Homebrew ARM based Linux system - Page 1
Forum section: electronics / projects

author: Dajgoro
author_rank: Frequent Contributor
timestamp: June 20, 2013, 07:54:11 pm
content: Hi.It is been almost a year since I have been looking at the AT91RM9200 ARM based chip from Atmel.I found out about the Linuxstamp computerhttp://opencircuits.com/Linuxstamp#Descriptionand it doesn't look that complicated. It has some ram, some sort of rom, and a bunch of connectors. So I was thinking about building something similar, but maybe a bit different, with more peripherals.The easiest thing would be to take the existing PCB and modify it, but that is like cheating.So I was wondering if the ultimate ARM travesty could be made! Place the chip in a adapter pcb to get though hole and then use point to point soldering. Obviously that is a terrible idea, but then I started investigating the timing diagrams and I didn't found a limit on how slow the clock can go. So theoretically I could bring the clock down to a reasonable amount and get the thing working.I already built computers with 6502/68000 cpu-s but never something as complex as an ARM based core.I am not planning to get into this project any time soon, since I gotta finish a 68030 system first. What I would like to do meanwhile is to collect info and see what is possible and what not. My ultimate goal is to build a system that can run some sort of Linux and that is usable. Such a device is a very neat way to build projects which require a bit more horsepower, and installing a raspberry pi into every such project is a bit silly to me. I also want to learn how to build stuff with such "modern" components.

author: andersm
author_rank: Super Contributor
timestamp: June 20, 2013, 08:15:55 pm
content: The CPU will probably work fine, but I suspect some peripherals may have problems at low clock speeds, especially memories. Signal integrity may also cause problems. The "normal" approach of placing the CPU and essential peripherals (clock, RAM, flash) on a PCB module is probably the easiest, but attempting what you suggested could be an interesting project even if the end result is a failure.

author: Dajgoro
author_rank: Frequent Contributor
timestamp: June 20, 2013, 08:40:37 pm
content: Using dynamic ram would be the easiest solution, but then I must provide a proper refresh. Otherwise I was thinking about using SRAM, that would be a bit expensive, but it could work.

author: InterestedTom
author_rank: Regular Contributor
timestamp: June 20, 2013, 09:06:10 pm
content: I don't understand, what's wrong with designing your own pcb?

author: Dajgoro
author_rank: Frequent Contributor
timestamp: June 20, 2013, 09:40:40 pm
content: The first problem is the price. I am a student, and paying for a expensive custom pcb is not an good option for me, and since this is something a bit more complex, there is a big chance it won't work on the first try.I also don't have the necessary stuff for making my own pcbs.Second problem is that I am not exactly sure what I am building will actually work, so if I mess up something I can easily fix it. Once I get a working prototype, then I can design a proper PCB and invest into it.

author: Dajgoro
author_rank: Frequent Contributor
timestamp: June 21, 2013, 10:18:07 am
content: A few megabytes of SRAM might be quite expensive.I was wondering how low can the clock be brought down on the CPU but to still have the SDRAM(MT48LC16M16A2P) refreshed on a acceptable rate?

author: millerb
author_rank: Regular Contributor
timestamp: June 21, 2013, 10:46:21 am
content: Have you looked into pseudo sram devices?

author: free_electron
author_rank: Super Contributor
timestamp: June 21, 2013, 10:54:21 am
content: You have not looked at he datasheet have you ? Or even the webpage...It clearly states the chip has an sdram controller on board as well as a nand flash controller. So just slap on some sdram and a flash and off you go.Besides linux has already been ported to that cpu. Theres schematics out there on what memories are used, how to wire it up ...But you will need to make a custom board as all those components are fine pitch.....

author: komet
author_rank: Unknown
timestamp: June 21, 2013, 11:02:07 am
content: If you are a student then surely your university has a very cheap PCB shop. Perhaps you just haven't found it yet?

author: Dajgoro
author_rank: Frequent Contributor
timestamp: June 21, 2013, 11:08:47 am
content: QuoteCan you give me any part numbers. I am not really an expert when it comes to modern components.QuoteI did.QuoteI know, but since the plan is to try to use point to point construction, I will have to slow down the clock a lot in order to make it work. I didn't find issues while looking into the cpu-s datasheet when it comes to slowing the clock, but if I slow the clock, will that slow down the dram refresh cycle, and if yes, how much can I then bring the clock down?QuoteThat is why I started looking into this chip into the first place.QuoteI found it already, the link to the Linuxstamp project is into my first post.QuoteActually ebay offers adapter pcb-s for all of those components, so I just have to solder the chip to the adapter board, and from there I can handle the chips.QuoteActually there is a PCB lab somewhere, but then again, if I make a pcb, I would have to scrap it all if I make a mistake somewhere. Also I am not sure if they allow students that are not on one of their classes.

author: millerb
author_rank: Regular Contributor
timestamp: June 21, 2013, 11:45:21 am
content: Quotegoogle "pseudo sram"http://www.mouser.com/search/refine.aspx?Ntk=P_MarCom&Ntt=170940226

author: Dajgoro
author_rank: Frequent Contributor
timestamp: June 21, 2013, 02:26:56 pm
content: I can't find any of a decent size that is not in a BGA package. I can't solder BGA components.

author: nctnico
author_rank: Super Contributor
timestamp: June 21, 2013, 09:21:05 pm
content: Designing a circuit like this without any prior experience will take many prototype runs before its right. And you'll need the proper equipment as well like a >400MHz oscilloscope. Even professionals usually need 2 prototypes before a design is ready for mass production.I'd start small and use an ARM Cortex Mx controller with SDRAM interface to make the first steps.

author: Dajgoro
author_rank: Frequent Contributor
timestamp: June 21, 2013, 10:22:59 pm
content: QuoteThat might be an issue...I only have an analog 60MHz oscilloscope.QuoteI did actually buy a LPC Xpresso board, but I think that doesn't have a SDRAM interface.