VHDL-Inertial Delay

The keyword INERTIAL may be used in the signal assignment statement to specify an inertial delay, or it may be left out because inertial delay is used by default in VHDL signal assignment statements which contain “after” clauses.

If the optional REJECT construct is not used, the specified delay is then used as both the ‘inertia’ (i.e. minimum input pulse width requirement) and the propagation delay for the signal. Note that in the example above, pulses on Input narrower than 10ns are not observed on Output.

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VHDL-Transport Delay

The keyword TRANSPORT must be used to specify a transport delay.

Transport delay is the simplest in that when it is specified, any change in an input signal value may result in a new value being assigned to the output signal after the specified propagation delay.

Note that no restrictions are specified on input pulse widths. In this example, Output will be an inverted copy of Input delayed by the 10ns propagation delay regardless of the pulse widths seen on Input .

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VHDL Timing Model

The VHDL timing model controls the stimulus and response sequence of signals in a VHDL model. At the start of a simulation, signals with default values are assigned those values. In the first execution of the simulation cycle, all processes are executed until they reach their first wait statement. These process executions will include signal assignment statements that assign new signal values after prescribed delays.

After all the processes are suspended at their respective wait statements, the simulator will advance simulation time just enough so that the first pending signal assignments can be made (e.g. 1 ns, 3 ns, 1 delta cycle).

After the relevant signals assume their new values, all processes examine their wait conditions to determine if they can proceed. Processes that can proceed will then execute concurrently again until they all reach their respective subsequent wait conditions.

This cycle continues until the simulation termination conditions are met or until all processes are suspended indefinitely because no new signal assignments are scheduled to unsuspend any waiting processes.

There are several types of delay in VHDL, and understanding of how delay works in a process is key to writing and understanding VHDL.

It bears repeating that any signal assignment in VHDL is actually a scheduling for a future value to be placed on that signal. When a signal assignment statement is executed, the signal maintains its original value until the time for the scheduled update to the new value. Any signal assignment statement will incur a delay of one of the three types listed below.

1. Transport Delay
2. Inertial Delay
3. Delta Delay

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Digital Logic in Analog Block - How To Test It?

Analog IP blocks these days have increasing amounts of digital control logic. With very small amounts of digital logic, it's possible to just draw gates on the schematic and run targeted tests that will hopefully catch any errors. But when you have several thousand digital gates, a new approach is needed, as I discovered in a recent discussion.

"2,000 gates is probably a good transition point where people switch from manually inserting gates in a schematic to synthesis," said Bob Melchiorre, director of field operations for digital implementation at Cadence. "Beyond 2,000 gates you have to start thinking about how you're going to test it, because you cannot guarantee that simulation or targeted testing will catch all the issues. Around 10,000 gates it starts getting completely unbearable, and you cannot write enough targeted tests to find all the faults in your digital logic."

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DDR4 SDRAM Standards published by JEDEC

The PC industry hasn't seen an updated memory spec in a while, and it was long past due. That upgrade came last week, as the memory standards group JEDEC revealed that it had published a spec for DDR4 SDRAM, defining "features, functionalities, AC and DC characteristics, packages and ball/signal assignments," that builds on the DDR3 spec, first published in 2007. The DDR4 spec applies to SDRAM devices from 2 GB through 16 GB for x4, x8 and x16 buses. Here's a look at some of the particulars.

“The new standard will enable next generation systems to achieve greater performance, significantly increased packaging density and improved reliability, with lower power consumption,” Macri said.

Double Data Rate

First and foremost, DDR4 memory doubles the maximum transfer rate of DDR3. The new spec supports a per-pin data rate of up to 3.2 giga transfers per second (GT/s), twice that of its predecessor's eventual maximum of 1.6 GT/s (the ceiling was raised over time). And, DDR4's max could likewise go higher, as necessary, to accommodate faster components and bus speeds. So far, the only processor roadmap we've seen in support of DDR4 has been Intel's, with its Haswell server processor slated for 2014; consumer-platform support isn't expected until sometime in 2015.

Meanwhile, JEDEC member company Samsung announced in July that it had begun sampling the "industry's first" 16-GB DDR4 RDIMMs, and that it will also offer a 32-GB module; and Samsung, Micron and other companies already offer smaller-denomination DIMMs that comply with the spec.

Lower Power

The DDR4 spec defines memory that operates on 1.2V, compared with DDR3's 1.5V and 1.35V low-voltage spec. According to Samsung, its DDR4 RDIMMs consume about 40 percent less power than DDR3 memory modules operating at 1.35V. We're not sure what math they used to arrive at that finding, but in a world increasingly mindful of power consumption and rising energy costs, 1.2V is better than 1.35V.

More, Wider Memory

While DDR3 supported DIMM sizes between 512 MB and 8 GB in as many as eight banks, DDR4 quadruples memory top-end by doubling the module maximum to 16 GB (with a 2-GB minimum) in as many as 16 banks. That's math we can handle. What's more, DDR4 can arrange memory banks into as many as four groups, providing faster burst access to memory and separate read, write, activation and refresh operations for each group.

Incidentally, memory speeds of DDR4 will start at 1,600MHz and balloon to 3,200MHz. DDR3 mobiles are available mostly at frequencies between 800MHz and 1,600MHz, although the spec supports 1,866MHz and 2,133MHz memory, according to a comparison chart published by memory maker Micron.

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DesignWare DDR4 Memory Interface IP from Synopsys

Highlights:

• Synopsys expands its industry-leading DesignWare® DDR Memory Interface IP family to include support for DDR4 SDRAMs
• Backward compatibility with DDR3 and LPDDR2/3 mobile SDRAMs gives SoC designers flexibility as they transition from one SDRAM standard to the next
• New DDR4 IP offers more features with up to 50 percent lower latency than the previous generation
• DDR4 memory controller and PHY are connected by a standard DFI 3.1 interface to streamline connections to custom PHYs and controllers

Synopsys, Inc. (SNPS), a world leader in software and IP used in the design, verification and manufacture of electronic components and systems, today announced the expansion of its DesignWare DDR interface IP portfolio to include support for next-generation SDRAMs based on the emerging DDR4 standard. By supporting DDR4 as well as DDR3 and LPDDR2/3 in a single core, the DesignWare DDR solution enables designers to interface with either high-performance or low-power SDRAMs in the same system-on-chip (SoC), which is a key requirement of many SoCs such as applications processors for smartphones and tablets.

"Synopsys' support for DDR4 memory is an important contribution to building a robust DDR4 ecosystem," said Robert Feurle, vice president of DRAM marketing for Micron Technology, Inc. "DDR4 brings substantial power and performance benefits to the industry, and Micron is aggressively driving its introduction. By implementing their DesignWare DDR Interface IP with backward compatibility in mind, Synopsys is enabling chip developers to bridge the transition from today's DDR3-based SoCs to the upcoming DDR4 designs."

Synopsys' DesignWare DDR4 IP solution consists of the DDR4 multiPHY and Enhanced Universal DDR Memory Controller (uMCTL2) that connect through a commonly used DFI 3.1 interface. The new DDR4 IP supports all key DDR4 features planned for the upcoming JEDEC standard and, compared to the previous version, includes a 13 percent increase in raw bandwidth, up to a 50 percent reduction in overall latency and new low-power features that provide intelligent system monitoring and control to power down elements of the IP as determined by the system's traffic patterns. Real-time scheduling features in Synopsys' unique CAM-based DDR controller can optimize the scheduling of data read/write traffic from multiple hosts, maximizing performance and minimizing latency.

"While the initial target markets for DDR4 are networking, server, and compute platforms, engineers designing for digital TVs, set-top-boxes, multi-function printing, smartphone and tablet applications will also adopt DDR4 DRAM as prices drop and performance improves," said Desi Rhoden, executive vice president, Montage Technology, and JEDEC memory chairman. "Synopsys has leveraged their participation at JEDEC to develop DDR4-compatible products before the actual standard has been released, which is a key benefit of JEDEC membership."

"Synopsys' complete DDR interface IP portfolio includes support for LPDDR, LPDDR2, LPDDR3, DDR, DDR2, and DDR3," said John Koeter, vice president of marketing for IP and systems at Synopsys. "With this announcement, we are broadening our portfolio to include support for DDR4 while maintaining backward compatibility with existing JEDEC standard SDRAMs. As new DDR standards evolve, designers look for reliable solutions. Synopsys' track record of over 320 DDR IP design wins demonstrates that we offer a low-risk path to silicon success."

Availability for the DesignWare DDR4 multiPHY and uMCTL2 with support for DDR4 is planned for Q4 2012.

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Smartphone is next stop for PCI Express

PCI Express, the I/O backbone of PCs and servers, is getting a low-power extension that will take it into Ultrabooks, tablets and smartphones starting next year. The enhanced interconnect will draw two to four times less power while helping mobile devices link to high-performance peripherals such as 60-GHz wireless networking controllers and solid-state drives.

The PCI Special Interest Group expects to approve by the end of the year a new version of its low level software for PCIe 3.0. The code will run on the M-PHY physical layer chips defined by the MIPI Alliance that creates handset interfaces.

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