GBT protocol implementation on Xilinx FPGAs Csaba SOOS PH-ESE-BE-OT

GBT frame 27 November, 2008 GBT Protocol Implementation on Xilinx FPGAs bits at 40 MHz => 4.8 Gb/s line rate User field of 84 bits, 3.36 Gb/s –4 bits slow control (SC) => 160 Mb/s –16 bits trigger, timing and control (TTC) => 640 Mb/s –64 data (DAQ) => 2.56 Gb/s Bandwidth can be shared between front-end devices HSCTTCDAQFEC

Forward Error Correction (FEC) Interleaved Reed-Solomon double error correction RS(15,11): 4 bits per symbol, 11 data symbols + 4 parity symbols (60 bits) 2 encoders are used in interleaved mode Error correction capability: 2 x 2 symbols ≈ 16 bits Code efficiency: 88/120 = 73% 27 November, 2008 GBT Protocol Implementation on Xilinx FPGAs 3

27 November, 2008 GBT Protocol Implementation on Xilinx FPGAs 4 GBT channel GT11 RX 40 bits 3:1 gearbox RX align RX slide De- interleave 40 MHz 120 bits RS decoder 120 bits84 bits De- scrambler 84 bits GBT receive data path RX in GT11 TX TX out 40 bits 120 MHz 1:3 gearbox Interleave 120 bits DCMPLL REFCLK 120 MHz RS encoder Scrambler 84 bits 88 bits 4 bits 120 bits GBT transmit data path MGT

27 November, 2008 GBT Protocol Implementation on Xilinx FPGAs 5 Virtex-4 RocketIO MGT SIPO Comma Detect Realign 8B/10B Decoder RX Buffer PISO TX Buffer 8B/10B Decoder Clock Manager Serial InterfaceParallel Interface Simplified block diagram of the transceiver module

27 November, 2008 GBT Protocol Implementation on Xilinx FPGAs 6 Word alignment received data recovered bit clock word clock During the alignment, we slide the sampling window with a Barrel shifter. The process is controlled by a state-machine. Out of Lock (OOL) In Locked (IL) Received 4 out of 64 frames with wrong header Received 15 out of 16 frames with good header

27 November, 2008 GBT Protocol Implementation on Xilinx FPGAs 7 Gearbox frame1, word3 frame2, word3 frame3, word3 frame4, word3 frame1, word2 frame2, word2 frame3, word2 frame4, word2 frame1, word1 frame2, word1 frame3, word1 frame4, word1 write pointer read pointer frame1, word3frame1, word2frame1, word1 word pointer frame3, word3frame3, word2frame3, word1 din, 120 bits at 40 MHz dout, 40 bits at 120 MHz Transmitter side example

Scrambling/Descrambling 27 November, 2008 GBT Protocol Implementation on Xilinx FPGAs Data inputData output Scrambled data Self-synchronizing (multiplicative) scrambler Defined by the polynomial of its LFSR (X 3 +X 2 +1 above) –should be a maximum sequence length polynomial and have the fewest taps possible GBT uses 21-bit scramblers (X 21 +X 19 +1) Can be implemented in parallel form (registers + XORs) –fewer taps result in simpler logic, hence higher speed

27 November, 2008 GBT Protocol Implementation on Xilinx FPGAs 9 ML421 board XC4VFX60 device: - 12 MGTs - 25,280 slices DSP48 blocks - 4,276,224 bits memory

Resource usage 27 November, 2008 GBT Protocol Implementation on Xilinx FPGAs 10 1 GBT channel requires about 3000 slices. Considering the XC4VFX60, it corresponds to about 12 % utilization. GT11 RX 40 bits 3:1 gearbox RX align RX slide De- interleave 40 MHz 120 bits RS decoder 120 bits84 bits De- scrambler 84 bits GBT receive data path RX in GT11 TX TX out 40 bits 120 MHz 1:3 gearbox Interleave 120 bits DCMPLL REFCLK 120 MHz RS encoder Scrambler 84 bits 88 bits 4 bits 120 bits GBT transmit data path

27 November, 2008 GBT Protocol Implementation on Xilinx FPGAs 11 MGT performance at 4.8 Gbps Jitter decomposition: = 74ps, RJ = 3.9ps, DJ = 19.6ps

27 November, 2008 GBT Protocol Implementation on Xilinx FPGAs 12 Application: BER tester ScramblerEncoderMGT TX DescramblerDecoderMGT RX Error inject Check Generator System error rate Line error rate ChipScope VIO/ILA JTAG UART RS232 Embedded CPU Internal bus (PLB)

27 November, 2008 GBT Protocol Implementation on Xilinx FPGAs 13 BER test system BERT RS232 DUT (e.g. OT) Attenuator or User interface and analysis

27 November, 2008 GBT Protocol Implementation on Xilinx FPGAs 14 BER test result Alignment is disabled Alignment is enabled

27 November, 2008 GBT Protocol Implementation on Xilinx FPGAs 15 Conclusion We gained lot of experience implementing the GBT protocol on the ML421 platform (Xilinx Virtex 4 FX) –The full data path has been tested The embedded transceivers are compatible with the GBT protocol BER testing can be performed using either the ChipScope or the UART –We need to develop the user interface for the second method (e.g LabView)