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Forum Index : Microcontroller and PC projects : Adding MMIO RAM On The RP2040

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lew247

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Joined: 23/12/2015
Location: United Kingdom
Posts: 1702
Posted: 01:43pm 21 Aug 2023
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  Quote  The problem

Can you use 8MB of external RAM with RP2040, memory mapped, like real memory?


The Answer

Towards a solution

First of all, how would you boot without persistent memory? I solved this by having both a flash and a RAM onboard. RP2040 only has a single nCS pin for SSI and only a single memory mapped address range, so we'll not be able to use them both. The idea is to boot from flash, copy flash to the start of RAM, and continue running from RAM. How do we make all of this work? It does not take much

the rest is in the article
 
Volhout
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Joined: 05/03/2018
Location: Netherlands
Posts: 4247
Posted: 02:37pm 21 Aug 2023
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  lew247 said  
  Quote  The problem

Can you use 8MB of external RAM with RP2040, memory mapped, like real memory?


The Answer

Towards a solution

First of all, how would you boot without persistent memory? I solved this by having both a flash and a RAM onboard. RP2040 only has a single nCS pin for SSI and only a single memory mapped address range, so we'll not be able to use them both. The idea is to boot from flash, copy flash to the start of RAM, and continue running from RAM. How do we make all of this work? It does not take much

the rest is in the article


When I understand correctly using external RAM for variables is not very efficient.
It takes 363 cyles (at 133MHz that is 3usec) to write a single 32 bit value ot RAM.
Block writes are seriously faster (up to 36Mbyte/second whis is around 0.25usec per word). But it is a nice "proof of concept". Nobody thought it could be done, and someone proves...it can be done..
PicomiteVGA PETSCII ROBOTS
 
Mixtel90

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Joined: 05/10/2019
Location: United Kingdom
Posts: 6798
Posted: 03:13pm 21 Aug 2023
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This is a "because it's there" project. :) How usable it would be with those restrictions on write speed is something else.
Mick

Zilog Inside! nascom.info for Nascom & Gemini
Preliminary MMBasic docs & my PCB designs
 
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