Designing Embedded Hardware, 2nd Edition

Memory

Memory is used to hold data and software for the processor. There is a variety of memory types, and often a mix is used within a single system. Some memory will retain its contents while there is no power, yet will be slow to access. Other memory devices will be high-capacity, yet will require additional support circuitry and will be slower to access. Still other memory devices will trade capacity for speed, yielding relatively small devices, yet will be capable of keeping up with the fastest of processors.

Memory chips can be organized in two ways, either in word-organized or bit-organized schemes. In the word-organized scheme, complete nybbles, bytes, or words are stored within a single component, whereas with bit-organized memory, each bit of a byte or word is allocated to a separate component (Figure 1-8).

Figure 1-8. Eight bit-organized 8×1 devices and one word-organized 8×8 device

Memory chips come in different sizes, with the width specified as part of the size description. For instance, a DRAM (dynamic RAM) chip might be described as being 4M×1 (bit-organized), whereas a SRAM (static RAM) may be 512K×8 (word-organized). In both cases, each chip has exactly the same storage capacity, but organized in different ways. In the DRAM case, it would take eight chips to complete a memory block for an 8-bit data bus, whereas the SRAM would only require one chip.However, because the DRAMs are organized in parallel, they are accessed simultaneously. The final size of the DRAM block is (4M×1)×8 devices, which is 32M. It is common practice for multiple DRAMs to be placed on a memory module. This is the common way that DRAMs are installed in standard computers.

The common widths for memory chips are x1, x4, and x8, although x16 devices are available. A 32-bit-wide bus can be implemented with thirty-two x1 devices, eight x4 devices, or four x8 devices.

RAM

RAM stands for Random Access Memory. This is a bit of a misnomer, since most (all) computer memory may be considered "random access." RAM is the "working memory" in the computer system. It is where the processor may easily write data for temporary storage. RAM is generally volatile, losing its contents when the system loses power. Any information stored in RAM that must be retained must be written to some form of permanent storage before the system powers down. There are special nonvolatile RAMs that integrate a battery-backup system, such that the RAM remains powered even when the rest of the computer system has shut down.

RAMs generally fall into two categories: static RAM (also known as SRAM) and dynamic RAM (also known as DRAM).

SRAMs use pairs of logic gates to hold each bit of data. SRAMs are the fastest form of RAM available, require little external support circuitry, and have relatively low power consumption. Their drawbacks are that their capacity is considerably less than DRAM, while being much more expensive. Their relatively low capacity requires more chips to be used to implement the same amount of memory. A modern PC built using nothing but SRAM would be a considerably bigger machine and would cost a small fortune to produce. (It would be very fast, however.)

DRAM uses arrays of what are essentially capacitors to hold individual bits of data. The capacitor arrays will hold their charge only for a short period before it begins to diminish. Therefore, DRAMs need continuous refreshing, every few milliseconds or so. This perpetual need for refreshing requires additional support and can delay processor access to the memory. If a processor access conflicts with the need to refresh the array, the refresh cycle must take precedence.

DRAMs are the highest-capacity memory devices available and come in a wide and diverse variety of subspecies. Interfacing DRAMs to small microcontrollers is generally not possible, and certainly not practical. Most processors with large address spaces include support for DRAMs. Connecting DRAMs to such processors is simply a case of "connecting the dots" (or pins, as the case may be). For those processors that do not include DRAM support, special DRAM controller chips are available that make interfacing the DRAMs very simple indeed.

Many processors have instruction and/or data caches , which store recent memory accesses. These caches are (often, but not always) internal to the processors and are implemented with fast memory cells and high-speed data paths. Instruction execution normally runs out of the instruction cache, providing for fast execution. The processor is capable of rapidly reloading the caches from main memory should a cache miss occur. Some processors have logic that is able to anticipate a cache miss and begin the cache reload prior to the cache miss occurring. Caches are implemented using very fast SRAM and are most often used in large systems to compensate for the slowness of DRAM.

ROM

ROM stands for Read-Only Memory. This is also a bit of a misnomer, since many (modern) ROMs can also be written to. ROMs are nonvolatile memory, requiring no power to retain their contents. They are generally slower than RAM, and considerably slower than fast static RAM.

The primary purpose of ROM within a system is to hold the code (and sometimes data) that needs to be present at power-up. Such software is generally known as firmware and contains software to initialize the computer by placing I/O devices into a known state. It may contain either a bootloader program to load an operating system off disk or network or, in the case of an embedded system, it may contain the application itself.

Many microcontrollers contain on-chip ROM, thereby reducing component count and simplifying system design.

Standard ROM is fabricated (in a simplistic sense) from a large array of diodes. The unwritten bit state for a ROM is all 1s, each byte location reading as 0xFF. The process of loading software into a ROM is known as burning the ROM. This term comes from the fact that the programming process is performed by passing a sufficiently large current through the appropriate diodes to "blow them," or burn them, thereby creating a zero at that bit location. A device known as a ROM burner can accomplish this, or, if the system supports it, the ROM may be programmed in-circuit. This is known as In-System Programming (ISP) or, sometimes, In-Circuit Programming (ICP).

One-Time Programmable (OTP) ROMs, as the name implies, can be burned once only. Computer manufacturers typically use them in systems where the firmware is stable and the product is shipping in bulk to customers. Mask-programmable ROMs are also one-time programmable, but unlike OTPs, they are burned by the chip manufacturer prior to shipping. Like OTPs, they are used once the software is known to be stable and have the advantage of lowering production costs for large shipments.

EPROM

OTP ROMs are great for shipping in final products, but they are wasteful for debugging, since with each iteration of code change, a new chip must be burned and the old one thrown away. As such, OTPs make for a very expensive development option. No sane person uses OTPs for development work.

A (slightly) better choice for system development and debugging is the Erasable Programmable Read-Only Memory, or EPROM. Shining ultraviolet light through a small window on the top of the chip can erase the EPROM, allowing it to be reprogrammed and reused. They are pin- and signal-compatible with comparable OTP and mask devices. Thus, an EPROM can be used during development, while OTPs can be used in production with no change to the rest of the system.

EPROMs and their equivalent OTP cousins range in capacity from a few kilobytes (exceedingly rare these days) to a megabyte or more.

The drawback with EPROM technology is that the chip must be removed from the circuit to be erased, and the erasure can take many minutes to complete. The chip is then inserted into the burner, loaded with software, and then placed back in-circuit. This can lead to very slow debug cycles. Further, it makes the device useless for storing changeable system parameters. EPROMs are relatively rare these days. You can still buy them, but flash-based memory (to be discussed shortly) is far more common and is the medium of choice.

EEROM

EEROM is Electrically Erasable Read-Only Memory, also known as EEPROM (Electrically Erasable Programmable Read-Only Memory). Very rarely, it is also called Electrically Alterable Read-Only Memory (EAROM). EEROM can be pronounced as either "e-e ROM" or "e-squared ROM," or sometimes just "e-squared" for short.

EEROMs can be erased and reprogrammed in-circuit. Their capacity is significantly smaller than standard ROM (typically only a few kilobytes), and so they are not suited to holding firmware. Instead, they are typically used for holding system parameters and mode information to be retained during power-off.

It is common for many microcontrollers to incorporate a small EEROM on-chip for holding system parameters. This is especially useful in embedded systems and may be used for storing network addresses, configuration settings, serial numbers, servicing records, and so on.

Flash

Flash is the newest ROM technology and is now dominant. Flash memory has the reprogrammability of EEROM and the large capacity of standard ROMs. Flash chips are sometimes referred to as "flash ROMs" or "flash RAMs." Since they are not like standard ROMs or standard RAMs, I prefer just to call them "flash" and save on the confusion.

Flash is normally organized as sectors and has the advantage that individual sectors may be erased and rewritten without affecting the contents of the rest of the device. Typically, before a sector can be written, it must be erased. It can't just be written over as with a RAM.

There are several different flash technologies, and the erasing and programming requirements of flash devices vary from manufacturer to manufacturer.