On Wednesday 30 September 2015 00:24:24 you wrote: > USB low/full speed allows for frequency tolerance of 1.5%/0.25%. At > maximum packet size (sync + PID + data + CRC16) of 12 bytes/1027 bytes > this amounts to 1.4 bits/20 bits, so the decoder has to lock to the > actual symbol frequency to avoid any symbol misdetections. > > The signal is sampled twice, once at the symbol center and once at > the expected edge position. Comparing the symbol at both positions gives > an indication if the current bit width is to low or to high. Adjust > accordingly. > --- > decoders/usb_signalling/pd.py | 33 ++++++++++++++++++++++++--------- > 1 file changed, 24 insertions(+), 9 deletions(-) > > See https://goo.gl/photos/P4SDN8oeXweW4GhF7 for a comparision of the > decoder with and without frequency tracking. The actual rate of the > sampled signal is 1.488 instead of 1.500 MHz, which is well within > spec for a low speed device.
I have attached the capture highlighting the problem. The device is a USB mouse. Kind regards, Stefan -- Stefan Brüns / Bergstraße 21 / 52062 Aachen home: +49 241 53809034 mobile: +49 151 50412019 work: +49 2405 49936-424
usb_lowspeed_plugin_trim.sr
Description: Zip archive
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