The display modules have several onboard memories:

• 32 MB external SDRAM (non-persistent)
• 16 MB external QSPI Flash
• 8 KB external configuration EEPROM
• 4032 byte internal EEPROM

External SDRAM¶

The DMSupport library has support for the SDRAM. It is initialized in the init() function of DMBoard and it is not a configurable option.

The SDRAM is attached to the heap and allows malloc(), realloc() and free() functions to use it.

External QSPI Flash¶

The display module has an SPI Flash Interface (SPIFI) allowing external serial flash memories to be connected with low performance penalties. A driver API included in on-chip ROM handles setup, programming and erasure and after initializing the SPIFI driver, the flash content is accessible as normal memory (in the 0x28000000..0x28ffffff memory range) using byte, halfword, and word accesses by the processor and/or DMA.Programming (i.e. writing data to the flash) is supported for up to 256 bytes at a time (which is the size of a "page" in the flash). Programming means that a bit can either be left at 0, or programmed from 1 to 0. Changing bits from 0 to 1 requires an erase operation. While bits can be individually programmed from 1 to 0, erasing bits from 0 to 1 must be done in larger chunks (manufacturer dependant but typically 4Kbyte to 64KByte at a time).

The 16 MB QSPI flash on the display modules can be used in three different ways (or a combination thereof): 1) To store program data/code for large programs. 2) To hold a file system 3) To store persistent data at runtime

To use the QSPI flash it must first be enabled in the dm_board_config.h file:

#define DM_BOARD_USE_QSPI

Program CODE/DATA¶

The linker scripts for the mbed online compiler as well as for Keil uVision automatically places read-only parts of the code in QSPI FLASH if the space in the internal flash is too small. As for normal compiling, the online compiler will generate one binary even if both internal flash and QSPI FLASH is used. The mbed HDK is responsible for writing the binary to the correct memory addresses so no special user action is needed when flashing.

The CODE/DATA will always spill over to the start of the flash.

QSPI File System¶

The DMSupport library has support for using the QSPI flash (or parts of it) as a file system. This is described in more detail here.

Note that the QSPI File System is always placed at the top/end of the flash so that it can coexist with any CODE/DATA placed by the linker scripts.

R/W Access at Runtime¶

The DMSupport library has a SPIFI class, allowing direct access to the QSPI flash at runtime:

The flash is still initialized by the init() function in DMBoard but it is then possible to use the SPIFI class to modify the flash.

#include "SPIFI.h"
...
void main() {
  // call DMBoard::instance().init() and handle return code
  ...
  
  // Write "Hello World!" into the first bytes of the SPIFI
  char buff[20] = "Hello World!";
  err = SPIFI::instance().program(0, strlen(buff)+1, buff, SPIFI::EraseAsRequired);
  if (err != SPIFI::Ok) {
    printf("Failed to write to SPIFI, error %d\n", err);
  }

  // Now verify that it can be read
  if (memcmp((char*)SPIFI::SpifiMemBase, buff, strlen(buff)+1) == 0) {
    printf("Readback from memory OK: '%s'\n", SPIFI::SpifiMemBase);
  } else {
    printf("Spifi does not contain the correct data!\n");
  } 
}

External Configuration EEPROM¶

The configuration EEPROM on the display module is used to store initialization code (i.e. the BIOS) and is reserved by the system.

Overwriting the contents of this EEPROM (address 0xAC/0xAD) will cause the board to fail in the init() function of DMBoard. To prevent this the BiosEEPROM class in the DMSupport library only have a read function.

Internal EEPROM¶

The LPC4088 MCU has an onchip 4032 byte EEPROM that can be used to store data persistently.

The DMSupport library has an InternalEEPROM class for direct access to it: