editor's blog
Subscribe Now

A Bit of Memory With Your Logic?

The recent ICCAD show had a session dedicated to MRAM and memristors. Spintec had a presentation on MRAM that went beyond the normal discussion of memory, and proposed a hybrid logic/memory (or logic-in-memory) usage of MRAM cells – or, more precisely, of magnetic tunnel junctions (MTJs).

At the very simplest end of this is a so-called non-volatile flip-flop (NVFF). It normally acts as a standard flip-flop or SRAM cell (with equivalent performance), but on the pull-up end of things are a couple of MTJs oppositely biased. Those can act as a permanently stored state: when an “auto-zero” function is performed using an additional transistor, the FF stops acting like a FF and acts like a sense amp instead. The unbalancing of what is essentially a diff pair biases the circuit in one direction so that, in a few hundred picoseconds, you can restore the stored state. Add a couple transistors needed to provide the heating current for thermally-assisted switching of the MTJs, and you have a way to dynamically change the non-volatile state.

This can help, for example, when powering down a block to save power. Before doing so, if you save the value in the MTJ pair, then the block can save its state and come back up with no loss of memory. Another example of this concept is a non-volatile L2 cache that can be powered-down and restored with no loss.

The Laboratoire d’Informatique, de Robotique et de Microélectronique de Montpellier (LIRMM) has gone so far as to put together an FPGA where the flip-flops and SRAM cells have been replaced by MTJs. Now, after capturing the FPGA content via a serial stream and storing it in the MTJs, you can have sub-1-ns configuration after power-up from then on – no serial loading.

The logic-in-memory concept embeds MTJs into standard logic circuits. They showed an adder example where two added transistors act as a latch above a current-mode adder. There’s an MTJ in each leg of the adder above the current source. The current source itself is dynamic, being clocked to reduce power.

This all works because the MTJ can be built above the logic rather than next to the logic, as is typical for CMOS or other memory. This allows more seamless mixing of MTJs and logic, and it reduces the distance between logic and computation, improving performance and increasing the opportunities for interconnectivity. The MTJ can be programmed quickly (on the order of 10 ns vs a 2-ns write time for a volatile SRAM bit).

In the adder example, area and performance improved nominally using the hybrid approach, but dynamic power went from over 70 µW to just over 16 µW; standby power went from 0.9 nW to 0 nW (since the current source can be shut off). The only metric that went in the wrong direction was the amount of energy needed to write the cell: it went from 4 pJ in the pure CMOS implementation to almost 7 pJ for the hybrid version – and just over 20 pJ for writing the MTJ.

Leave a Reply

featured blogs
Jan 17, 2020
I once met Steve Wozniak, or he once met me (it's hard to remember the nitty-gritty details)....
Jan 17, 2020
[From the last episode: We saw how virtual memory helps resolve the differences between where a compiler thinks things will go in memory and the real memories in a real system.] We'€™ve talked a lot about memory '€“ different kinds of memory, cache memory, heap memory, vi...
Jan 16, 2020
While Samtec started as a connector company with a focus on two-piece, pin-and-socket board stacking systems, High-Speed Board Stacking connectors and High-Speed Cable Assemblies now make up a significant portion of our sales. To support development in this area, in December ...
Jan 16, 2020
Betting on Hydrogen-Powered Cars On-demand DRC within P&R cuts closure time in half for MaxLinear Functional Safety Verification For AV SoC Designs Accelerated With Advanced Tools Automating the pain out of clock domain crossing verification Mentor unpacks LVS and LVL iss...

Featured Video

RedFit IDC SKEDD Connector

Sponsored by Wurth Electronics and Mouser Electronics

Why attach a header connector to your PCB when you really don’t need one? If you’re plugging a ribbon cable into your board, particularly for a limited-use function such as provisioning, diagnostics, or testing, it can be costly and clunky to add a header connector to your BOM, and introduce yet another component to pick and place. Wouldn’t it be great if you could plug directly into your board with no connector required on the PCB side? In this episode of Chalk Talk, Amelia Dalton chats with Ben Arden from Wurth Electronics about Redfit, a slick new connector solution that plugs directly into standard via holes on your PCB.

Click here for more information about Wurth Electronics REDFIT IDC SKEDD Connector