## If you are using the STM32L476, check out my repository which has a working example: https://github.com/peter-tanner/eMMC-USB-mass-storage-device-STM32L476
TLDR: Minimum configuration you should try for troubleshooting is:
- High clock divider (try 4 or 8 or even larger values)
- 1 bit wide MMC bus
- Don't initialize MMC peripheral in the USB MSC initialization function.
## 1. USB connects but then disconnects after a few seconds. The device shows momentarily in the tray, but is not assigned a driver letter.
This also shows in wireshark when the response from the device is a `GET MAX LUN Response[Malformed Packet]` packet.
Try return `USBD_OK` for the all functions except `STORAGE_GetMaxLun_FS`. It should show up in explorer with a drive letter and should not disconnect.
This means the functions interacting with the storage device are not working.
---
Make sure you are not re-initializing the MMC peripheral in the `STORAGE_Init_FS` function. This is because in the initialization of the MMC peripheral, `HAL_Delay` is called twice. The use of `HAL_Delay` while the USB device is active appears to cause issues with mounting the device.
<!-- ### 1.1 Try disable eMMC or SD card first
In my board I had eMMC, by commenting out code I isolated the issue to the `SDMMC_PowerState_ON` function which is called during the device initialization. When this code was not called the drive showed up with a drive leter, but when the code was called the drive was not mounted.
```c
(void)SDMMC_PowerState_ON(hmmc->Instance);
```
From this I concluded that any calls to `HAL_Delay` were causing the USB stack to break.
Commenting out the `HAL_Delay` in the `SDMMC_PowerState_ON` function and the one to wait 74 cycles (`HAL_Delay(1U + (74U * 1000U / (sdmmc_clk)))`) caused it to mount properly. I still have to think about what is happening since obviously the delays appear to have good reasons for existing. -->
## 2. "This request is not supported" when formatting drive, files not readable
Try changing the clock division to something larger like `4` or greater. Once it is stable try decreasing the divider to maximize speed. When I used lower values, the bus was unstable and I could not retrieve files or perform format operations.
> **EDIT: It's now working with 4-bit bus width and 0 clock div, not sure what I did to make it work but it works. I think it might be that I increased my `SYSCLK` since when I started the project it was at the default `16 MHz` since it was using the HSI instead of the HSE.**
## 3. Random files appearing when formatting AND using 4-bit mode
When a drive has already been formatted in 1-bit mode and you try to reformat it, the format fails and there appears to be random files in the apparently formatted drive
The format fails with:
> Windows was unable to complete the format
I guess my board doesn't work in 4-bit mode so I just compromised with 1-bit mode and it worked again.
> **EDIT: It's now working with 4-bit bus width and 0 clock div, not sure what I did to make it work but it works. I think it might be that I increased my `SYSCLK` since when I started the project it was at the default `16 MHz` since it was using the HSI instead of the HSE.**
## 4. DMA/Multiple block DMA not working on STM32L476RET6 STM32L476
Enable "SDMMC hardware flow control" under the SDMMC peripheral.
Make sure to switch the DMA direction each for RX and TX
Since we are doing bi-directional transfer just use the SDMMC1 DMA request type. Don't use SDMMC1_TX and SDMMC1_RX simulatneously since you need to switch the direction each transfer anyways.
The order of preemption priority is crucial, otherwise the transfer will hang. For example `HAL_MMC_GetCardState()`, will be stuck in the `HAL_MMC_CARD_SENDING` state since the interrupt priority is wrong.
> **EDIT: It's now working with 4-bit bus width and 0 clock div, not sure what I did to make it work but it works. I think it might be that I increased my `SYSCLK` since when I started the project it was at the default `16 MHz` since it was using the HSI instead of the HSE.**
>
> This caused a sizeable performance increase! However it's still quite terrible, since the eMMC module I used claims read/write speeds of 45/35 MB/s at 52 MHz in SDR mode at 3.3V. Obviously having a HAL puts overhead and this figure is probably dependant on the block size.
## 5. MMC performance is very bad, slow write speeds
One last thing to change is the packet size. Larger packet sizes will perform better (at the cost of more memory).
Change the packet size under the `USB_DEVICE` middleware parameters:
Alternatively, in `usbd_conf.h`, change:
```c
#define MSC_MEDIA_PACKET 512U
```
to some **multiple of 512**
For example:
```c
#define MSC_MEDIA_PACKET 32678U // MUST BE A MULTIPLE OF 512
```
You may need to modify your minimum stack/heap size to accomodate for the larger packet size.
The maximum is `32678` bytes
The read/write speeds improve on the previous tests, however it is still no where near the limit of the eMMC module. Currently I have not tried anything else to improve the speed, if there is any way then let me know.
## Conclusion
Overall, eMMC is mechanically robust, however from a cost perspective I wouldn't choose this in a future project unless necesary. While it appears to be cheaper than SD cards and holders, the tight tolerances require JLCPCB's 4-wire kelvin testing for the smaller vias (Unless you use 6-layer which has the pad in via for free), and requires X-ray inspection which is another fee. I wouldn't attempt soldering them manually since I've never dealt with BGA so this is a fee I had to accept.
In my next project I will try out those embedded SD card modules, they come in DFN-8 packages and should be much cheaper than eMMC while offerring high capacity than NOR flash, but they are less widely known and I would not use them in a project which is going into production, only a hobby project.
I will update this post with more cases as I run into them. Comment on your experiences debugging USB on STM32 and I'll quote them in the article.
## Sample code for eMMC and USB mass storage class device (`#define BLOCKING` for simple, non-DMA implementation)
