Spartan-6 FPGA GTP - PLL feedback divider settings in GTP
Description
Keywords: GTP, DUAL, INTDATAWIDTH, PLL
If I am sharing the PLL in the DUAL, can I select INTDATAWIDTH0 different from INTDATAWIDTH1.
Solution
In UG386, pg. 66, Table 3-1, it is claimed that:<< If the PLL is shared between both GTP transceivers, then INTDATAWIDTH0 must equal INTDATAWIDTH1>>.
The intention of the note is to cover general cases where both lanes within a single DUAL are being used.
However, there could be some cases where INTDATAWIDTH1 has to be different from INTDATAWIDTH0 and the PLL is shared in the DUAL.
As an example, imagine that only one GTP (GTP1) is used in the DUAL, the 8B10B codec is enabled, and that the ratio between the datarate and the reference clock frequency is 8.
A feedback divider of 4 is needed to achieve the correct datarate from the reference clock oscillator frequency, but an INTDATAWIDTH1 of 5 is needed because the 8B10B encoder is used in GTP1.
In this particular case it is possible to multiply by 4 in GTP0 PLL (INTDATAWITH0=0) and then to share the PLL0 output with GTP1 (INTDATAWITH1=1).