Fix SDIO DMA (finally)

git-svn-id: svn://svn.code.sf.net/p/nuttx/code/trunk@4409 42af7a65-404d-4744-a932-0658087f49c3

1 files changed, 14 insertions, 0 deletions

diff --git a/nuttx/configs/stm3240g-eval/README.txt b/nuttx/configs/stm3240g-eval/README.txt
index ef26c0088..e4902947e 100755
--- a/nuttx/configs/stm3240g-eval/README.txt
+++ b/nuttx/configs/stm3240g-eval/README.txt
@@ -720,9 +720,23 @@ Where <subdir> is one of the following:
        configuration.
 
     2. RS-232 is disabled, but Telnet is still available for use as a console.
+       Since RS-232 and SDIO use the same pins (one controlled by JP22), RS232
+       and SDIO cannot be used concurrently.
 
     3. This configuration requires that jumper JP22 be set to enable SDIO operation.
 
+    4. In order to use SDIO without overruns, DMA must be used.  The STM32 F4
+       has 192Kb of SRAM in two banks:  112Kb of "system" SRAM located at
+       0x2000:0000 and 64Kb of "TCM" SRAM located at 0x1000:0000. It appears
+       that you cannot perform DMA from TCM SRAM.  The work around that I have now
+       is simply to omit the 64Kb of TCM SRAM from the heap so that all memory is
+       allocated from System SRAM.  This is done by setting: 
+       
+       CONFIG_MM_REGIONS=1
+
+       Then DMA works fine. The downside is, of course, is that we lose 64Kb
+       of precious SRAM.
+
   ostest:
   ------
     This configuration directory, performs a simple OS test using