LPC1788 versus LPC2478


I have a project running on a LPC2478 with a QVGA LCD, similar to the Olimex LPC2478-STK (a subset ). It runs at 72MHz.

The size of my code is approaching the 500k limit, so I have tried to cross load the code from a serial flash to external SD-RAM and execute the code from the external SD-RAM. In this way my code can exceed the 500k limit.

Unfortunately, the execution speed is slower when running from the SD-RAM. It runs app. tree times slower. The speed reduction comes probably because the LCD uses the SD-RAM, and therefore occupies the bus, resulting in reduced access speed for normal read/write operations.

Does anybody know if the LPC1788 have similar speed reduction on executing code from the external bus? Or has NXP solved this infirmity in the LPC1788?

Thanks in advance!

Regards Jens

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Are you using Thumb ?

How can they "solve" this ? If both LCD and your program need access to external SDRAM, there's no way they can make it fast.

How about only moving part of the code to SDRAM, and keep all timing critical stuff in Flash ?

Reply to
Arlet Ottens

Well that depends on the details doesn't it? QVGA should not have *that* high a bandwidth, I am running "full" VGA on a LPC2478, so there is at least 3/4 left over on his! :)

I think the reason is more likely that SDRAM code execution is simply slower, even with no bandwidth consumed by the LCD.

Yes, good idea, that should be a good solution.


John Devereux
Reply to
John Devereux

On the STM32, accesses to external memory go through the external bus a word at a time. Internal Flash uses wider-bus accesses and runs more quickly. Is it possible that the LPC chips do something similar?

The contention with other devices using DMA on the same bus is going to have the same penalty on all chips, unless you have separate buses for different memory segments.

When you set up your external SDRAM, you may also have to specify wait states depending on the speed of the SDRAM. It's been a year or so since I worked with LCD and camera code, so the details are a bit fuzzy in my mind.

Mark Borgerson

Reply to
Mark Borgerson

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