X-Git-Url: https://sigrok.org/gitweb/?p=libsigrokdecode.git;a=blobdiff_plain;f=decoders%2Fusb%2F__init__.py;h=313c4955adf0fe2dd8dac50b5bbd173e9dad0159;hp=068cadff5c5291111824c19e238baaeca6b749ae;hb=2fcd7c22852436c3226de9007e88cb305cce1b00;hpb=64c29e28e0efa184319f7831b3eca18c7f73f7d0

diff --git a/decoders/usb/__init__.py b/decoders/usb/__init__.py
index 068cadf..313c495 100644
--- a/decoders/usb/__init__.py
+++ b/decoders/usb/__init__.py
@@ -18,5 +18,52 @@
 ## Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301 USA
 ##
 
+'''
+USB (low-speed and full-speed) protocol decoder.
+
+Electrical/signalling layer (USB spec, chapter 7):
+
+USB signalling consists of two signal lines, both driven at 3.3V
+logic levels. The signals are DP (D+) and DM (D-), and normally operate in
+differential mode.
+
+Low-speed: The state where DP=1,DM=0 is K, the state DP=0,DM=1 is J.
+Full-speed: The state where DP=1,DM=0 is J, the state DP=0,DM=1 is K.
+
+A state SE0 is defined where DP=DM=0. This common mode signal is used to
+signal a reset or end of packet. A state SE1 is defined where DP=DM=1.
+
+Data transmitted on the USB is encoded with NRZI. A transition from J to K
+or vice-versa indicates a logic 0, while no transition indicates a logic 1.
+If 6 ones are transmitted consecutively, a zero is inserted to force a
+transition. This is known as bit stuffing.
+
+Data is transferred at a rate of 1.5Mbit/s (low-speed) / 12Mbit/s (full-speed).
+
+The SE0 transmitted to signal an end-of-packet is two bit intervals long
+(low-speed: 1.25uS - 1.50uS, full-speed: 160ns - 175ns).
+
+Protocol layer (USB spec, chapter 8):
+
+Bit/byte ordering: Bits are sent onto the bus LSB-first. Multibyte fields
+are transmitted in little-endian order (i.e., LSB to MSB).
+
+SYNC field: All packets begin with a SYNC field (8 bits).
+
+Packet field format: Packets start with an SOP (Start Of Packet) delimiter
+that is part of the SYNC field, and end with an EOP (End Of Packet).
+
+PID: A PID (packet identifier) follows the SYNC field of every packet. A PID
+consists of a 4-bit packet type field, and a 4 bit check field.
+The check field is the one's complement of the packet type field.
+
+Protocol output format:
+TODO
+
+Details:
+https://en.wikipedia.org/wiki/USB
+http://www.usb.org/developers/docs/
+'''
+
 from .usb import *