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ATMEGA640 Datasheet(PDF) 34 Page - ATMEL Corporation |
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ATMEGA640 Datasheet(HTML) 34 Page - ATMEL Corporation |
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34 / 449 page  34 ATmega640/1280/1281/2560/2561 2549K–AVR–01/07 • Bit 0 – EERE: EEPROM Read Enable The EEPROM Read Enable Signal EERE is the read strobe to the EEPROM. When the correct address is set up in the EEAR Register, the EERE bit must be written to a logic one to trigger the EEPROM read. The EEPROM read access takes one instruction, and the requested data is available immediately. When the EEPROM is read, the CPU is halted for four cycles before the next instruction is executed. The user should poll the EEPE bit before starting the read operation. If a write operation is in progress, it is neither possible to read the EEPROM, nor to change the EEAR Register. General Purpose registers GPIOR2 – General Purpose I/O Register 2 GPIOR1 – General Purpose I/O Register 1 GPIOR0 – General Purpose I/O Register 0 External Memory registers XMCRA – External Memory Control Register A • Bit 7 – SRE: External SRAM/XMEM Enable Writing SRE to one enables the External Memory Interface.The pin functions AD7:0, A15:8, ALE, WR, and RD are activated as the alternate pin functions. The SRE bit over- rides any pin direction settings in the respective data direction registers. Writing SRE to zero, disables the External Memory Interface and the normal pin and data direction set- tings are used. • Bit 6:4 – SRL2:0: Wait-state Sector Limit It is possible to configure different wait-states for different External Memory addresses. The external memory address space can be divided in two sectors that have separate wait-state bits. The SRL2, SRL1, and SRL0 bits select the split of the sectors, see Table 7 and Figure 13. By default, the SRL2, SRL1, and SRL0 bits are set to zero and the entire external memory address space is treated as one sector. When the entire SRAM Bit 7654 321 0 0x2B (0x4B) MSB LSB GPIOR2 Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0000 000 0 Bit 7654 321 0 0x2A (0x4A) MSB LSB GPIOR1 Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0000 000 0 Bit 7654 321 0 0x1E (0x3E) MSB LSB GPIOR0 Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0000 000 0 Bit 7654 321 0 “(0x74)” SRE SRL2 SRL1 SRL0 SRW11 SRW10 SRW01 SRW00 XMCRA Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0000 000 0 |
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