TAILIEUCHUNG - Program C Ansi Programming Embedded Systems in C and C++ phần 4

Tham khảo tài liệu 'program c ansi programming embedded systems in c and c++ phần 4', công nghệ thông tin, kỹ thuật lập trình phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | DRAM Controllers If your embedded system includes DRAM there is probably a DRAM controller on board or on-chip as well. The DRAM controller is an extra piece of hardware placed between the processor and the memory chips. Its main purpose is to perform the refresh operations required to keep your data alive in the DRAM. However it cannot do this properly without some help from you. One of the first things your software must do is initialize the DRAM controller. If you do not have any other RAM in the system you must do this before creating the stack or heap. As a result this initialization code is usually written in assembly language and placed within the hardware initialization module. Almost all DRAM controllers require a short initialization sequence that consists of one or more setup commands. The setup commands tell the controller about the hardware interface to the DRAM and how frequently the data there must be refreshed. To determine the initialization sequence for your particular system consult the designer of the board or read the databooks that describe the DRAM and DRAM controller. If the DRAM in your system does not appear to be working properly it could be that the DRAM controller either is not initialized or has been initialized incorrectly. When deciding which type of RAM to use a system designer must consider access time and cost. SRAM devices offer extremely fast access times approximately four times faster than DRAM but are much more expensive to produce. Generally SRAM is used only where access speed is extremely important. A lower cost per byte makes DRAM attractive whenever large amounts of RAM are required. Many embedded systems include both types a small block of SRAM a few hundred kilobytes along a critical data path and a much larger block of DRAM in the megabytes for everything else. Types of ROM Memories in the ROM family are distinguished by the methods used to write new data to them usually called programming and the number of .

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