There's no such thing as a one-size-fits all automotive memory solution. Rather, diverse and advanced automotive information and entertainment electronics have forced designers to seek out consistently more and different system memory solutions.
In many cases, system architects must strive to find the proper balance of durability, functional potency, power consumption, and priceweighing a variety of factors from BOM (bill of materials) costs to space limitations. Given this potential complexity, it's nice to know that, while no panacea exists, there are some simple and solid memory choices you can make.
In this article, we focus on one such choice, Mobile DDR (double data rate) SDRAM (synchronous dynamic random access memory) (Mobile DDR)first looking at automotive temperature Mobile DDR, which is brand new to the market and makes Mobile DDR a lot more useful for automotive and other embedded applications. Then we'll examine Mobile DDR's low power consumption; its flexibility in terms of specifications, like density, physical form (stacking or configuration), and, finally, its long-term availability.
We will also look at a real-world example of where Mobile DDR is currently being used in Continental's module for the Ford Sync voice-activated infotainment/telematics system.
Automotive temp Mobile DDR
Put simply, automobiles are not the most memory friendly environment in the world. Consider the case of basic DRAM architecture wherein each cell must be constantly refreshed and is temperature sensitive. When DRAM cells heat up, charge loss begins to accelerate as the charge moves across the capacitor. So putting DRAM in an in-dash satellite or navigation system where temperatures inside the system may reach 90C or more can be challenging, to say the least.
Given this limitation, Mobile DDR manufacturers, like Micron, have been striving to make qualified automotive temperature components that can operate at temperatures as cold as "40C or as blazing hot as 105C. This more robust memory can survive not just a hot summer in Arizona, but that hot summer plus the system heat that an adaptive drive assistance system is likely to generate.
Mobile DDR conserves power
One might think that automobiles, with their relatively large batteries and powerful alternators, don'tnor would they everreally have power constraints, but this simply isn't true. First, there are limits. As the shear volume of power-hungry electronic or mechanical systems in a vehicle increases, designers will be forced to ration power.
Consider the case of memory in cars. In 2000, the worldwide automotive industry consumed about 88 million megabits of DRAM. But this year, the industry is expected to use more than 3.6 billion megabits of DRAM or about 41 times as much DRAM as just eight years ago [Source: Micron market research and Gartner Research]. Now consider processors, image sensors, etc., and it seems ridiculous to imagine that vehicles won't soon have power constraintsespecially as more hybrid or electric models are introduced. And even now, designers are bumping into their power budget on new designs.
Mobile DDR anticipates this need by running at lower voltage supply levels than some standard DRAM1.8V for a typical Mobile DDR device. Mobile DDR also offers several low-power operating modes for automotive applications:Temperature-compensated self refresh (TCSR): DRAM is temperature sensitive. If the ambient temperature around the memory device rises, individual cells lose their charge more quickly, and if the temperature falls, cells become hardier, keeping their charge for a longer time. Mobile DDR's TCSR takes advantage of this characteristic and uses information from an on-board temperature sensor to adjust its internal refresh rate. So each cell is refreshed when it needs to be using precisely the right amount of power.
Partial-array self refresh (PASR): What about the portion of the array that is not being used? It would seem like a waste to constantly refresh bits that are not being used, right? So Mobile DDR, when in PASR mode, only refreshes the portion of memory array where READ and WRITE activity is focused (see figure below). Typically, PASR in Mobile DDR can be controlled to refresh half of the array (i.e., banks 0 and 1), a quarter of the array (bank 0), an eighth of the array (bank 0 with address MSB = 0), or one-sixteenth array (bank 0 with row address MSB = 0 and row address MSB -1 = 0).
This plot of self-refresh current vs. operating temperature illustrates how TCSR and PASR work together to reduce the self-refresh current.
PSAR across temperature
Deep power-down (DPD): Sometimes an automotive design will not require Mobile DDR to retain data during certain operation modes. DPD exploits this non-retention time to save power.
Variable output drive strength: This mode allows designers to specify the drive strength at the output buffers for point-to-point connections, potentially saving power. In a typical Mobile DDR device, drive strength can be adjusted to 25, 37, 55, and 80Ω of internal impedance.