srd: usb: Preliminary USB low-speed support.

Some of the differences of USB low-speed (compared to full-speed) are:

 - 1.5Mb/s datarate, not 12MBit/s.
 - The J and K states are swapped/inverted.

Note: This is work in progress, further changes may still be needed.

Add a 'signalling' option, which allows the user to select whether the
PD should decode using the low-speed or full-speed signalling protocol.
If unspecified, the default is full-speed.

1 files changed, 25 insertions, 7 deletions

diff --git a/decoders/usb/__init__.py b/decoders/usb/__init__.py
index a7204db..1a16f8f 100644
--- a/decoders/usb/__init__.py
+++ b/decoders/usb/__init__.py
@@ -19,21 +19,39 @@
 ##
 
 '''
-USB (full-speed) protocol decoder.
+USB (low-speed and full-speed) protocol decoder.
 
-Full-speed USB signalling consists of two signal lines, both driven at 3.3V
+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.
-The state where DP=1,DM=0 is J, the state DP=0,DM=1 is K.
+
+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.
+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 12Mbit/s. The SE0 transmitted to signal an end-of-packet is two bit
-intervals long.
+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).
+
+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