ZYNQ 7000 : AXI4-Lite based MMCM dynamic configuration

Start my hardware project on Xilinx ZYNQ 7000 MPSoC device!

I have got a Z-turn board (designed by Myir Technology) equipped with XC7Z020 SoC chip.

To briefly introduce the ZYNQ devices, we can define it as a combination of FPGA and a dual-core ARM Cortex-A9 processor(also called hard core compared to soft core like MicroBlaze) with on-chip programmable interconnection. The internal connection make the data transfer easier and faster than the previous scheme that place an external ARM processor near a FPGA chip. In the ZYNQ scheme, they call the ARM part Processing System(PS), and the FPGA part Processing Logic(PL). Many convenient implementation of AXI4 bus is provided by Xilinx as IP cores. With the block design function in Vivado, we can quickly integrate our IP cores into the system. Also thanks to the powerful ARM core, we can run Linux operating systems on the PS and directly control all the hardware implementation on the PL.

In Xilinx devices, the clock can be generated by a module called Mixed-Mode Clock Manager(MMCM), which includes a PLL. This module have fractional multiplier and divider thus is quite versatile to meet any need for digital clock generation. Another good thing is that it can be dynamically reconfigured through a set of registers.

The MCMM_DRP is connected to the PS GP0 port through an AXI-interconnection module. I have allocated a small 4KB virtual space for the registers starting at 0x40000000(which is right after my 1GB DDR3 RAM). Because of the limited bandwidth of my Mini-DSO, I have put a 1/4096 clock divider at the output.

In the Linux side, I have two choices to make modifications on registers. The first way is to write a kernel driver that well handles the requests from user space and do the reconfiguration on registers. However, this is not so easy to implement. The another way is to use mmap to map the registers to a virtual address and modify that. This way is easy and can be done in the user space(root privilege is required, though), but not safe. As a simple test and our ‘Hello world!’ project to ZYNQ, I chose the second plan.