[time-nuts] ``direct'' RS-232 vs. RS-232 via USB vs.PPSdecoding cards

jimlux jimlux at earthlink.net
Wed Feb 22 19:20:44 EST 2017

On 2/22/17 3:37 AM, Hal Murray wrote:

> We could get a USB logic analyzer.  That would give us some more data on the
> USB timings.  (There are probably some limits in the specs, but I won't be
> surprised if somebody doesn't get it right..)

USB has a basic timing interval of 125 microseconds (8 kHz) and most 
devices follow that in some way (e.g. they'll burp out a buffer every 
125 microseconds, or do a bulk transfer, etc.)

The stuff I've done with microcontrollers with USB onboard (mostly 
Teensy devices, which is the Freescale micro) seem to have this sort of 
125 microsecond granularity in timing.

I just ran across this, which has lots of useful timing info

There's a whole lot of complexity in how the bus switches directions 
(since there's only two wires), how it detects the speed, etc.

(and let's not even get started on hub and device power management, 
which was my particular bete noir a few years ago...)

I make no claims about USB 3.0..

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