[etherlab-users] Calculation of time_diff in ec_fsm_master_dc_offset()

[Jun Yuan]

Hello Florian,

OK, I believe that I've found the final real solution. The variable
'correction' is necessary. Please ignore my last change. Find below my
final patch for this problem. I'd love to hear any feedback.

diff -r 9cdd7669dc0b master/fsm_master.c
--- a/master/fsm_master.c    Thu Jan 10 17:36:41 2013 +0100
+++ b/master/fsm_master.c    Wed Jan 30 11:49:59 2013 +0100
@@ -1068,7 +1068,7 @@

     system_time = EC_READ_U64(datagram->data);     // 0x0910
     old_offset = EC_READ_U64(datagram->data + 16); // 0x0920
-    jiffies_since_read = jiffies - datagram->jiffies_sent;
+    jiffies_since_read = jiffies - datagram->jiffies_received;

     if (slave->base_dc_range == EC_DC_32) {
         new_offset = ec_fsm_master_dc_offset32(fsm,


On Wed, Jan 23, 2013 at 5:36 PM, Jun Yuan <j.yuan at rtleaders.com> wrote:

> Hello Florian,
>
> I think there might be a bug in the calculation of time_diff between the
> master clock and the slave clock in fsm_master.c. My temporary solution is
> to comment out the line 'system_time32 += correction;' in the function
> ec_fsm_master_dc_offset32() and the line 'system_time += correction;' in
> ec_fsm_master_dc_offset64().
>
> Here is my story. I've been testing the etherlab master on Xenomai. After
> several times restart of my master process, I occasionally found something
> interesting: the DC sync between the master and the slave always takes a
> long time (approx. 5 second) by each start, and it seems by each time the
> time diff between the master and the slave is always approx. -4000000 ns.
> [16165.453658] EtherCAT DEBUG 0-0: DC 32 bit system time offset
> calculation: system_time=xxxxxxxxxxx (corrected with 4000000),
> app_time=xxxxxxxxxx, diff=-3954618
>
> And the checking synchrony will be started with a difference around that
> diff time.
> [16165.521874] EtherCAT DEBUG 0-0: Checking for synchrony.
> [16165.529891] EtherCAT DEBUG 0-0: Sync after    4 ms:     3941732 ns
>
> This is odd. Since the master has synchronized with the slave just several
> seconds ago before the restart, their clocks should not have 4 ms
> difference in such a short time. And it is strange that the value is always
> around 4000000 ns. So I digged into the master's source code to find out
> the reason.
>
> I have etherlab master rev 2498:9cdd7669dc0b at stable-1.5 on Xenomai 2.6.1,
> and my rt task is something like the following:
>
> ecrt_master_application_time(master, dummy_time);
>
> while(true) { // run loop
>         wait_period();
>         ecrt_master_receive(master);
>         ecrt_domain_process(domain1);
>         ...
>         ecrt_domain_queue(domain1);
>         sync_distributed_clocks();
>         ecrt_master_send(master);
> }
>
> In the function sync_distributed_clocks() there are three calls:
>     ecrt_master_application_time(master, dc_time_ns);
>     ecrt_master_sync_reference_clock(master);
>     ecrt_master_sync_slave_clocks(master);
>
> And here are what I've found:
>
> 1. I must call ecrt_master_application_time() once somewhere before my run
> loop, otherwise I'll get error "No app_time received up to now", and
> ec_fsm_master_state_dc_read_offset will not be executed. The app time given
> to the ecrt_master_application_time() at this point is not important, it
> will not be used anywhere. Calling the function
> ecrt_master_application_time(master, dummy_time) can avoid
> master->has_app_time = 0 in ec_fsm_master_enter_write_system_times().
>
> 2. The first app_time *dc_time_ns* given to the
> ecrt_master_application_time() within the sync_distributed_clocks() in my
> run loop will be used as slave->master->app_time in the function
> ec_fsm_master_dc_offset.
>
> 3. In ec_fsm_master_dc_offset, there is a variable 'correction', which is
> somehow always equal to my fsm interval(4000000 ns on my master). I believe
> this correction should be the time interval since the last read. It is
> calculated using the variable jiffies_since_read.
> *jiffies_since_read = jiffies - datagram->jiffies_sent;*
> jiffies is the current time, datagram->jiffies_sent is the time of the
> last call ecrt_master_send(master).
>
> The system_time from the ref_sync_datagram is the time of my ref slave.
> The following line calculates the current system_time of the ref slave
> clock.
> *system_time += correction;*
>
> Then the time_diff is calcuated as following
> *time_diff = fsm->slave->master->app_time - system_time;*
>
> The problem here however is that the fsm->slave->master->app_time is not
> the current app_time of the master. It is the app time set in
> ecrt_master_application_time() just before ecrt_master_send(master). So the
> current app_time of master should also be approximately calculated as
> fsm->slave->master->app_time + correction.
>
> So the correct calculation for the current time_diff should be
> time_diff = fsm->slave->master->app_time + correction - system_time;
>
> Now the question is, why bother adding the correction(time interval since
> read) to the system_time, as the app_time was set by the user at about the
> same time when the system_time was read?
>
> After commenting out the line '*system_time += correction;', *I have now
> very small time diff between the master and the slave after restart, and
> the ec_fsm_slave_config_state_dc_sync_check goes way faster because the
> initial difference becomes small.
>
> I hope this mail could also help those having the warning 'Slave did not
> sync after 5000 ms'.
>
> Regards,
> Jun
>
>
-- 
Jun Yuan
[Aussprache: Djün Üän]

Robotics Technology Leaders GmbH
Am Loferfeld 58, D-81249 München
Tel: +49 89 189 0465 24
Mobile: +49 176 2176 5238
Fax: +49 89 189 0465 11
mailto: j.yuan at rtleaders.com

Umlautregel in der chinesischen Lautschrift Pinyin: Nach den Anlauten y, j,
q, und x wird u als ü ausgesprochen, z.B. yu => ü,  ju => dschü,  qu =>
tschü,  xu => schü.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.etherlab.org/pipermail/etherlab-users/attachments/20130130/b36b8f77/attachment-0004.htm>