title: MDMA & internal FLASH R/W on STM32H7
body: <p>Good morning,</p>
  
  <p>We're using an STM32H7 to acquire simoultaneously 3 ADC channels 16-bit @ 5MSPS, and we store data in an external eMMC. Everything worth seamlessly as follow :
  3 slave-SPI with 3 DMA to internal flip/flop buffers ; MDMA to interleave data to 2 flip flop eMMC buffers ; IDMA of SDMMC peripheral to eMMC.
  We have some unexpected wait-times introduced by the eMMC that requires to buffer more data internally than expected, and, as we do not have enough internal RAM to buffer enough during eMMC waits (and cannot add external RAM on FMC, or nvSRAM on Quad-SPI, ... due to integration constraints), we looked at using MDMA with internal FLASH memory to buffer data temporarily.
  Is this such a bad idea ? FLASH on STM32H7 is splitted in 2 banks, the first one can be used for program memory (so no conflict and no processor halt during DMA access), and there is 2 AXI controllers, MDMA is also an AXI peripheral...Maybe excessive slowness ? Has any of you ever tried this ?</p>
  
  <p>As we read the reference manual (chapter 13 : MDMA controller (MDMA)) :</p>
  
  <blockquote>
    <p>The master direct memory access (MDMA) is used in order to provide high-speed data transfer between memory and memory or between peripherals and memory. Data can be quickly moved by the MDMA without any CPU action. This keeps the CPU resources free for other operations.</p>
  </blockquote>
  
  <p>But MDMA may only be able to move data from AXI to AHB or AHB to AXI, not AXI to AXI ?</p>
  
  <p>Best regards,
  Clément</p>