Wrote SDP design document on timing based on station2_sdp_firmware_design.txt....

Wrote SDP design document on timing based on station2_sdp_firmware_design.txt. Text snippets that were not yet used are kept in station2_tmp.txt. Hence station2_sdp_firmware_design.txt is now obsolete.

applications/lofar2/doc/prestudy/station2_sdp_timing.txt

@@ -95,13 +95,13 @@ and any M&C upon the data, because:
 The actual resolution T_sub of the Station BSN in LOFAR2 depends on the ADC sample frequency and on the
 subband filterbank:
 
-  N_blk  T_adc    T_sub       T_sub_i
-  1024 * 5   ns = 5120   ns = 25600 [0.2 ns] for critical sampled filterbank at 200 MHz
-  1024 * 6.4 ns = 6553.6 ns = 32768 [0.2 ns] for critical sampled filterbank at 160 MHz
-   864 * 5   ns = 4320   ns = 21600 [0.2 ns] for oversampled filterbank R_os = 32/27 = 1.185 at 200 MHz
-   864 * 6.4 ns = 5529.6 ns = 27648 [0.2 ns] for oversampled filterbank R_os = 32/27 = 1.185 at 160 MHz
-   800 * 5   ns = 4000   ns = 20000 [0.2 ns] for oversampled filterbank R_os = 32/25 = 1.28 at 200 MHz
-   800 * 6.4 ns = 5120   ns = 25600 [0.2 ns] for oversampled filterbank R_os = 32/25 = 1.28 at 160 MHz
+  N_blk  T_adc    T_sub
+  1024 * 5    ns = 5120 ns for critical sampled filterbank at 200 MHz
+  1024 * 6.25 ns = 6400 ns for critical sampled filterbank at 160 MHz
+   864 * 5    ns = 4320 ns for oversampled filterbank R_os = 32/27 = 1.185 at 200 MHz
+   864 * 6.25 ns = 5400 ns for oversampled filterbank R_os = 32/27 = 1.185 at 160 MHz
+   800 * 5    ns = 4000 ns for oversampled filterbank R_os = 32/25 = 1.28 at 200 MHz
+   800 * 6.25 ns = 5000 ns for oversampled filterbank R_os = 32/25 = 1.28 at 160 MHz
 
 In LOFAR2 the timestamp should be independent of:
 
@@ -110,35 +110,39 @@ In LOFAR2 the timestamp should be independent of:
 
 If T_sub was fixed then T_sub could be used as timestamp resolution (like in APERTIF). However T_sub depends
 on the type of subband filterbank with a resolution of T_adc. If T_adc was fixed then T_adc could be used
-as timestamp resolution. However T_adc depends on the sample clock rate. Therefore the timestamp resolution
-needs to be as fine as the greatest common time resolution of T_adc = 5 ns and T_adc = 6.4 ns, which is 0.2 ns.
-A 64 bit timestamp with 0.2 ns resolution can count 2**64 / (365.25 * 24 * 3600 / 0.2e-9) = 116 years. Hence
-for t_epoch = 1970 this is until 2086, which is sufficient for the lifetime of LOFAR2.0. Internally in SDP
+as timestamp resolution. However T_adc depends on the sample clock rate. At most the timestamp resolution
+needs to be as fine as the greatest common time resolution of T_adc = 5 ns and T_adc = 6.25 ns, which is
+0.25 ns. However typical R_os = p/q have p is a power of 2 and p <= 64, because N_FFT = 1024 = 2**10. Hence
+N_FFT / p >= 16, so typical subband periods can always be expressed as a multiple of 16* 6.25 = 100 ns or 
+16 * 5 = 80 ns. Therefore the minimum required time stamp resolution is gcd(100, 80) = 20 ns. For human 
+readability choose 1 ns as timestamp resolution
+A 64 bit unsigned timestamp with 1 ns resolution can count 2**64 / (365.25 * 24 * 3600 / 1e-9) = 584 years.
+Hence for t_epoch = 1970 this is more than enough for the lifetime of LOFAR2.0. Internally in SDP
 firmware use the BSN to count T_sub. Externally at the SDP interface use timestamp values with a resolution
-of 0.2 ns such that they are:
+of 1 ns such that they are:
 
  * integer values, and
  * independent of the sample period.
 
-The actual timestamp in fractional seconds of 0.2 ns follows from:
+The actual timestamp in units of 1 ns follows from:
 
-  timestamp = Station BSN * T_sub_i * 0.2 [ns].
+  timestamp = Station BSN * T_sub [ns].
 
-The BSN and T_sub_i can be specified as:
+The BSN and T_sub can be specified as:
 
-- single 64 bit integer timestamp value of BSN * T_sub_i [0.2 ns]
-- two separate fields with an incrementing BSN and resolution given by T_sub_i [0.2 ns]
+- single 64 bit unsigned integer timestamp value of BSN * T_sub [ns]
+- two separate fields with an incrementing BSN and resolution given by T_sub [ns]
 
-To cover 116 years for a BSN with smallest T_sub = 4000 ns for R_os = 32/25 = 1.28 requires:
+To cover 100 years for a BSN with smallest T_sub = 4000 ns for R_os = 32/25 = 1.28 requires:
 
-  log2( 116 * (365.25 * 24 * 3600 / 4000e-9) ) = 49.7, so 50 bits
+  log2( 100 * (365.25 * 24 * 3600 / 4000e-9) ) = 49.7, so 50 bits
 
 Therefore allocate 64b in a packet header to send the BSN information. The BSN and timestamp are direcly
-related via T_sub_i, but the advantage of providing the BSN separately is that it increments by 1 for
+related via T_sub, but the advantage of providing the BSN separately is that it increments by 1 for
 each block period T_sub, so it can be used as block index.
 
-The range of T_sub is 4000 ns - 5120 ns, so the range of T_sub_i is 20000 - 25600. These T_sub_i values
-can be covered in a 16 bit number. Alternatively T_sub_i can be derived from the four possible
+The range of T_sub is 4000 ns - 5120 ns. These T_sub values
+can be covered in a 16 bit number. Alternatively T_sub can be derived from the four possible
 combinations of f_adc = 200M or 160M and R_os = 1 or 32/25, that can be represented with 2 bits.
 
 
@@ -148,7 +152,8 @@ combinations of f_adc = 200M or 160M and R_os = 1 or 32/25, that can be represen
 *******************************************************************************
 
 Together the initial BSN, counting blocks and the order of the data within a block uniquely define the timing
-of the data in a Station. However counting blocks is not sufficient to maintain the data timing, because:
+of the data in a Station. However counting blocks is not sufficient to maintain the data timing, because
+there can be gaps due to:
 
 - The data flow at the SDP input may be stopped and restarted,
 - at the external interfaces of the FPGAs in SDP it is possible that blocks of data get lost.
@@ -172,7 +177,7 @@ FPGA in the SDP firmware has a BSN source, that all run synchronously within a S
 Stations, because they have been started by the external PPS from the Timing Distributor (TD). The sync is a
 periodic signal with period larger than the maximum latency of the data within the SDP, to ensure
 that a sync at any FPGA refers to the same time instant. If somewhere in an FPGA the sync comes along, then
-the BSN can be recreated by directly using the BSN that was held at the sync by the BSN source and start
+the BSN can be recreated by directly using the BSN that was held at the sync by the local BSN source and start
 continue counting blocks from there.
 
 

applications/lofar2/doc/prestudy/station2_tmp.txt

+Remaining texts that have not been used yet in official SDP detailed design documents
+
+*******************************************************************************
+* From station2_sdp_timing
+*******************************************************************************
+
+- It is not necessary to represent fine group delays of digital filters or analogue electronics and
+  cables in the BSN, because these delays are all accounted for after calibration.
+  . Course group delays and cable delay differences can be compensated for in steps to T_adc via the signal
+    input buffer of every ADC input in SDP.
+  . Fine group delay differences within a Station can be calibrated via the subband calibration weights.
+- Group delay differences between Stations need to be calibrated at CEP, and can be compensated at CEP or
+  at Station via the input buffers and the subband calibration weights.
+
+
+
+Key ideas:
+- Use Ethernet CRC and DP CRC to ensure detection of packet errors and to ensure error free blocks
+  within FPGA firmware
+
+
+  RCU2                          Subband        Ring
+                                PFB
+
+                        data            data
+  data ------> BSN    --------> Move, -------> Packet
+  PPSH ------> source   sync    DSP     sync   encoding
+                        BSN .........>  BSN    ring
+
+  Ring                                                            BF, XC
+             data                                         data            data
+  Packet   --------> Validate  --> Validate --> BSN     --------> Move, --------> Packet
+  decoding   sync    CRC           BSN          aligner   sync    DSP     sync    encoding
+  ring       BSN .......................................................> BSN     output
+
+
+