
 categories: ["Tutorial"]
 contentCopyright: false
@@ -15,6 +14,8 @@ LED. However, it is my belief that this leads to confusion for beginners and
 only opens the door to misunderstandings. That being said, we will be using
 timers and their associated GPIO ports with Alternate Function modes.
 
+{{< img src="/static/img/stm32-examples/part1/blinky.gif" >}}
+
 <!--more-->
 
 # Straight to the Chase
@@ -45,7 +46,7 @@ int main(void) {
     gpio_set_output_options(LED_PORT, GPIO_OTYPE_PP, GPIO_OSPEED_HIGH, LED_PIN_BLU | LED_PIN_GRN);
     gpio_set_af(LED_PORT, GPIO_AF0, LED_PIN_BLU | LED_PIN_GRN);
 
-    timer_set_mode(TIM3, TIM_CR1_CKD_CK_INT, TIM_CR1_CMS_CENTER_1, TIM_CR1_DIR_UP);
+    timer_set_mode(TIM3, TIM_CR1_CKD_CK_INT, TIM_CR1_CMS_EDGE, TIM_CR1_DIR_UP);
 
     // The math for seconds isn't quite right here
     timer_set_prescaler(TIM3, (rcc_apb1_frequency/TIM_PSC_DIV)/2*SECONDS);
@@ -60,11 +61,9 @@ int main(void) {
 
     for (i = 0; i < (sizeof(tim_oc_ids)/sizeof(tim_oc_ids[0])); ++i) {
         timer_set_oc_value(TIM3, tim_oc_ids[i], (TIM_PSC_DIV/2));
+        timer_enable_oc_output(TIM3, tim_oc_ids[i]);
     }
 
-    timer_enable_oc_output(TIM3, TIM_OC3);
-    timer_enable_oc_output(TIM3, TIM_OC4);
-
     timer_enable_counter(TIM3);
 
     while (1) {
@@ -76,11 +75,17 @@ int main(void) {
 ```
 {{< /admonition >}}
 
+
 # Set up the GPIO
 Assuming the reader is either familiar with GPIO setup for the STM32F0, or has
 reviewed [**Part 0**](/post/stm32-part0) of this series we will set up the GPIO
 pins tied to the LEDs (_port C, pins 8 and 9_) in the Alternate Function mode.
 
+Knowing that we'll be using `GPIOC`, we should enable this peripheral:
+```C
+rcc_periph_clock_enable(RCC_GPIOC);
+```
+
 ## Alternate Functions
 The STM32 microcontroller's GPIO has a hardware feature allowing you to tie
 certain port's pins to a different register as part of the output or input
@@ -113,3 +118,181 @@ gpio_mode_setup(GPIOC, GPIO_MODE_AF, GPIO_PUPD_NONE, GPIO8 | GPIO9);
 gpio_set_output_options(GPIOC, GPIO_OTYPE_PP, GPIO_OSPEED_HIGH, GPIO8 | GPIO9);
 gpio_set_af(GPIOC, GPIO_AF0, GPIO8 | GPIO9);
 ```
+
+# Set up the General Purpose Timer
+From the previous section we chose the two on-board LEDs on the STM32F0
+Discovery board tied to `PC8` and `PC9`. From the Alternate Function GPIO
+mapping, we know these will be Timer 3 (_channels 3, and 4_).
+
+Knowing that we'll be using `TIM3`, we should enable this peripheral:
+```C
+rcc_periph_clock_enable(RCC_TIM3);
+```
+
+## Timer Mode
+The first step in setting up the timer, similar to GPIO, is setting the timer
+mode. The encompass the divider amount (_dividing the peripheral clock_),
+alignment for capture/compare, and up or down counting:
+
+| Divider Mode            | Description                                    |
+|-------------------------|------------------------------------------------|
+| `TIM_CR1_CKD_INT`       | No division (_use peripheral clock frequency_) |
+| `TIM_CR1_CKD_INT_MUL_2` | Twice the the timer clock frequency            |
+| `TIM_CR1_CKD_INT_MUL_4` | Four times the timer clock frequency           |
+
+| Alignment Mode         | Description                                                                                        |
+|------------------------|----------------------------------------------------------------------------------------------------|
+| `TIM_CR1_CMS_EDGE`     | Edge alignment, counter counts up or down depending on direction                                   |
+| `TIM_CR1_CMS_CENTER_1` | Center mode 1: counter counts up and down alternatively (_interrupts on counting down_)            |
+| `TIM_CR1_CMS_CENTER_2` | Center mode 2: counter counts up and down alternatively (_interrupts on counting up_)              |
+| `TIM_CR1_CMS_CENTER_3` | Center mode 3: counter counts up and down alternatively (_interrupts on both counting up or down_) |
+
+| Direction          | Description   |
+|--------------------|---------------|
+| `TIM_CR1_DIR_UP`   | Up-counting   |
+| `TIM_CR1_DIR_DOWN` | Down-counting |
+
+For our purpose, it's easier to have no division (_multiplication_), edge
+alignment, using up counting direction (_can be down-counting, too_):
+
+```C
+timer_set_mode(TIM3, TIM_CR1_CKD_CK_INT, TIM_CR1_CMS_EDGE, TIM_CR1_DIR_UP);
+```
+
+## Timer Prescaler
+In addition to the timer clock, set by the peripheral clock (internal), each
+timer has a perscaler value. This determines the counter clock frequency and is
+equal to `Frequency/(Prescaler + 1)`. This is the value the timer will count to prior
+resetting (default behavior). We can get the exact value of this frequency,
+provided we didn't change the clock divisions via `rcc_apb1_frequency` (_unsigned
+integer value_).
+
+For the sake of simplicity in dividing the clock into easy decimal values, we
+will utilize setting up the High Speed Internal clock to 48MHz and dividing by
+48,000:
+```C
+rcc_clock_setup_in_hsi_out_48mhz(); // Place at the beginning of your int 'main(void)'
+...
+
+// SECONDS: integer value of period (seconds) of LED blink
+timer_set_prescaler(TIM3, (rcc_apb1_frequency/48000)/2*SECONDS));
+```
+
+## Timer Period
+Having set the prescaler to determine the maximum count of the timer, there is
+an additional period we need to set. For our purposes, this will simply be the
+same value of the prescaler:
+```C
+timer_set_period(TIM3, 48000);
+```
+
+## Timer Additional Configuration
+There are two minor settings we want to configure for the timer:
+<div style="display: none;">[^1]</div>
+
+1. Disable preloading the ARR<sup id="fnref:1" class="footnote-ref"><a href="#fn:1">1</a></sup> (auto-reload register) when the timer is reset
+1. Run the timer in continuous mode (never stop counting, clear the status
+   register automatically)
+
+```C
+timer_disable_preload(TIM3);
+timer_continuous_mode(TIM3);
+```
+
+## Timer Channel Output Compare Mode
+Since we are utilizing Timer 3's channel 3 (`GPIOC8`), and channel 4 (`GPIOC9`)
+we need to determine the output compare mode we want to use for each channel. By
+default the mode for each channel is frozen (unaffected by the comparison of the
+timer count and output compare value).
+
+| Output Compare Mode  | Description                                                                        |
+|----------------------|------------------------------------------------------------------------------------|
+| `TIM_OCM_FROZEN`     | (default) Frozen -- output unaffected by timer count vs. output compare value      |
+| `TIM_OCM_ACTIVE`     | Output active (high) when count equals output compare value                        |
+| `TIM_OCM_TOGGLE`     | Similar to active, toggles the output state when count equals output compare value |
+| `TIM_OCM_FORCE_LOW`  | Forces the output to low regardless of counter value                               |
+| `TIM_OCM_FORCE_HIGH` | Forces the output to high regardless of counter value                              |
+| `TIM_OCM_PWM1`       | Output is active (high) when counter is **less than** output compare value         |
+| `TIM_OCM_PWM2`       | Output is active (high) when counter is **greater than** output compare value      |
+
+Essentially, what we will be doing is using PWM (pulse-width modulation) at a
+very slow speed to create an alternating "blinky" effect on the LEDs. Using the
+alternating PWM output-compare modes will yield this effect:
+
+```C
+timer_set_oc_mode(TIM3, TIM_OC3, TIM_OCM_PWM1);
+timer_set_oc_mode(TIM3, TIM_OC4, TIM_OCM_PWM2);
+```
+
+In layman's terms: _only one LED will be on at a time, alternating._
+
+## Timer Channel Output Compare Value
+Lastly, we need to set the values that the output compare looks to for it's
+comparison. For this example, we want a 50%-on/50%-off time for ease of timing
+the duration of LEDs on-time determined by the frequency and period of the
+timer:
+
+```C
+// (48,000 / 2) = 24,000
+timer_set_oc_value(TIM3, TIM_OC3, 24000);
+timer_set_oc_value(TIM3, TIM_OC4, 24000);
+```
+
+### Exercise for the Reader
+A fun exercise in C to reduce repetition would be by creating an array of timer
+output compare address values and looping through them to set them to the same
+value.
+
+Garbage collection _may_ be discussed in a future post in this series, however
+this is not intended to be a "How-To C" series and should instead focus on the
+microcontroller. That being said, there is still some fun to have.
+
+The following snippet will be provided as a note and exercise for the reader in
+exploring memory allocation and garbage collection:
+```C
+int tim_oc_ids[2] = { TIM_OC3, TIM_OC4 };
+
+for (i = 0; i < (sizeof(tim_oc_ids)/sizeof(tim_oc_ids[0])); ++i) {
+    timer_set_oc_value(TIM3, tim_oc_ids[i], 24000);
+}
+```
+<center><sub>_Determining the 'length' of an array in C is different than in
+other languages.[^2]_</sub></center>
+
+## Enable the Timer
+Lastly, to kick everything off we need to enable both the timer and the relevant
+output-compare outputs.
+
+```C
+// Note: these cannot be OR'd together
+timer_enable_oc_output(TIM3, TIM_OC3);
+timer_enable_oc_output(TIM3, TIM_OC4);
+
+timer_enable_counter(TIM3);
+```
+
+### Another Exercise for the Reader
+The same for loop for `timer_set_oc_value()` can be appended to
+for `timer_enable_oc_output()` as discussed previously:
+
+```C
+int tim_oc_ids[2] = { TIM_OC3, TIM_OC4 };
+
+for (i = 0; i < (sizeof(tim_oc_ids)/sizeof(tim_oc_ids[0])); ++i) {
+    timer_set_oc_value(TIM3, tim_oc_ids[i], 24000);
+    timer_enable_oc_output(TIM3, tim_oc_ids[i]);
+}
+```
+
+# Fin
+Lastly, as always, we should not forget to place the microcontroller in an
+infinite loop:
+```C
+while (1);
+```
+
+The reasons for why this is done was discussed in [**Part
+0: Turn it on!**](/post/stm32-part0/#turn-it-on)
+
+[^1]: The Auto-Reload Register is the value automatically loaded into the timer when it finishes counting
+[^2]: [Determining the size of an array in C](https://stackoverflow.com/a/37539)