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+---
+title: Debugging steps for CDC USB on STM32 and Platformio
+author: peter
+date: 2024-03-02 21:58:54 +0800
+categories: [Electronics] # Blogging | Electronics | Programming | Mechanical
+tags: [electronics, STM32, platformio] # systems | embedded | rf | microwave | electronics | solidworks | automation
+# image: assets/img/2024-03-02-Debugging-steps-for-/preview.png
+---
+
+Progress to the next step if it still isn't working
+
+## Step 1: Have you put the required defines in your build flags?
+
+The build flags enable the USB CDC functionality. Make sure to also set the CPU frequency, on my board I've used a 16 MHz crystal.
+
+```ini
+[env:genericSTM32F103C8]
+platform = ststm32
+board = genericSTM32F103C8
+framework = arduino
+upload_protocol = stlink
+debug_tool = stlink
+board_build.f_cpu = 16000000L   ; This should match your crystal oscillator frequency (On my board I've used a 16 MHz crystal)
+monitor_dtr = 1
+
+build_flags =
+   -D PIO_FRAMEWORK_ARDUINO_ENABLE_CDC
+   -D USBCON
+```
+
+## Step 2: Does your MCU require a 1.5kR pullup resistor?
+
+Consult ST application note [AN4879](https://www.st.com/resource/en/application_note/dm00296349-usb-hardware-and-pcb-guidelines-using-stm32-mcus-stmicroelectronics.pdf), Table 3. Find your MCU series and check the "Embedded pull-up resistor on USB_DP line" column. In my case, unfortunately, I forgot to put the 1.5kR pullup resistor and I am using an STM32F103, which has a `-` in that column. The note (2) states:
+
+> To be compliant with the USB 2.0 full-speed electrical specification, the USB_DP (D+) pin must be pulled up to a voltage in the 3.0 to 3.6 V range with a 1.5 kΩ resistor.
+
+Try putting a resistor on `USB_DP`, then reset and re-connect the MCU to your host device. It should work with any low-value resistor, in my case I used a 10kR through hole resistor since I was not at my lab. This worked and I received characters as long as the resistor was held there.
+
+## Step 3: ???
+
+I've solved my issue with my custom board, but if you took more debugging steps feel free to leave your solutions in the comments below and I'll add them here.