usb15: add protocol notes

doc/usb15-protocol.txt

@@ -0,0 +1,35 @@
+STC15 series USB ISP protocol
+=============================
+
+- 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
+