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Analysis done by Denis Kalashnikov:
It seems that some ROS versions on some routerboard models have this bug:
after silence boot (no output to uart, no beeps) beeper clicks when wireless traffic is.
https://forum.mikrotik.com/viewtopic.php?f=3&t=92269
https://forum.mikrotik.com/viewtopic.php?t=63399
From these links:
1)
Hello, I have RB951G-2HnD and I noticed strange thing
when I loaded the device with some wireless traffic it
produced strange sound - like hissing, fizzing etc.
2)
Same problem still on 6.33, with silent boot enabled
I hear buzzing noise on wireless load.
3)
The sound is fixed in v5.19, it was a bug that caused beeper to make clicks.
It also got fixed in RouterOS:
* What's new in 5.19 (2012-Jul-16 10:51):
fix ticking sound on RB411UAHL;
* What's new in 6.38.3 (2017-Feb-07 09:52):
rb3011 - fixed noise from buzzer after silent boot;
I've checked with an oscilloscope that:
* When on the ssr beeper pin is 0,
on the beeper itself is 1 (~5V),
and when on the ssr beeper pin is 1,
on the beeper is 0
The beeper doesn't consume power,
so 1 should be a default/idle value for the ssr beeper pin).
* When there is wireless traffic (ping packets)
in the background and the beeper clicks, I see
pulses on the beeper itself,
but no pulses on the ssr beeper pin (Q5 pin of 74hc595).
When I manually toggle the ssr beeper pin I see pulses on both.
So, it is likely that the phantom beeper clicks are caused by the EMI.
Suggested-by: Denis Kalashnikov <denis281089@gmail.com>
Reviewed-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Signed-off-by: Koen Vandeputte <koen.vandeputte@ncentric.com>
(cherry picked from commit
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