【正文】 s achieved by observing that their features are enabled Figure 211 Programming the Flash Figure 222 Verifying the Flash 22 Chip Erase The entire Flash array is erased electrically by using the proper bination of control signals and by holding ALEPROG low for 10 ms The code array is written with all 1s The chip erase operation must be executed before the code memory can be reprogrammed 23 Reading the Signature Bytes The signature bytes are read by the same procedure as a normal verification of locations 030H 031H and 032H except that P36 and P37 must be pulled to a logic low The values returned areas follows 030H 1EH indicates manufactured by Atmel 031H 51H indicates 89C51 032H FFH indicates 12V programming 032H 05H indicates 5V programming 24 Programming Interface Every code byte in the Flash array can be written and the entire array can be erased by using the appropriate bination of control signals The write operation cycle is selftimed and once initiated will automatically time itself to pletion A microputer interface converts information between two forms Outside the microputer the information handled by an electronic system exists as a physical signal but within the program it is represented numerically The function of any interface can be broken down into a number of operations which modify the data in some way so that the process of conversion between the external and internal forms is carried out in a number of steps An analogtodigital converter ADC is used to convert a continuously variable signal to a corresponding digital form which can take any one of a fixed number of possible binary values If the output of the transducer does not vary continuously no ADC is necessary In this case the signal conditioning section must convert the ining signal to a form which can be connected directly to the next part of the interface the inputoutput section of the microputer itself Output interfaces take a similar form the obvious difference being that here the flow of information is in the opposite direction it is passed from the program to the outside world In this case the program may call an output subroutine which supervises the operation of the interface and performs the scaling numbers which may be needed for digitaltoanalog converter DAC This subroutine passes information in turn to an output device which produces a corresponding electrical signal which could be converted into analog form using a DAC Finally the signal is conditioned usually amplified to a form suitable for operating an actuator The signals used within microputer circuits are almost always too small to be connected directly to the outside world and some kind of interface must be used to translate them to a more appropriate form The design of section of interface circuits is one of the most important tasks facing the engineer wishing to apply microputers We have seen that in microputers information is represented as discrete patterns of bits this digital form is most useful when the microputer is to be connected to equipment which can only be switched on or off where each bit might represent the state of a switch or actuator To solve realworld problems a microcontroller must have more than just a CPU a program and a data memory In addition it must contain hardware allowing the CPU to access information from the outside world Once the CPU gathers information and processes the data it must also be able to effect change on some portion of the outside world These hardware devices called peripherals are the CPUs window to the outside The most basic form of peripheral available on microcontrollers is the general purpose I70 port Each of the IO pins can be used as either an input or an output The function of each pin is determined by setting or clearing corresponding bits in a corresponding data direction register during the initialization stage of a program Each output pin may be driven to either a logic one or a logic zero by using CPU instructions to pin may be viewed or read by the CPU using program instructions Some type of serial unit is included on microcontrollers to allow the CPU to municate bitserially with external devices Using a bit serial format instead of bitparallel format requires fewer IO pins to perform the munication function which makes it less expensive but slower Serial transmissions are performed either synchronously or asynchronously