嵌入式系统常见内存种类

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Many types of memory devices are available for use in modern computer systems. As an embedded software engineer, you must be aware of the differences between them and understand how to use each type effectively. In our discussion, we will approach these devices from a software viewpoint. As you are reading, try to keep in mind that the development of these devices took several decades and that there are significant physical differences in the underlying hardware. The names of the memory types frequently reflect the historical nature of the development process and are often more confusing than insightful.
Most software developers think of memory as being either random-access (RAM) or read-only (ROM). But, in fact, there are subtypes of each and even a third class of hybrid memories. In a RAM device, the data stored at each memory location can be read or written, as desired. In a ROM device, the data stored at each memory location can be read at will, but never written. In some cases, it is possible to overwrite the data in a ROM-like device. Such devices are called hybrid memories because they exhibit some of the characteristics of both RAM and ROM.
Figure 6-1
provides a classification system for the memory devices that are commonly found in embedded systems.
Figure 6-1. Common memory types in embedded systems

1 Types of RAM
There are two important memory devices in the RAM family: SRAM and DRAM. The main difference between them is the lifetime of the data stored. SRAM (static RAM) retains its contents as long as electrical power is applied to the chip. However, if the power is turned off or lost temporarily then its contents will be lost forever. DRAM (dynamic RAM), on the other hand, has an extremely short data lifetime-usually less than a quarter of a second. This is true even when power is applied constantly.
In short, SRAM has all the properties of the memory you think of when you hear the word RAM. Compared to that, DRAM sounds kind of useless. What good is a memory device that retains its contents for only a fraction of a second? By itself, such a volatile memory is indeed worthless. However, a simple piece of hardware called a DRAM controller can be used to make DRAM behave more like SRAM. (See
DRAM Controllers
later in this chapter.) The job of the DRAM controller is to periodically refresh the data stored in the DRAM. By refreshing the data several times a second, the DRAM controller keeps the contents of memory alive for as long as they are needed. So, DRAM is as useful as SRAM after all.
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.
2 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 times they can be rewritten. This classification reflects the evolution of ROM devices from hardwired to one-time programmable to erasable-and-programmable. A common feature across all these devices is their ability to retain data and programs forever, even during a power failure.
The very first ROMs were hardwired devices that contained a preprogrammed set of data or instructions. The contents of the ROM had to be specified before chip production, so the actual data could be used to arrange the transistors inside the chip! Hardwired memories are still used, though they are now called "masked ROMs" to distinguish them from other types of ROM. The main advantage of a masked ROM is a low production cost. Unfortunately, the cost is low only when hundreds of thousands of copies of the same ROM are required.
One step up from the masked ROM is the PROM (programmable ROM), which is purchased in an unprogrammed state. If you were to look at the contents of an unprogrammed PROM, you would see that the data is made up entirely of 1's. The process of writing your data to the PROM involves a special piece of equipment called a device programmer. The device programmer writes data to the device one word at a time, by applying an electrical charge to the input pins of the chip. Once a PROM has been programmed in this way, its contents can never be changed. If the code or data stored in the PROM must be changed, the current device must be discarded. As a result, PROMs are also known as one-time programmable (OTP) devices.
An EPROM (erasable-and-programmable ROM) is programmed in exactly the same manner as a PROM. However, EPROMs can be erased and reprogrammed repeatedly. To erase an EPROM, you simply expose the device to a strong source of ultraviolet light. (There is a "window" in the top of the device to let the ultraviolet light reach the silicon.) By doing this, you essentially reset the entire chip to its initial-unprogrammed-state. Though more expensive than PROMs, their ability to be reprogrammed makes EPROMs an essential part of the software development and testing process.
3 Hybrid Types
As memory technology has matured in recent years, the line between RAM and ROM devices has blurred. There are now several types of memory that combine the best features of both. These devices do not belong to either group and can be collectively referred to as hybrid memory devices. Hybrid memories can be read and written as desired, like RAM, but maintain their contents without electrical power, just like ROM. Two of the hybrid devices, EEPROM and Flash, are descendants of ROM devices; the third, NVRAM, is a modified version of SRAM.
EEPROMs are electrically-erasable-and-programmable. Internally, they are similar to EPROMs, but the erase operation is accomplished electrically, rather than by exposure to ultraviolet light. Any byte within an EEPROM can be erased and rewritten. Once written, the new data will remain in the device forever-or at least until it is electrically erased. The tradeoff for this improved functionality is mainly higher cost. Write cycles are also significantly longer than writes to a RAM, so you wouldn't want to use an EEPROM for your main system memory.
Flash memory is the most recent advancement in memory technology. It combines all the best features of the memory devices described thus far. Flash memory devices are high density, low cost, nonvolatile, fast (to read, but not to write), and electrically reprogrammable. These advantages are overwhelming and the use of Flash memory has increased dramatically in embedded systems as a direct result. From a software viewpoint, Flash and EEPROM technologies are very similar. The major difference is that Flash devices can be erased only one sector at a time, not byte by byte. Typical sector sizes are in the range of 256 bytes to 16 kilobytes. Despite this disadvantage, Flash is much more popular than EEPROM and is rapidly displacing many of the ROM devices as well.
The third member of the hybrid memory class is NVRAM (nonvolatile RAM). Nonvolatility is also a characteristic of the ROM and hybrid memories discussed earlier. However, an NVRAM is physically very different from those devices. An NVRAM is usually just an SRAM with a battery backup. When the power is turned on, the NVRAM operates just like any other SRAM. But when the power is turned off, the NVRAM draws just enough electrical power from the battery to retain its current contents. NVRAM is fairly common in embedded systems. However, it is very expensive-even more expensive than SRAM-so its applications are typically limited to the storage of only a few hundred bytes of system-critical information that cannot be stored in any better way.
Table 6-1
summarizes the characteristics of different memory types.
Table 6-1. Memory Device Characteristics
Memory Type
Volatile?
Writeable?
Erase Size
Erase Cycles
Relative Cost
Relative Speed
SRAM
yes
yes
byte
unlimited
expensive
fast
DRAM
yes
yes
byte
unlimited
moderate
moderate
Masked ROM
no
no
n/a
n/a
inexpensive
fast
PROM
no
once, with programmer
n/a
n/a
moderate
fast
EPROM
no
yes, with programmer
entire chip
limited (see specs)
moderate
fast
EEPROM
no
yes
byte
limited (see specs)
expensive
fast to read, slow to write
Flash
no
yes
sector
limited (see specs)
moderate
fast to read, slow to write
NVRAM
no
yes
byte
none
expensive
fast


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