Honored Contributorcyclone3 : maximum DDR input rate with LVDS
I'm designing a 10 megapixel still-and-video camera, and am trying to determine whether cyclone3 FPGAs are fast enough to interface to the HiSPi interface of this MT9J003 image sensor (CCD). This chip transmits the data via the HiSPi interface, which has 1 clock lane (signal) and 4 data lanes (signals). All 5 lanes are subLVDS, which I run through 3.125Gbps LVDS repeater chips (one DS25BR440 and one DS25BR100) to generate very clean, high-quality LVDS signals to feed into the cyclone3 FPGA (EP3C5 or EP3C10 or larger). The HiSPi interface is DDR, which means one data-bit must be captured from each data lane on each edge of the clock lane (rising and falling edges). In other words, the clock signal is 360MHz, but due to DDR sampling, the data-capture rate on each data lane is 720MHz.
Since the image sensor IC generates a clock signal with rising and falling edges perfectly aligned with the data signals, presumably the cyclone3 PLL would not be involved in the data capture circuit. Presumably the clock signal would be fed directly into the LVDS clock on the same bank of the FPGA as the LVDS data signals. Also note that the image sensor is only 10mm from the LVDS repeater ICs, and the LVDS repeater ICs are only 10mm from the FPGA... AND... all 10 traces on the PCB can be kept exactly the same length between the image-sensor-pads and the LVDS-repeater-pads AND between the LVDS-repeater-pads and the FPGA-input-pads. Also note that we can send the data clock through two LVDS-repeaters with opposite input-polarity to generate two clocks that are exactly 180-degrees out of phase (so FPGA can sample data on rising edge of each clock, if this helps the FPGA). The output capacitance of the LVDS repeaters are only 1.2pF (typical). When I read the cyclone3 handbook, I see the following [maybe] relevant information (all the following information is given for C6, C7, C8 speed parts in that order). page 1-15 : clock tree performance: 402 MHz, 437 MHz, 500 MHz page 1-15 : PLL input clock frequency: 472 MHz page 1-22 : LVDS receiver timing (?clock cycle rate?): 320 MHz, 370 MHz, 437 MHz page 1-22 : LVDS data receive rate: 640 Mbps, 740 Mbps, 875 Mbps (must be via DDR) page 1-22 : data sampling window: 400ps, 400ps, 400ps page 1-22 : input jitter tolerance: 500ps, 500ps, 550ps So far, so good, though it appears I must adopt C7 parts to achieve my 720 Mbps rate. My problem is, I originally read the older cyclone3 handbook, which has vastly more detailed timing specifications than the current (new, "final") handbook. In that handbook was listed best-case setup and hold times for sampling input data, and the absolute best case setup + hold time was about 2 nanoseconds (even worse for LVDS inputs). Now, assuming we sample on both the rising and falling edges, we need the data to be valid for 2 nanoseconds + 2 nanoseconds (for the two edges), plus we need to allow 100ps ~ 1000ps for the two rise-and/or-fall times in the data lanes. Even if we allow zero time for data rise-and-fall, 4 nanoseconds implies a maximum LVDS speed of 250MHz and a maximum data receive rate of 500Mbps. Clearly the contradiction is the setup + hold time, which is about 2.5 to 3.0 nanoseconds in the older handbook, as compared to only 0.4 nanoseconds in the current handbook (the so-called "sample window", which must be just another name for "setup + hold" time). # 1: Should I supply complementary clocks to the FPGA (to sample data on both edges)? # 2: Will my design work reliably (assuming the logic in the FPGA is correct)? # 3: Why the *huge* difference between the older versus new handbook? # 4: Is the cyclone4 better for this (5 LVDS inputs = 4 data + 1 clock). Thanks in advance to anyone who explains these issues to me.
