[STM32F4] C++ Project Tutorial – Step 5-1

SPI-Based M95M04 Serial Flash Read/Write

In the previous post, we discussed the basic SPI interface structure and how to design device-specific extension classes. In this post, we’ll extend that concept into a practical example — implementing the M95M04 Serial Flash device as a C++ class and demonstrating how to read and write data through SPI.

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M95M04 Serial Flash Overview

The M95M04 is a 4-Mbit (512KB) SPI-based serial flash memory device provided by STMicroelectronics.

• Capacity: 4Mbit (512KB)
• SPI Interface: up to 20MHz
• Page size: 256 bytes per write
• Command Set: Read(0x03), Write(0x02), Write Enable(0x06), Read Status Register(0x05), etc.
• Data Retention: Over 20 year and Over 1,000,000 write/erase cycles
• Protection: WP(Write Protect), HOLD pin supported.

Thanks to these features, M95M04 is ideal for firmware storage, log data recording, or retaining system configuration data.

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Class Design

The M95M04SPI class inherits from BaseSPI and implements the M95M04-specific protocol.

Main Member Variables

• pSPI : Pointer to the SPI interface instance
• nCS : Chip Select pin
• nWP : Write Protect pin
• nHOLD : Hold pin
• CTask : Console task for debug log output

Main Member Functions

• WriteEnable() / WriteDisable() : Switch between writable and protected states
• WriteByte(), ReadByte() : Single-byte transfer functions
• sendCommand(), sendAddress() : Send command and 24-bit address sequences
• WriteData(addr, buf, len) : Write data to a specific address
• ReadData(addr, buf, len) : Read data from a specific address
• EEPROM_Test(addr, size) : Test routine including write, read, and data verification

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Code Examples

1. Object Declaration and Initialization

#include "M95M04SPI.h"

M95M04SPI eepSpi;

...
void InitSerialFlash(void);
...
/*
 *
 */
void InitSerialFlash(void)
{
#if defined(_SPI_EEP_)
	  eepSpi = M95M04SPI(SPI6, EEP_nCS_1, &CTask);
	  eepSpi.setWPPin(EEP_nWP_1);
	  eepSpi.setHOLDPin(EEP_nHOLD_1);
#endif
}

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2. Writing Data

this->pEEPSPI->WriteData(this->pEEPos->FlashingSize.getPosition(), tmpBuf, this->pEEPos->FlashingSize.getLength());

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3. Reading Data

this->pEEPSPI->ReadData(this->pEEPos->bDebug.getPosition(), (uint8_t *)&(this->pSysinfo->General.bDebug), this->pEEPos->bDebug.getLength());

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4. Using the Test Function

The EEPROM_Test() routine automatically writes and reads data at a specified address, verifying correctness and printing logs to the console.

void AT24CTask::EEPROMEraseAll(void)
{
	int i, j;

	uint8_t data[32];
	for(i=0; i<32; i++)
		data[i] = 0xFF;

	for(i=0; i<16*1024; i++)
	{
	      this->pEEPSPI->WriteData(32*i, (uint8_t *)&i, 1);
	}

	for(i=0; i<16*1024; i++)
	{
		this->pEEPSPI->ReadData(32*i, data, 32);
		for(j=0; j<32; j++)
			this->CTask->PRINTF((char *)"%02X ", data[j]);
		this->CTask->PRINTF((char *)"\r\n");
		this->CTask->flushTxBuf();
	}

}

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Troubleshooting

1. Write not working
   • Cause: Missing Write Enable (WREN) command
   • Solution: Verify WriteData() handles WREN before data transmission
2. Read data mismatch
   • Cause: Address transmission error (24-bit addressing issue)
   • Solution: Check implementation of sendAddress() function
3. No SPI response
   • Cause: CS pin not toggled correctly
   • Solution: Inspect csEnable() and csDisable() functions

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Conclusion

In this post, we implemented the M95M04SPI class derived from BaseSPI and demonstrated how to communicate with a real SPI-based Serial Flash device.

Key Takeaways:

• M95M04 is a 512KB SPI flash that requires command-based access.
• M95M04SPI class implements full read, write, and test routines.
• Using routines like EEPROM_Test() makes it easy to verify data integrity in real-world projects.