AN 755: Implementing JESD204B IP Core System Reference Design with ARM HPS As Control Unit (Baremetal Flow)
1.3.2. Core PLL
The core PLL generates clocks for the FPGA core fabric. The core PLL uses the device_clk external clock signal as its reference clock to generate two derivative clocks from a single VCO:
- Link clock—from output C0
- Frame clock—from output C1
Clock |
Formula |
Description |
---|---|---|
Link Clock |
Serial data rate/40 |
Clocks the JESD204B IP core link layer and the link interface of the transport layer. |
Frame Clock |
Serial data rate/(10 × F) |
Clocks the transport layer, test pattern generators and checkers, and any downstream modules in the FPGA core fabric. |
For the frame clock, when F=1 and F=2, the resulting frame clock value can easily exceed the capability of the core PLL to generate and close timing. The top level HDL file (jesd204b_ed.sv) defines the frame clock division factor parameters, F1_FRAMECLK_DIV (for cases with F = 1) and F2_FRAMECLK_DIV (for cases with F = 2). This factor enables the transport layer and test pattern generator to operate at a divided factor of the required frame clock rate by widening the data width accordingly. For this reference design, the F1_FRAMECLK_DIV is set to 4 and F2_FRAMECLK_DIV is set to 2. Based on the default settings of the reference design, the frame clock for a serial data rate of 6.144 Gbps and F = 1 equals to:
(6144/(10 × 1)) / F1_FRAMECLK_DIV = 614.4 / 4 = 153.6 MHz