NOR flash and ARM

[Alison Chaiken]

Andy Tai <at...@atai.org> inquires:

ARM hardware cannot offer support for PC style RAM slots?

Chuck McManis <chuck....@gmail.com> Nov 03 05:59PM -0700  

There isn't anything particularly difficult about adding a JEDEC compliant memory bus to the chip,

Chuck, Andy asked about RAM, and I believe you're replying about ROM.   Probably Andy wants to add NAND flash for filesystem storage.

As far as JEDEC-compliant NOR ROM goes, newer kernels require a devicetree description of devices like flash, so designers might want to see if the kernel they want to use has support for JEDEC flash.    Since parallel NOR (for XIP boot) is less used than SPI-NOR, the kernel support situation is a bit iffier.  

HTH,

Alison

--
Alison Chaiken
(650) 279-5600  (cell)                       {she-devel.com, exerciseforthereader.org}
From what I've known of desire, I hold with those who favor fire.

[Chuck McManis]

JEDEC does a lot of standards, one of them is connector standards for dynamic memory modules (aka DIMMs, or SODIMMS). Just for grins and giggles here is a link to several of them : http://www.jedec.org/standards-documents/results/taxonomy%3A2873 

Anyway, there is IP (pre-designed circuits) for JEDEC compatible DIMM modules, which are what things like your PC and laptop use. But most SoC (system on chip) vendors don't design them into their ARM designs because the added expense of adding a socket, termination capacitors, and all the pins isn't something that folks look for in a SoC, they solder a single DRAM chip, often 256MB x 32 or 512MB x 16 to the chip and they are 'done'. You can see that design on the Pandaboard which uses a 'package on package' (POP) pinout to mount the memory chip underneath the OMAP chip. (kinda cool tech). 

Anyway, AMD will offer 'PC style' RAM slots on their Opteron ARM chips for servers. And presumably if we see more ARM type units going into laptops we may get a chip from Samsung or TI which lets you hook up a SODIMM slot rather than using the POP stuff like they do now. 

--Chuck