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Created: 7/12/2021Last Updated: 7/06/2023
Introduction
Clocks are needed for various functions of a computing device containing processor, memory, and input and output peripherals. Each of these components requires different clock speeds specified by different standards. The core operating frequency is in the gigaherts range, memory clocks speed is in hundreds of megahertz range, and I/O transceivers interface can be slow as 10s of KHz to fast in the giga herts range.
All these clock speeds comes from a signal source of clocks that can be scale up or down to meet the needs of each components in a computing device. a clock generator takes a high quality clock source such as a crystal oscillator and scales up using a phase locked loop and then divided down to various clock speeds needed in the system. Then various clock source are distributed to the target hardware blocks such as core, transceivers, and peripherals devices.
Pracical Design
Designing a clock tree for an FPGA (Field-Programmable Gate Array) involves several steps to ensure proper clock distribution and minimize skew. Here's a general outline of the process:
Analyze clock requirements: Understand the timing requirements of your design and the clock specifications provided by the FPGA vendor. Determine the required clock frequencies, clock domains, and any special constraints.
Identify clock sources: Identify the primary clock sources for your design. These are typically external clock signals or on-chip oscillators. Ensure that the clock sources meet the timing requirements and have suitable frequencies for your application.
Define clock domains: Divide your design into different clock domains based on the functionality and timing requirements. Each clock domain will have its own associated clock tree. For example, a Xilinix Ultrascale FPGA contains two parts, programable logic (PL), and processing system (PS), and these two domains has its own clock requirements.
Create clock distribution network: Develop a plan for the clock distribution network. This includes selecting clock distribution ICs such as external crystal oscillotrs, clock buffers, and clock generators, and clock sources inside the FPGA, such as global clock networks, clock routing resources, clock buffers, and clock dividers.
Excercise
Play around with TI Clock generator to understand how each frequency is generated and distributed.
https://www.ti.com/tool/CLOCK-TREE-ARCHITECT
An example of Clock Generator IC
Summary
In reality, clock trees are more complex and involve multiple levels of buffering and distribution to account for factors like clock skew, fanout, and clock gating. Additionally, the clock tree may have a hierarchical structure with primary and secondary clock domains.
To design an efficient clock tree, various techniques are employed, such as buffer insertion, clock balancing, and clock meshing, to minimize clock skew and delay.
Further Reading:
An example of a Clock Tree: Page 3 https://www.mouser.com/pdfdocs/clock-tree-101-timing-basics.pdf
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