RAM chip naming/identification/nomenclature

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RAM chip naming/identification/nomenclature

Postby ghost » February 5th, 2013, 5:23 pm

I'm thinking of trying to use an external RAM module with a microcontroller (LPC1788 is the current plan). Said micro supports an 8-bit memory bus (at least in the 144-pin package).

So I'm left with a newbie question: How does one identify the memory bus size of a RAM chip? For instance, this is taken off a Micron 64 Mb SDRAM datasheet:

MT48LC16M4A2 – 4 Meg x 4 x 4 Banks
MT48LC8M8A2 – 2 Meg x 8 x 4 Banks
MT48LC4M16A2 – 1 Meg x 16 x 4 Banks

I'm guessing the first number is the number of memory elements (addresses) and the second is the bus width. Is that correct? (so "2 Meg x 8" is what I'd want if I'm looking for an 8-bit bus)

I don't want to get the wrong part and have to wait another ~week to get the right one ;)
ghost
 
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Re: RAM chip naming/identification/nomenclature

Postby GaspingSpark » February 6th, 2013, 12:40 pm

Yes, the "x 8" indicates the size of the data bus of the RAM chip. It looks like the LPC1788 (at least from my quick glance at the manuals) can be configured to use 16 and 32 bit buses as well. More bits makes memory faster, but gives you less GPIOs to play with.

But there are other considerations. Does your microcontroller support the same SDRAM interface protocol as the RAM? You will need to check the timing diagrams from both datasheets to be sure they are compatible. The RAM chip looks like it uses the standard PC100/PC133 signalling. You'll need to look up the specs for the EMC or "External Memory Controller" (Chapter 9 of UM10740.pdf manual) and make sure it supports the same signalling as your RAM. (Nearly all of them support PC100/PC133 chips...)

Then you'll need to use your RAM datasheet to figure out the proper values to write into those 54 EMC configuration registers. :)
GaspingSpark
 
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Re: RAM chip naming/identification/nomenclature

Postby ghost » February 6th, 2013, 7:40 pm

Thanks! That should be enough to get me going... at least going through datasheets, and knowing what to look for. ;)
ghost
 
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Re: RAM chip naming/identification/nomenclature

Postby ghost » August 4th, 2013, 9:59 am

One more thing came up (yes, messing with this has been delayed a good bit): I've got 16 address, 2 chip select, and 8 data lines available. Is it possible to use a 256k x 8 module?

By UM10470 table 110, it looks like the chip select lines cover discrete address spaces. So I think I should be able to use use 4 separate 64k chunks on the chip by telling the EMC it has four 64k x 8 memories attached?


In case what I'm asking isn't clear, here's what I'm trying to do: I'm making a GPS device, with map tiles stored on an SD card. The tiles are standard OpenStreetMap tiles (converted to BMP), 256x256x8, so each one is 64kB. Since accessing SD cards is relatively slow and power-hungry, I'd like to be able to cache 4 tile in memory. So having four "separate" 64k x 8 memories would be fine, but I don't think I'd have space on my board for four separate chips.

Thanks!
ghost
 
Posts: 11
Joined: October 19th, 2012, 8:42 am


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