The sections below indicate how many inputs and outputs (I/Os) are needed for the main blocks of the system.
The FPGA has two types of GT quads:
The LEAP transceiver can operate up to 25 Gbit/s and therefore require GTY transceivers. There are 12 TX and 12 RX channels in total.
There are 8 high-speed data outputs from each ASIC that can operate up to ~6 Gbit/s. For each ASIC there is also one GT clock input which the FPGA has to provide a clock to (6 Gbit/s = 3 GHz clock). In total each ASIC needs 8 RX and 1 TX channel of the FPGA. There are 6 ASICs in total.
This is more than what is available in GTH or GTY separately, which means that some ASIC GT channels will be in GTH and some in GTY. It would also be beneficial to have the TX and RX channels separated to avoid constraining the resources and there are enough FPGA GTs available for this. As shown in the table below, there are 8x ASIC RX channels that are placed in two GTY quads.
From FPGA to PC:
Reverse calculation assuming the current PGPv4 running at 15 Gbit/s:
Quads can source their reference clock from a quad that is at most two quads away. This is to ensure the best jitter performance.
In the table below there are the following reference clock inputs (marked with X and purple/blue):
This means at least one reference clock is needed for the LEAP channels and four reference clocks are needed for the ASIC channels. See 3x2 readout board overview above for more details on the system clock structure.
Important note on clocking of GTY transceivers from UG578 page 48: "QPLL0 must use GTREFCLK0 and QPLL1 must use GTREFCLK1 when the channel is operating above 16.375 Gb/s"
Quad 0 | Quad 1 | Quad 2 | Quad 3 | Quad 4 | Quad 5 | Quad 6 | Quad 7 | Quad 8 | Quad 9 | Quad 10 | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
TX | RX | TX | RX | TX | RX | TX | RX | TX | RX | TX | RX | TX | RX | TX | RX | TX | RX | TX | RX | TX | RX | |
GTY | 4x LEAP | 4x LEAP | 4x LEAP | 4x LEAP | 4x LEAP | 4x LEAP | N/C | 4x ASIC | N/C | 4x ASIC | 3x ASIC | N/C | 3x ASIC | N/C | ||||||||
Clock | |← | ← X → | →| | | X → | →| | | X → | →| | |||||||||||||||
GTH | N/C | 4x ASIC | N/C | 4x ASIC | N/C | 4x ASIC | N/C | 4x ASIC | N/C | 4x ASIC | N/C | 4x ASIC | N/C | 4x ASIC | N/C | 4x ASIC | N/C | 4x ASIC | N/C | 4x ASIC | ||
Clock | |← | - | ← X → | - | →| | |← | - | ← X → | - | →| |
There is one GTY quad and one GTH quad left over.