[ntp:hackers] Reduce jitter of refclocks (connected via USB)

Terje Mathisen terje at tmsw.no
Mon Mar 14 19:25:10 UTC 2011


David J Taylor wrote:
>> Hi,
>>
>> I am running a Meinberg USB 5131 DCF77 Radio Clock. The clock works
>> quite good. I know that USB is a very poor solution for timekeeping, but
>> I had no other choice since my mini pc has no serial interface and
>> there's no free PCI slot.
>>
>> I noticed that there's a jitter of about (exactly?) 1 ms. As the device
>> works with USB 1.1 (Full Speed), I assume that the jitter is a result of
>> an additional interrupt interval in some cases, leading to an additional
>> error of 1 ms.

USB by design operates with a 1 ms polling interval, so this is exactly 
the jitter you should get, best case.

Terje

-- 
- <Terje at tmsw.no>
"almost all programming can be viewed as an exercise in caching"


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