STC15 series USB ISP protocol
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- host does OUT and IN control transfers for write and read
- IN transfer with wLength = 132, wValue = 0, wIndex = 0, bRequest = 0 are used for all reads
- OUT transfers with arbitrary size are used for writes

- packets from MCU
  always start with 0x46 0xb9, similar to serial protocols
  third byte is packet length
  followed by data bytes
  8 bit checksum at the end, looks like 8 bit modular subtraction

- packet types
  most likely derived from the serial protocol, at least partially

  info packet
  - same as with serial protocol

  option packet
  - generally same as with serial protocol, some header stuff omitted

- flash data
  wIndex specifies write address
  wValue is 0xa55a
  bRequest is 0x22 for first packet, 0x02 for the following ones
  unusually encoded: a total of 128 bytes per packet, with every 7 byte checksummed in some way, for a total of 18x7 byte segments and a final 2 byte segment
  checksum: 8 bit inverted modular sum

- option packet
  wIndex is 0
  wValue is 0xa55a
  bRequest is 4
  seems to use the same checksumming scheme