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AVR100: Accessing the EEPROM Features • Random Read/Write Sequential Read/Write Runable Test/Example Program Introduction This application note contains routines for access of the EEPROM memory in the AVR Microcontroller. Two types of Read/Write access has been implemented: Random Read/Write: The user must set up both data and address before calling the Read or Write routine. Sequential Read/Write: The user needs only to set up the data to be Read/Written. The current EEPROM address is automatically incremented prior to access. The address has to be set prior to writing the first byte in a sequense. The application note contains four routines which are described in detail in the follow- ing sections. This application note contains routines for accessing the EEPROM in all AVR devices. Note: In the latest devices the EEWE bit in EECR is called EEPE, and the EEMWE is called EEMPE. Also in the latest devices the EECR consist of two extra bits to set the Programming Mode, EEPM0 and EEPM1. These two bits needs to be initialized before EEPE is set. EEdwr – Data to be written EEawr – Address low byte to write EEawrh – Address high byte to write Random Write – Subroutine “EEWrite” Three register variables must be set up prior to calling this routine: The subroutine waits until the EEPROM is ready to be programmed by polling the EEPROM Write Enable – EEWE bit in the EEPROM Control Register – EECR. When EEWE is zero, the contents of EEdwr is transferred to the EEPROM Data Register – EEDR, and the contents of EEawrh:EEawr is transferred to the EEPROM Address Register – EEARH:EEARL. First the EEPROM Master Write Enable – EEMWE is set, followed by the EEPROM write strobe EEWE in EECR. See Figure 1. 8-bit Microcontroller Application Note Rev. 0932C–AVR–09/05 1

Figure 1. “EEWrite” Flow Chart EEWrite EEWE = 0? N Y EEARH:EEARL ! EEawh:EEawr EEDR ! EEdwr Global Interrupt Disable Set EEMWE Set EEWE Global Interrupt Enable RETURN Random Read – Subroutine “EERead” Prior to calling this routine, two register variables must be set up: EEard – Address of low byte to read from EEardh – Address of high byte to read from The subroutine waits until the EEPROM is ready to be accessed by polling the EEWE bit in the EEPROM Control Register – EECR. When EEWE is zero, the subroutine and transfers the contents of EEardh:EEard to the EEPROM Address Register – EEARH:EEARL. It then sets the EEPROM Read Strobe – EERE. In the next instruction the content of the EEDR Register is tranferred to the register vari- able EEdrd. See Figure 2. 2 AVR100 0932C–AVR–09/05

Figure 2. “EERead” Flow Chart AVR100 EERead EEWE = 0? N Y EEARH:EEARL ! EEardh:EEard SET EERE EEdrd ! EEDR RETURN Sequential Write – Subroutine “EEWrite_seq” Prior to calling this routine, one register variable must be set up: EEdwr_s – Data to write The subroutine waits until the EEPROM is ready to be programmed by polling the EEWE bit in the EEPROM Control Register – EECR. When EEWE is zero and the con- tents of the EEPROM Address Register – EEARH:EEARL are read into the register variable EEWTMPH:EEWTMP. EEwtmp is incremented and written back to EEARH:EEARL. This increments the current EEPROM address by one. The contents of EEdwr is then transferred to the EEPROM Data Register – EEDR, before EEWE in EECR is set, and then EEMWE is set. See Figure 3. 0932C–AVR–09/05 3

Figure 3. “EEWrite_seq” Flow Chart EEWrite_seq EEWE = 0? N Y EEARH:EEARL ! EE ARH:EEARL +1 EEDR ! EEdwr Disable Global Interrupt SET EEMWE SET EEWE Enable Global Interrupt RETURN Sequential Read – Subroutine “EERead_seq” The subroutine waits until the EEPROM is ready to be accessed by polling the EEWE bit in the EEPROM Control Register – EECR. The subroutine then increments the current EEPROM address by performing the following operation: Transfer EEAR to the register variable EERTMPH:EERTMP, increments this register and writes the new address back to EEARH:EEARL. The routine then sets the EEPROM Read Strobe – EERE twice. Finally, the EEPROM data is transferred from EEDR to the register variable EEdrd_s. See Figure 4. 4 AVR100 0932C–AVR–09/05

Figure 4. “EERead_seq” Flow Chart for 8515 AVR100 EERead_seq EEWE = 0? N Y EEARH:EEARL ! EEARH:EEARL +1 SET EERE EEdrd ! EEDR RETURN Optimization for different devices Test Program Not all the instructions are necessary for all devices. If the device has an EEPROM of 256 bytes or less, the high address of the EEPROM Address Register doesn’t need to be changed. On the AT90S1200, the EEMWE bit in the EEGR doesn’t have to be set. See the section EEPROM Read/Write in the datasheet for further information. The application note assembly file contains a complete program which calls the four subroutines as a test of operation, and also as an example of usage. The test program is suitable for running in AVR Studio®. The test programs contains comments on how to port the code to work on any AVR- part. Note: If the code initiates a write to EEPROM shortly after Reset, keep in mind the following: If EEPROM contents are programmed during the manufacturing process, the MCU might change the code shortly after programming. When the programmer then verifies the EEPROM contents, this might fail because the EEPROM contents have already been modified by the MCU. Also notice that some In-System Programmers will allow the MCU to execute a short time between each step in the programming and verification process. Table 1. CPU and Memory Usage Function Code Size Cycles Example Register Usage Description EEWrite EERead 10 words 7 words EEWrite_seq 13 words EERead_seq 10 words 15 11 19 17 R16, R17, R18 EEPROM Random Location Write R0, R17, R18 EEPROM Random Location Read R24, R25, R18 R0, R24, R25 EEPROM Sequential Location Write EEPROM Sequential Location Read 0932C–AVR–09/05 5

Table 1. CPU and Memory Usage (Continued) Function Code Size Cycles Reset Main TOTAL 8 words 39 words 87 words 8 – – Example Register Usage R16 Description Example Initialisation R16, R19, R20 Example Program R0, R16, R17, R18, R19, R20, R24, R25 – Table 2. Peripheral Usage Peripheral 8 I/O Pins 1 I/O Pin Description LEDs (example only) Button (example only) 10 bytes EEPROM Target EEPROM Locations (example only) Interrupts Enabled – – – 6 AVR100 0932C–AVR–09/05

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