Fpga clock domain crossing pdf

Sep 30, 2014 as shown in figure 1 and figure 2, flip flop a and b1 are operating in asynchronous clock domain. Unless you leverage exhaustive, automated clockdomain crossing cdc analyses to identify and correct problem areas, you will inevitably suffer unpredictable. Fpgas have build in pll blocks, which can be used to create a stable clock. Refer to the clocking section of the spartan6 fpga memory controller user guide ref 1. Pdf synchronization techniques for crossing multiple clock. Clock signal is connected only to flipflops and not to basic gates flipflops are the start and end point of critical path all flipflops within one clock domain have the same clock signal same frequency use the longest path delay to calculate the frequency critical path 1. Crossing clock domains inside of an fpga is a common task, but it is one that many digital designers have trouble with. Clock outputs have the ability to fanout without the use of a bridge. In an asynchronous design, the read pointer is kept in the read clock domain and the write pointer in a separate write clock domain. The lut can be configured as either a 64 x 1 or 32 x 2 memory.

Synchronization techniques for crossing multiple clock domains in fpgabased tmr circuits article pdf available in ieee transactions on nuclear science 576. Intel provides fifo intel fpga ip core through the parameterizable singleclock fifo scfifo and dualclock fifo dcfifo functions. In this method the system module a sends data to systemmodule b based on the handshake signals ack and req signals. Im using a flexrio 7966r for digital signal manipulation and need to buffer data across clock domains. Various design techniques are presented together with reallife. Also shows how to use fifos to cross boundaries with large amounts of data. Generating clock domain crossing fifos fpga developer. Synchronization failure is caused by an output going metastable and not converging to a legal stable state by the time the output must be sampled again. I have a slight problem with my clock domain crossing timing constraints.

Updated references in appendix a, additional resources and legal notices. A clock domain crossing cdc takes place anytime the inputs to a given flipflop were set based upon something other than the clock edge used by that flipflop. Averting clockdomain crossing issues in fpga design. Fifo intel fpga ip user guide archives on page 32 provides a list of user guides for previous versions of the fifo intel fpga ip core. Article pdf available in ieee transactions on nuclear. Each clock forms a clock domain inside the fpga, and care needs to be taken. It is very clear than none of the three output signals, y, cb, or cr, change at a faster rate than 27 mhz half of 54. You can configure and build the fifo intel fpga ip core with methods shown in the following table. Fig 2 illustrates three examples of this that well discuss below.

Metastabilty from the intermixing of multiple clock signals is not modeled by simulation. The signal delay is between one and two clock periods in the new clock domain. It can be sufficient in many applications, as long as the signal generated in the fast clock domain is wider than the cycle time of the slow clock. The fifo functions are mostly applied in data buffering applications that comply with the firstinfirstout data flow in synchronous or asynchronous clock domains. Clock domain crossing using systemverilog from sunburst design, inc. For example, a clock and its derived clock via a clock divider are in the same clock domain because they have a constant phase relationship. In figure 1, signal a is launched by the c1 clock domain and needs to be captured properly by the c2 clock domain. Systemverilog assertions for clockdomaincrossing data. Unless you leverage exhaustive, automated clockdomain crossing cdc analyses to identify and correct problem areas. Questa verification solution datasheet pdf, 1mb questa clockdomain crossing datasheet datasheet pdf, 510kb how to avoid metastability on reset signal networks, aka reset check is the new cdc blog post. In this tutorial, we will generate fifos with independent read and write clocks, and nonsymmetric aspect ratios. The issue that i am facing is that of clock domain crossing. Clock domain crossing design 3 part series verilog pro. Fpga projects basic music box led displays pong game rc servos text lcd module quadrature decoder pwm and onebit dac debouncer crossing clock domains the art of counting external contributions fpga projects interfaces rs232 jtag i2c epp spi sd card pci pci express ethernet hdmi sdram fpga projects advanced.

Using alintpro to verify clock domain crossing issues in fpga designs description. Design guidelines and timing closure techniques for hardcopy asics july 2010 an5452. Fpga tools do guarantee that within a clock domain, it will never happen thats the entire point of checking setuphold times. This invites problems with metastability, where a design might fine work in simulation but not on actual hardware. This paper details some of the latest strategies and. Updated references in 3d ic devices using stacked silicon interconnect ssi technology and added second generation 3d ic interconnect subsection. The risk of cdc related failure is on the rise as fpga based soc design complexity continues to mount thanks to an ever increasing drive to make designs faster while reducing power consumption. For transfering data ive used asynchronous fifos, and it worked in hardware with no problem. The direct inputs x and i serve as the data inputs. Aug 09, 2017 how to go from slow to fast, fast to slow clock domains inside of an fpga with code examples. A clock crossing fifo is one mechanism for doing this.

A clock domain crossing occurs whenever data is transferred from a flop driven by one clock to a flop driven by another clock. It is extremely hard to understand the cause of such issues. The clock domain crossing issues cause functional instability of fpga designs. Then youd only need to synchronize that changed signal to the other clock domain to create a clock enable to transfer the actual parallel data, which should not be changing at that time. Clock domain crossing cdc paths crossings between registers on asynchronous clock domains reset domain crossing rdc paths crossings between registers on asynchronous reset domains voltage domain crossing vdc paths crossings between registers on. Dec 12, 2018 a clock domain crossing occurs whenever data is transferred from a flop driven by one clock to a flop driven by another clock. How to go from slow to fast, fast to slow clock domains inside of an fpga with code examples.

A clock domain crossing occurs whenever data is trans ferred from a flop driven by one clock to a flop driven by another clock. Feb 05, 2019 this course presents some considerations when crossing clock domains in intel fpgas. The industrys most comprehensive and easytouse clockdomain crossing verification solution metastabilty from the intermixing of multiple clock signals is not modeled by simulation. Synchronization techniques for crossing multiple clock domains in fpga based tmr circuits. Oct 20, 2017 a clock domain crossing cdc takes place anytime the inputs to a given flipflop were set based upon something other than the clock edge used by that flipflop. Multipleclock domain fpga designs indico at esa estec indico. Reduce metastability by using a user grey cell methodology. A clock domain crossing occurs whenever data is transferred from a flipflop driven by one clock to a flipflop driven by another clock. Oct 25, 2017 fpga tools do guarantee that within a clock domain, it will never happen thats the entire point of checking setuphold times. Clock domain crossing is a global problem and leda currently has an effective solution for cdc verification. This week we will look at standard synchronization techniques for multiclock domain socs. Clock domain crossing analysis asic, fpga and ip verification overview the blue pearl software visual verification suite provides enhanced lint, debug, clock domain crossing cdc and automated sdc generation flows to accelerate asic, fpga and ip rtl verification on your choice of linux or windows.

Transmitter asserts the req request signal, asking the receiver to accept the data on the data bus. Spyglass cdc helps designers find clock domain crossing errors spyglass and vc formal combined enable designers and verification engineers to. I was looking through some of the labview fpga examples and i came across the crossing clock domains example. Wilson research group and mentor graphics, 2016 functional verification study 5. Dec 24, 2007 a clock domain crossing occurs whenever data is transferred from a flop driven by one clock to a flop driven by another clock. It will occasionally assert high for several consecutive clock cycles and then go low again for many many clock cycles. The automated code conversion allows fpga designers to load complex fpga projects into the alintpro environment in seconds. Request pdf research on metastability based on fpga multi clock is commonly used in complex systems. Designers increasingly use advanced multiclocking and reset signal distribution architectures to meet the highperformance, low bringup latency, and lowpower requirements of. For example, if you just need to synchronize a slow changing status signal, this may work.

Using alintpro to verify clock domain crossing issues in. Method description using the fifo parameter editor. A failing fpga device in the field with a clock domain crossing cdc issue is a true nightmare. Fpga vendors resulting in more efficient and accurate analysis. I have enough resolution on the clock ticks that being off by one, or even a few ticks has no real impact. A pulse cannot be synchronized directly using 2 ff synchronizer. Sep 23, 2009 in some fpga designs, it is necessary to interface two devices that operate in different clock domains. So if synchronous signals in one clock domain are transferred to another clock domain, they. In fact, designers can perform cdc analysis through the fpga vendor protectedgenerated ips.

Clock domain crossing cdc issues cause significant amount of failures in asic and fpga devices. Clock domain crossing 0% 5% 10% 15% 20% 25% 30% 35% 40% 45% 1 2 34 510 1120 2 3150 50 s fpga number of independent clock domains 2012 2014 2016 sb, cdc verification, 2017 source. The risk of cdc related failure is on the rise as fpgabased soc design complexity continues to mount thanks to an ever increasing drive to make designs faster while reducing power consumption. Hello ive read amount of material about clock domain crossing. Clock domain crossing analysis blue pearl software inc.

As fpga complexity and performance grows, the influence of cdc issues on design functionality grows even more. Apr 20, 2017 questa clockdomain crossing datasheet datasheet pdf, 510kb how to avoid metastability on reset signal networks, aka reset check is the new cdc blog post power aware cdc verification of dynamic frequency and voltage scaling dvfs artifacts. Basic cdc analysis before we get into the details of user grey cell, let us do a quick refresher on basic cdc analysis. You only need a bridge if you want a clock from an external exported source to connect internally to more than one source. Help understand multibit cdc what you want for something that doesnt change often is a signal that indicates that a new value is available. Synchronization techniques for crossing multiple clock domains in fpga based tmr circuits article pdf available in ieee transactions on nuclear science 576.

The enhanced clock domain crossing verification with alintpro is able to reveal hardto find issues causing functional instability of fpga designs. In some fpga designs, it is necessary to interface two devices that operate in different clock domains. Ultrascale architecture configurable logic block user guide. This white paper describes metastability in fpgas, why it happens, and how it can cause design failures. I have a signal that i need to send between two asynchronous clock domains in an fpga. If so, can i safely sample the signals at 27 mhz and not worry about clock domain crossing issues, setup and hold, etc. By buffer i mean i need to be able to store in memory a variable amount of data before its read back out in order to achieve a data delay. In clock domain crossing cdc design part 2, i discussed potential problems with passing multiple signals across a clock domain, and one effective and safe way to do so. Spyglass cdc helps designers find clock domain crossing errors spyglass and vc formal combined enable designers and verification engineers to quickly analyze and check rtl designs. Pdf synchronization techniques for crossing multiple. That circuit, however, does hot handle the case when the destination side logic cannot accept data and needs to backpressure the source side. The processing logic in the peripheral is required to operate at a relatively slow clock frequency, but the clock frequency of the apb interface can be 4 to 5 times that of the slow clock domain.

The protocol for this uses the same old method that is found with 8155 chip used with 8085. These can be avoided by following a few critical guidelines and using wellestablished verification techniques. Added asynchronous clock domain crossing to chapter 5, advanced topics. A rule of thumb is that a synchronizer circuit causes two clock cycles of delay in the new clock domain, and a designer needs to consider how synchronization delay impacts timing of signals crossing clock domains. Current webinar explains this methodology and demonstrates it on the. Depending on the relationship between the two clocks, there could be different. Edn synchronizer techniques for multiclock domain socs. Xilinx xapp496 creating wider memory interfaces using. Design guidelines and timing closure techniques for. If the transition on signal a happens very close to the active edge of clock c2, it could lead to setup or hold violation at. Power aware cdc verification of dynamic frequency and voltage scaling dvfs artifacts. One solution to crossing from one clock domain to another is by using fifos with independent read and write clocks. Problems can occur if the digital designer does not understand all of the details involved in crossing from one clock domain into another. Ultrascale architecture configurable logic block user.

The slicem shares a common write address and write clock across all 8 luts. Instance browser clock violations code browser figure 4. A clock domain is a part of a design that has a clock that operates asynchronous to, or has a variable phase relationship with, another clock in the design. I can successfully write to the fifo in one clock domain and read data from the fifo in another clock domain, but as soon as i introduce the. Crossing clock domains an fpga design can use multiple clocks. Research on metastability based on fpga request pdf. The distributed ram can be combined across the eight luts in the slicem to create memories of up to 512 bits. The fifos in the user interface manage the necessary clock domain crossing.

Clock domain crossing cdc errors can cause serious design failures. Both clock domains have the same nominal frequency. Each clock forms a clock domain inside the fpga, and care needs to be taken if a signal generated in a clock domain is needed in another clock domain. A friend suggested i use a dual clock fifo, write into it from the counter side, read from it on the 50mhz to solve the clock domain crossing problem.

Crossing clock domains in an fpga dealing with metastability and timing errors. Question about clock domain crossing i have a signal that i need to send between two asynchronous clock domains in an fpga. A simple two flipflop synchronizer is the fastest way to pass signals across a clock domain crossing. When data crosses from one clock domain to the other, it is called as clock domain crossing cdc. However i would like to constrain a few of these paths as. Even though design is working perfectly in functional simulations, it may still fail during lab testing. Clock domain crossing issues this section describes three main issues, which can possibly occur whenever there is a clock domain crossing. In each of these vis there is a timedloop that runs at 160 mhz. The industrys most comprehensive and easytouse clockdomain crossing verification solution. Otherwise the routertiming tool will try to infer a relationship, and if it cant find one itll default to a setup requirement of 0. Averting clockdomain crossing issues in fpga design blog. Mar 28, 2016 a simple two flipflop synchronizer is the fastest way to pass signals across a clock domain crossing.