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etching the last metal layer during the wafer manufacturing process. When an order comes in for a batch of microcontrollers with a ROM with a customer-speci ed application, these wafers are pulled from stock and the last metal layer is exposed to a custom mask made from the customer-supplied software program, which makes the connections to the memory cells that turns them into ones or zeros. This is known as mask ROM programming. With the program put into the chip, the customer will have a device they can use in their product without having to load a program into it later. ROM contents typically cannot be read out of the microcontroller to thwart others trying to pirate or reverse engineer the product. There are some signi cant downsides to buying microcontrollers with mask ROM. The rst two are the cost and lead time required to have the customized chips built. While the actual piece price of a ROM program memory chip is less than a device with a customer (or eld) programmable program memory, the nonrecurring expense (NRE) costs of getting the mask made makes this process cost effective in lot sizes of 10,000 or more chips. The lead time for getting mask ROM devices built is typically on the order of six to ten weeks. For certain applications, such as for the automotive market, the downsides of mask ROM microcontrollers do not take away from the cost advantages; here, the parts are ordered well in advance and with one or more per vehicle, a large guaranteed order is assured. It should be obvious that going straight to mask ROM for a product or project is not an ef cient method of nding out whether the program works. To provide a method of loading a program into a device outside the factory in short order, programmable read-only memory (PROM) was invented. The most popular form of PROM is known as fuseable link, in which high current is optionally passed through small metal connections to burn them out and cause the memory cell they are associated with to be programmed to a one or zero. These chips fell out of favor for two reasons: the part can only be used once and cannot be reprogrammed, and after a period of time some of the links will regrow back, changing the value of the cell (and ruining the program contained within the chip). Erasable PROM (EPROM) program memory quickly eclipsed PROM-based memory because it was reprogrammable. The microcontrollers using this type of program memory became available in the late 1970s. EPROM uses ultraviolet light to erase its memory cells, which consist of a transistor that can be set to always on or off. Fig. 1.14 shows the side view of the EPROM transistor. The EPROM transistor is a MOSFET-like transistor with a oating gate surrounded by silicon dioxide above the substrate of the device. To program the oating gate, the
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Figure 1.14 EPROM memory, which is programmed when the control gate forces a charge onto the oating gate.
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Figure 1.15 The quartz window on a ceramic EPROM chip package allows ultraviolet light through to erase the chip.
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control gate above the oating gate is raised to a high voltage potential that causes the silicon dioxide surrounding it to break down and allow a charge to pass into the oating gate. With a charge in the oating gate, the transistor is turned on at all times. Before programming, all the oating gates of all the cells are uncharged. The act of programming the program memory will load a charge into some of the oating gates of these cells. By convention, the memory cell acts as a switch to a pulled-up bit. If an unprogrammed memory cell is read, a 1 will be returned because the switch is off. After the cell is programmed and pulls the line to ground, a 0 is returned. To erase a programmed EPROM cell, ultraviolet (UV) light energizes the trapped electrons in the oating gate to an energy level where they can escape the silicon oxide barrier. In some manufacturer s devices, you nd that some EPROM cells are protected from UV light by a metal layer over them. The purpose of this metal layer is to prevent the cell from being erased. This is often done in memory protection schemes in which critical bits, if erased, will allow reading out of the software in the device. By placing the metal shield over the bit, UV light targeted to just the code protection bit cannot reach the oating gate and the programmed cell cannot be erased. This may seem like an unreliable method of storing data, but EPROM memories are normally rated as being able to keep their contents without any bits changing state for 30 years or more. This speci cation is based on the probability of the charge in one of the cells leaking away enough in 30 years to change the state of the transistor from on to off. Microcontrollers with EPROM program memory can be placed in two types of packages. If you ve worked with EPROM before, you probably have seen the ceramic packages with a small window built in for erasing the device (Fig. 1.15). EPROM microcontrollers are also available in black plastic packages that do not have a window, known as one-time programmable (OTP, see Fig. 1.16).
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Figure 1.16 The plastic encapsulant of the OTP package does not allow ultraviolet light through to the EPROM chip inside.
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