Bug 227687 - PPS echo pulse on ppbus too short
Summary: PPS echo pulse on ppbus too short
Status: New
Alias: None
Product: Base System
Classification: Unclassified
Component: kern (show other bugs)
Version: 11.1-RELEASE
Hardware: Any Any
: --- Affects Only Me
Assignee: freebsd-bugs mailing list
URL:
Keywords:
Depends on:
Blocks:
 
Reported: 2018-04-22 09:05 UTC by Lukas Senger
Modified: 2018-04-25 00:08 UTC (History)
2 users (show)

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Description Lukas Senger 2018-04-22 09:05:13 UTC
PPS echo signals are not detectable on the parallel port. The irq handler in ppbus/pps.c creates the echo pulse by setting AUTOFEED, handling the pulse and immediately unsetting AUTOFEED. The "handling the pulse" phase is not long enough to create a usable echo pulse. Adding a DELAY of 100ms before disabling the bit creates good pulses, however this is not a feasible solution in an interrupt handler.

Fix:
Add a delay (maybe 100ms) before disabling the AUTOFEED bit.

How-To-Repeat:
Connect a PPS source (e.g. GPS receiver, signal generator) via the parallel port (pin 10). Set PPS_ECHOASSERT (with pps-api utility in ntp). Try to read the echo on pin 14 (e.g. with oscilloscope, LED, GPS receiver...).