Data is read/written from/to SPI Flash through wear-levelling APIs. Wear leveling is a technique that helps to distribute wear and tear among sectors more evenly without requiring any attention from the user. As a result, it helps in extending the life of each sector of the Flash memory.
1. USB which accesses the ESP MSC Partition is unplugged initially and the board is powered-on.
- Result: Host PC can't access the partition over USB MSC. Application example can perform operations (read, write) on partition.
2. USB which accesses the ESP MSC Partition is already plugged-in at boot time.
- Result: Host PC recongnize it as removable device and can access the partition over USB MSC. Application example can't perform any operation on partition.
- Result: Host PC can't access the partition over USB MSC. Application example can perform operations (read, write) on partition.
4. USB which accesses the ESP MSC Partition is plugged-in at boot-up. It is then unplugged(removed) from Host PC manually by user.
- Result: The behaviour is different for bus-powered devices and self-powered devices
- (a) Bus-Powered devices - Both Host PC as well as application example can't access the partition over USB MSC. Here, the device will be Powered-off.
- (b) Self-Powered devices - Here, the device can be powered-on even after unplugging the device from Host PC. These behaviour can be further categorize in two ways:
- (i) Self-Powered Devices without VBUS monitoring - Both Host PC as well as application example can't access the partition over USB MSC.
- (ii) Self-Powered Devices with VBUS monitoring - Host PC can't access the partition over USB MSC. Application example can perform operations (read, write) on partition. Here, in ``tinyusb_config_t`` user must set ``self_powered`` to ``true`` and ``vbus_monitor_io`` to GPIO number (``VBUS_MONITORING_GPIO_NUM``) that will be used for VBUS monitoring.
_Note:_ In case your board doesn't have micro-USB connector connected to USB-OTG peripheral, you may have to DIY a cable and connect **D+** and **D-** to the pins listed below.
See common pin assignments for USB Device examples from [upper level](../../README.md#common-pin-assignments).
Next, for Self-Powered Devices with VBUS monitoring, user must set ``self_powered`` to ``true`` and ``vbus_monitor_io`` to GPIO number (``VBUS_MONITORING_GPIO_NUM``) that will be used for VBUS monitoring.
### Additional Pin assignments for ESP32-S3 for accessing SD MMC Card
On ESP32-S3, SDMMC peripheral is connected to GPIO pins using GPIO matrix. This allows arbitrary GPIOs to be used to connect an SD card. In this example, GPIOs can be configured in two ways:
1. Using menuconfig: Run `idf.py menuconfig` in the project directory, open "USB DEV MSC Example Configuration" and select "SDMMC CARD" for "Storage Media Used".
2. In the source code: See the initialization of ``sdmmc_slot_config_t slot_config`` structure in the example code.
The table below lists the default pin assignments.
When using an ESP32-S3-USB-OTG board, this example runs without any extra modifications required. Only an SD card needs to be inserted into the slot.
ESP32-S3 pin | SD card pin | Notes
--------------|-------------|------------
GPIO36 | CLK | 10k pullup
GPIO35 | CMD | 10k pullup
GPIO37 | D0 | 10k pullup
GPIO38 | D1 | not used in 1-line SD mode; 10k pullup in 4-line mode
GPIO33 | D2 | not used in 1-line SD mode; 10k pullup in 4-line mode
GPIO34 | D3 | not used in 1-line SD mode, but card's D3 pin must have a 10k pullup
By default, this example uses 4 line SD mode, utilizing 6 pins: CLK, CMD, D0 - D3. It is possible to use 1-line mode (CLK, CMD, D0) by changing "SD/MMC bus width" in the example configuration menu (see `CONFIG_EXAMPLE_SDMMC_BUS_WIDTH_1`).
Note that even if card's D3 line is not connected to the ESP chip, it still has to be pulled up, otherwise the card will go into SPI protocol mode.
1. By default, the example will compile to access SPI Flash as storage media. Here, SPI Flash Wear Levelling WL_SECTOR_SIZE is set to 512 and WL_SECTOR_MODE is set to PERF in Menuconfig.
2. In order to access SD MMC card as storage media, configuration has to be changed using `idf.py menuconfig`:
Mass Storage Devices are one of the most common USB devices. It use Mass Storage Class (MSC) that allow access to their internal data storage.
In this example, ESP chip will be recognised by host (PC) as Mass Storage Device.
Upon connection to USB host (PC), the example application will initialize the storage module and then the storage will be seen as removable device on PC.
