Crystal cap calculation

README.md

@@ -90,12 +90,15 @@ The microcontroller can boot in different modes, similar to how you can enter th
 
 ### 6) Crystal
 
-The crystal will resonate at a specified frequency. Its waveform will be used as an input to the PLL (Phase Locked Loop) that will multiply the crystal’s frequency. This is the crystal that will be used: https://www.elfa.se/en/quartz-surface-mount-12mhz-iqd-lfxtal003210/p/17452154
+The crystal will resonate at a specified frequency. Its waveform will be used as an input to the PLL (Phase Locked Loop) that will multiply the crystal’s frequency. This is the crystal that will be used: https://www.elfa.se/sv/kristall-ytmontering-16mhz-iqd-lfxtal029370/p/30131284
+
+The crystal and its associated capacitors are part of a "Pierce oscillator" circuit connected to two special pins on the MCU - `PH0 - OSC_IN` and `PH1 - OSC_OUT`. To calculate value of the load capacitors required for proper operation, use the following formula:
+`C = 2*(Cload - Cstray)` where `Cload` is the load capacitance value specified in the crystal's [datasheet](https://www.elfa.se/Web/Downloads/_t/ds/LFXTAL029370REEL_eng_tds.pdf) (18pF) and `Cstray` is the stray capaciatance on the PCB, usually in the range of 2-10pF (pick something in-between, like 6pF). This gives us a capacitance value of `2*(18 - 6) = 24pF`. The closest standard value would be `22pF`.
 
 - Click on the Place component button.
 - Find the device/Crystal.
-- Place it close to `PH0` and `PH1`, pin 5 and 6 (`RCC_OS_CIN` and `RCC_OSC_OUT`).
-- Connect the component with wires to the mentioned pins. Value 12MHz.
+- Place it close to `PH0 - OSC_IN` and `PH1 - OSC_OUT`, pin 5 and 6.
+- Connect the component with wires to the mentioned pins. Value 16MHz.
 - Connect one 22pF capacitor to each pin on the crystal and connect the other end of the capacitors to GND.
 
 ### 7) Serial Wire Debug (SWD) connector
@@ -128,7 +131,7 @@ Download the general [datasheet](https://www.pixart.com/products-detail/4/PMW338
 For some components (like the PMW3389DM-T3QU) there might not exist a pre-made library, so you will now learn how to draw your own component.
 
 - Click on Tools/ Symbol Library Editor.
-- Create a new library for your project and click on Create New symbol inthe new library).
+- Create a new library for your project and click on Create New symbol in the new library).
 - Component name ”PMW3389”. OK.
 - Add pins and name them according to the datasheet. Remember to assign the correct pin number as well.
 - Make sure to set the ”Electrical type” on each pin according to usage (signal in/out, power in/out etc.) `X` marks a non connected pin. In our setting we don't use `GPO` so you can set it `X`.
@@ -139,7 +142,7 @@ Hint. Never, never ever, change the grid settings when placing the pins in the s
 
 ### 11) 1.9v Power Supply
 
-As you can see in the PWM3389DM-T3QU datasheet and reference schematics, the sensor requires a 1.9v power supply. This is much less common voltage for a fixed output LDO (try finding one from Elfa or rs-online and you will likely fail).
+As you can see in the PWM3389DM-T3QU datasheet and reference schematics, the sensor requires a 1.9v power supply. This is a much less common voltage for a fixed output LDO (try finding one from Elfa or rs-online and you will likely fail).
 
 To that end, you can use an adjustable voltage regulator, such as [MIC5235YM5-TR](https://www.elfa.se/en/ldo-voltage-regulator-24-20v-sot-23-microchip-mic5235ym5-tr/p/30099685), which will let you set your desired output voltage using external resistors.
 
@@ -196,11 +199,11 @@ It is very important that the pin numbering on your component is correct to the
 
 ### 17) Electric Rule Check (ERC)
 
-ERC is a check that detects if you have any ”lose ends”, meaning that it will give you a warning if you have a net with only one connection. It will also give you a warning if you have drawn a net to a component but the net and the component haven’t connected. To get the ERC to work as intended, you have to mark all the unused pins with a Not-connected flag, found to the right.
+ERC is a check that detects if you have any ”loose ends”, meaning that it will give you a warning if you have a net with only one connection. It will also give you a warning if you have drawn a net to a component but the net and the component haven’t connected. To get the ERC to work as intended, you have to mark all the unused pins with a Not-connected flag, found to the right.
 
 - 1. ”Perform electrical rules check” button (Ladybug button).
 - 2. Do a ERC.
-- 3. Fix errors.4.
+- 3. Fix errors.
 - 4. Repeat point 1 until errors == 0 && warnings == 3. The GND net is not connected to a Power output, the 5 voltage neither and the capacitor connected to VCAP1 is not connected to a power output.
 
 ## First Submission