short intro

README.md

@@ -1,5 +1,9 @@
 # WCH RISC-V UF2 Bootloader
 
+This code implements a [uf2](https://github.com/microsoft/uf2) bootloader for RISC-V microcontrollers from [WCH](https://www.wch-ic.com). This allows user-friendly uploading of code by dragging files onto a virtual USB Mass Storage device (i.e. a pretend thumb drive). This means that, unlike the vendor ISP bootloader, no special software is required.
+
+## Usage notes
+
 - Triggered by a double reset within 0.5 seconds (like Adafruit bootloaders)
 - Needs usage of `R16_BKP_DATAR10`
     - Write 0x4170 ('Ap') to boot into the application immediately without delay
@@ -12,6 +16,7 @@
     - Note that USBD requires a USB A-A cable if using the official devkit
 - Runs off of internal 8 MHz oscillator (despite stability concerns)
     - The HSI oscillator's tolerance (-1.0% to +1.6%) is well outside of the range specified for USB full speed ($\pm$ 0.25%), but it seems to work in practice n=1 ¯\\\_(ツ)\_/¯
+    - The vendor's ISP ROM bootloader also does this
 - Should work across most of the CH32V2xx and CH32V3xx family with appropriate changes to hardcoded constants.
 - Allows both download to flash and to SRAM with the "not main flash" flag (this is how the RP2040 bootrom works)
     - Flash download address must be 08xxxxxx (i.e. not starting at 0)