【正文】 nated by reset, the instruction following the one that invokes Idle should not be one that writes to a port pin or to external memory. Powerdown Mode: In the powerdown mode, the oscillator is stopped, and the instruction that invokes powerdown is the last instruction executed. The onchip RAM and special function registers retain their values until the powerdown mode is terminated. The only exit from powerdown is 13 a hardware reset. Reset redefines the special function registers but does not change the onchip RAM. The reset should not be activated before VCC is restored to its normal operating level and must be held active long enough to allow the oscillator to restart and stabilize. Program Memory Lock Bits: When lock bit 1 is programmed, the logic level at the EA pin is sampled and latched during reset. If the device is powered up without a reset, the latch initializes to a random value, and holds that value until reset is activated. It is necessary that the latched value of EA be in agreement with the current logic level at that pin in order for the device to function properly. Programming the Flash: The AT89C51 is normally shipped with the onchip Flash memory array in the erased state (that is, contents = FFH) and ready to be programmed. The programming interface accepts either a highvoltage (12volt) or a lowvoltage (VCC) program enable signal. The lowvoltage programming mode provides a convenient way to program the AT89C51 inside the user’s system, while the highvoltage programming mode is patible with conventional third party Flash or EPROM AT89C51 is shipped with either the highvoltage or lowvoltage programming mode enabled. The AT89C51 code memory array is programmed bytebybyte in either programming mode. To program any nonblank byte in the onchip Flash memory, the entire memory must be erased using the chip erase mode. Programming Algorithm: Before programming the AT89C51, the address, data and control signals should be set up according to the Flash programming mode table .To program the AT89C51, take the following steps: 1. Input the desired memory location on the address lines. 2. Input the appropriate data byte on the data lines. 3. Activate the correct bination of control signals. 4. Raise EA/VPP to 12V for the highvoltage programming mode. 5. Pulse ALE/ PROG once to program a byte in the Flash array or the lock bits. The bytewrite cycle is selftimed and typically takes no more than . Repeat steps 1 through 5, changing the address and data for the entire array or until the end of the object file is reached. Data Polling: The AT89C51 features Data Polling to indicate the end of a write cycle. During a write cycle, an attempted read of the last byte written will result in the plement of the written datum on . Once the write cycle has been pleted, true data are valid on all outputs, and 14 the next cycle may begin. Data polling may begin any time after a write cycle has been initiated. Program Verify: If lock bits LB1 and LB2 have not been programmed, the programmed code data can be read back via the address and data lines for verification. The lock bits cannot be verified directly. Verification of the lock bits is achieved by observing that their features are enabled. Chip Erase: The entire Flash array is erased electrically by using the proper bination of control signals and by holding ALE/ PROG low for 10 ms. The code array is written with all “1”s. The chip erase operation must be executed before the code memory can be reprogrammed. 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 and must be pulled to a logic low. The values returned are as follows: (030H) = 1EH indicates manufactured by ATMEL (031H) = 51H indicates AT89C51 singlechip (032H) = FFH indicates 12V programming (032H) = 05H indicates 5V programming 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 self timed and once initiated, will automatically time itself to pletion. （ 2） The sensor DS18B20 In the traditional analog signal distance temperature measuring system, need good solve lead error pensation, multipoint measurement error and amplifying circuit switching technologies such as zero drift error problem, can achieve high measuring accuracy. Another general monitoring site of the electromagic environment is very bad, all kinds of jamming signal is stronger, the simulated temperature signal interference and vulnerable to produce measurement error and measuring precision [5]. Therefore, in temperature measuring system, the strong antijamming capability of the new digital temperature sensor is the most effective to solve these problems, pared with other temperature sensor DSl820 has the following features: 15 (1) the unique singleline interface way. DSl820 in connection with the microprocessor only need one interface to implement line DSl820 microprocessors and twoway munication. (2) more function simplifies distributed temperature detection application. (3) DSl820 in use without any peripheral devices. (4) power, voltage range data available from V to V. (5) can measure temperature range from 55 degrees c + + to 125, incremental value 0. 5 176。 c, start fan circuit. Because the job is great, so use current amplifier circuit to drive, to enlarge current transistor. When the temperature is above 80 176。 如有侵權(quán)，請(qǐng)告知，我看到會(huì)立刻處