It is possible that one of the blocks in the middle of your flowgraph is
not handling the tags properly. What is between the sender and receiver?
On Mon, May 17, 2021 at 11:17 AM Jeff Long <willco...@gmail.com> wrote:
> When there is a rate change, tag offsets are adjusted to match. Otherwise,
> the tag spacing should stay the same, taking into account that the tags may
> end up in different calls to work().
>
> On Mon, May 17, 2021 at 11:07 AM jmfriedt <jean-michel.fri...@femto-st.fr>
> wrote:
>
>> I am trying to get an accurate timing of a given sample (1-PPS
>> transition). To do so, a marker is injected in the AD9831 datastream in
>> the B210 FPGA. When this tag is detected in the UHD source block, a tag
>> is created in the stream:
>>
>> for (item=0;item<noutput_items;item++) // check all samples
>> {if (ch==0) // check if channel 0
>> {if (in[item]==_tag_pps_int) // check marker detection
>> {add_item_tag(0, // Port number
>> nitems_written(0) + item, // Offset
>> pmt::string_to_symbol("1PPS"), // Key
>> pmt::from_double(3.14159)); // Value
>> ...
>>
>> Displaying the item number when the marker is detected, I get things
>> like 444331/524288 or 364347/524288 (item/noutput_items) which seems
>> realistic.
>>
>> Downstream the flowchart, after many other processing blocks, I check
>> for the tag in the window of samples that were just transferred
>> std::vector<gr::tag_t> PPS_tag;
>> get_tags_in_window(PPS_tag,0,
>> 0, // abs_start,
>> noutput_items, // uint64_t abs_end);
>> pmt::string_to_symbol("1PPS")) ;
>> if (PPS_tag.size() != 0)
>> { printf("\n** PPS_tag received: %ld (%ld/%d):
>> message=%.9f",PPS_tag.size(), noutput_items,
>> to_double(PPS_tag[0].value));
>> ...
>> and there I *always get* PPS_tag.offset=1.
>>
>> If on the emitter side I create two tags separated by 42 i.e.
>> add_item_tag(0,
>> nitems_written(0) + item,
>> pmt::string_to_symbol("1PPS"),
>> pmt::from_double(3.14159));
>> add_item_tag(0,
>> nitems_written(0) + item + 42,
>> pmt::string_to_symbol("1PPS"),
>> pmt::from_double(3.14159));
>> then the receiver indeed detects PPS_tag.size() = 2 but still
>> both PPS_tag[0].offset and PPS_tag[1].offset are equal to 1 !
>> I am obviously misunderstanding the behviour of tag.offset: can anyone
>> enlighten me on how to identify the index of the sample carrying the
>> tag ?
>>
>> Thanks, JM
>>
>>
