Problems with AVR reading and writing external RAM-EEWORLD

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Problems when AVR reads and writes external RAM

Question:
1. For AT90S8515, the address of external RAM must start from 0x0260? 2. I use ICE200 to emulate 8515 to read and write external RAM. The RAM size is 32K Bytes, and the address is mapped to 0x0000-0x7fff. PC7 is used as its chip select. Set SRE to 1. When I access 0x0300, PC7 is low. This is correct, but when I do not access the 0x0000-0x7fff address space (for example, entering an infinite loop), the problem occurs. At this time, PC7 is still low, which should be high. Is this the difference between AVR and 51, or am I wrong? Answer: 1. For AT90S8515, the address of external RAM must start from 0x0260. This is no problem. The address accessed from 0x0000-0x025F is the internal RAM. 2. When you have accessed 0x0300, PC7 is low. When you do not access 0x0000-0x7FFF, there is no instruction to set PC7 high, so PC7 is still low. But it does not matter, because when you access 0x0000-0x025F, RD and WR are not there, so even if the external RAM is selected, no reading or writing will occur. When you access 0x0260-0x7FFF, RD and WR are automatically generated. So it is OK to directly ground the CS of the external RAM.

Keywords：AVR Reference address：Problems with AVR reading and writing external RAM

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