From 01b750604221027d318e7f38284e7668bad3adcb Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Henrik=20Alse=CC=81r?= <henrik.alser@me.com> Date: Mon, 18 Jan 2021 00:04:31 +0100 Subject: [PATCH] Cleanup --- README.md | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) diff --git a/README.md b/README.md index 5ef0e4a..4a48450 100644 --- a/README.md +++ b/README.md @@ -93,7 +93,9 @@ The microcontroller can boot in different modes, similar to how you can enter th 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. @@ -157,8 +159,7 @@ Now add a reset button. Suitable value for capacitor is found in the MCU data sh ### 13) Power LEDs and test points -In order to see that your system is correctly powered, it is recommended to add test points for easy probing in the form of exposed pads on the board. It is recommended to use a pin header footprint for GND test points so you can easily clip the scope's alligator clip to it. -Place at least one GND clip for each subcircuit block on your boards. You don't need to stuff them with pins by default during assembly, but it is nice to have the option to add these when troubleshooting. +In order to see that your system is correctly powered, it is recommended to add test points for easy probing, in the form of exposed pads on the board. It is recommended to use a pin header footprint for GND test points so you can easily clip the scope probe's GND alligator clip to it. Place at least one GND clip for each subcircuit block on your boards. You don't need to stuff them with pins by default during assembly, but it is nice to have the option to add those in when troubleshooting. It is also recommended you add LEDs to your power rails so you can see they're powered up correctly. You can use an LED such as [this red one](https://www.elfa.se/en/smd-led-645nm-0805-65cd-30ma-kingbright-kptd-2012surck/p/30118951), together with a suitable current limiting resistor. Aim for a forward current of about 2mA. You can use this [LED calculator](https://ohmslawcalculator.com/led-resistor-calculator) to calculate suitable resistor values for your LEDs. Use resistor values from the E12 series. -- GitLab