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ST6208CB1 Arkusz danych(PDF) 9 Page - STMicroelectronics |
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ST6208CB1 Arkusz danych(HTML) 9 Page - STMicroelectronics |
9 / 70 page 9/70 ST62T08C/T09C ST62T10C/T20C/E20C MEMORY MAP (Cont’d) 1.3.5 Data Window Register (DWR) The Data read-only memory window is located from address 0040h to address 007Fh in Data space. It allows direct reading of 64 consecutive bytes locat- ed anywhere in program memory, between ad- dress 0000h and 0FFFh (top memory address de- pends on the specific device). All the program memory can therefore be used to store either in- structions or read-only data. Indeed, the window can be moved in steps of 64 bytes along the pro- gram memory by writing the appropriate code in the Data Window Register (DWR). The DWR can be addressed like any RAM location in the Data Space, it is however a write-only regis- ter and therefore cannot be accessed using single- bit operations. This register is used to position the 64-byte read-only data window (from address 40h to address 7Fh of the Data space) in program memory in 64-byte steps. The effective address of the byte to be read as data in program memory is obtained by concatenating the 6 least significant bits of the register address given in the instruction (as least significant bits) and the content of the DWR register (as most significant bits), as illustrat- ed in Figure 5 below. For instance, when address- ing location 0040h of the Data Space, with 0 load- ed in the DWR register, the physical location ad- dressed in program memory is 00h. The DWR reg- ister is not cleared on reset, therefore it must be written to prior to the first access to the Data read- only memory window area. Data Window Register (DWR) Address: 0C9h — Write Only Bits 6, 7 = Not used. Bit 5-0 = DWR5-DWR0: Data read-only memory Window Register Bits. These are the Data read- only memory Window bits that correspond to the upper bits of the data read-only memory space. Caution: This register is undefined on reset. Nei- ther read nor single bit instructions may be used to address this register. Note: Care is required when handling the DWR register as it is write only. For this reason, the DWR contents should not be changed while exe- cuting an interrupt service routine, as the service routine cannot save and then restore the register’s previous contents. If it is impossible to avoid writ- ing to the DWR during the interrupt service routine, an image of the register must be saved in a RAM location, and each time the program writes to the DWR, it must also write to the image register. The image register must be written first so that, if an in- terrupt occurs between the two instructions, the DWR is not affected. Figure 5. Data read-only memory Window Memory Addressing 70 - - DWR5 DWR4 DWR3 DWR2 DWR1 DWR0 DATA ROM WINDOW REGISTER CONTENTS DATA SPACE ADDRESS 40h-7Fh IN INSTRUCTION PROGRAM SPACE ADDRESS 765432 0 543210 543210 READ 1 6 7 8 9 10 11 0 1 VR01573C 12 1 0 DATA SPACE ADDRESS : : 59h 0 0 0 0 1 00 1 1 1 Example: (DWR) DWR=28h 11 00 0 00 00 1 ROM ADDRESS:A19h 11 13 0 1 97 |
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