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====== USB CDC driver for RP2040 ======

===== The idea =====

This is a compact and universal MSP430 USB CDC driver.
Every effort has been made to make it portable.

===== What we need to do in short =====

  * Initialising the USB hardware on the RP2040.
  * Receiving and responding to setup packets from the host (PC). This determines which driver the OS will load.
  * If the host is satisfied with the setup, then sending and receiving data packets may start.


===== Pseudo code =====

<code>
hex
Function DEVICE-DECRIPTOR \ 18 bytes
12 c, 01 c, 10 c, 01 c, ( 1.10 ) EF c, 02 c, 01 c, 40 c, 66 c, 66 c, ( 6666 )
10 c, 66 c, ( 6610 ) 00 c, 01 c, ( vsn 1.00 ) 00 c, 02 c, 00 c, 01 c,  align

Function CONFIGURATION-DESCRIPTOR \ 9 or 75 bytes
9 c,  2 c,  4B c,  0 c,  2 c,  1 c,  0 c,  80 c,  FA c, \ Maximum power = 500mA
8 c,  0B c,  0 c,  2 c,  2 c,  2 c,  1 c,  0 c,         \ Interface Association Descriptor - CDC 0
9 c,  4 c,  0 c,  0 c,  1 c,  2 c,  2 c,  1 c,  0 c,    \ Interface 1: Control - 1 - 0 -
5 c,  24 c,  0 c,  10 c,  1 c, ( CDC vsn 1.10 )         \ CDC Header functional
5 c,  24 c,  1 c,  0 c,  1 c,                           \ CDC Call management functional
4 c,  24 c,  2 c,  2 c,                                 \ CDC ACM functional ( two commands )
5 c,  24 c,  6 c,  0 c,  1 c,                           \ CDC Union functional ( one interface )
7 c,  5 c,  81 c,  3 c,  8 c,  0 c,  10 c,              \ Endpoint 1 IN descriptor
9 c,  4 c,  1 c,  0 c,  2 c,  0A c,  0 c,  0 c,  0 c,   \ Interface 2: DATA
7 c,  5 c,  82 c,  2 c,  40 c, 0 c,  0 c,               \ Endpoint 2 IN descriptor
7 c,  5 c,  3 c,  2 c,  40 c, 0 c,  0 c,  align         \ Endpoint 3 OUT descriptor

decimal
Function  USB-STATE  0 ,           \ Hold current USB state in first half word: 3 = ready
                                   \ In second half word are memory for the 900/880 requests
          4 c, 3 c, 9 c, 4 c,      \ English/US = language ID
          115200 ,  0 c,  0 c,  8 c,  align  \ Line data: 115k2, Stop bits, Parity, Data bits

hex
</code>