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【文章內(nèi)容簡介】 and 80C52 instruction set and onchip Flash allows the program memory to be user programmed by a conventional nonvolatile memory programmer. By bining a versatile 8bit CPU with Flash on a monolithic chip, the Atmen AT89C55 is a powerful microputer which provides a highly flexible and cost effective solution to many embedded control applications. The AT89C55 provides the following standard features: 20K bytes of flash, 256 bytes of RAM, 32 I/O lines, three 16bit timer/counters, a sixvector, twolevel interrupt architecture, a fullduplex serial port, onchip oscillator, and clock circuitry. In addition, the AT89C55WD is designed with static logic for operation down to zero frequency and supports two software selectable power saving modes. The Idle Mode stops the CPU while allowing the RAM, timer/counters, serial port, and interrupt system to con tine functioning. The Powerdown Mode saves the RAM contents but freezes the oscillator, disabling all other chip functions until the next external interrupt or hardware reset. Block Diagram Pin Description VCC： Supply voltage. GND： Ground. Port 0： Port 0 is an 8bit opendrain bidirectional I/O port. As an output port, each pin can sink eight TTL inputs. When 1s are written to port 0 pins, the pins can be used as high impedance inputs. Port 0 may also be configured to be the multiplexed low order address/data bus during accesses to external program and data memory. In this mode P0 has internal pull ups. Port 0 also receives the code bytes during Flash programming and outputs the code bytes during program verification. External pull ups are required during program verification. Port 2: Port 2 is an 8bit bidirectional I/O port with internal pull ups .The Port 2 output buffers can sink/source four are written to Port 2 pins they are pulled high by the internal pull ups and can be used as inputs. As inputs, Port 2 pins that are externally being pulled low will source current (IIL) because of the internal pull ups .Port 2 emits the highorder address byte during fetches from external program memory and during accesses to external data memory that use 16bit addresses (MOVX @DPTR). In this application, it uses strong internal pull ups when emitting 1s. During accesses to external data memory that use 8bit addresses (MOVX @ RI), Port 2 emits the contents of the P2 Special Function Register. Port 2 also receives the highorder address bits and some control signals during Flash programming and verification. Port 3 : Port 3 is an 8bit bidirectional I/O port with internal pullups. The Port 3 output buffers can sink/source four TTL inputs. When 1s are written to Port 3 pins, they are pulled high by the internal pullups and can be used as inputs. As inputs, Port 3 pins that are externally being pulled low will source current (IIL) because of the pullups。 Port 3 receives some control signals for Flash programming and verification. Port 3 also serves the functions of various special features of the AT89C55, as shown in the following table. XTAL1: Input to the inverting oscillator amplifier and input to the internal clock operating circuit. XTAL2: Output from the inverting oscillator amplifier. Oscillator Characteristics: XTAL1 and XTAL2 are the input and output, respectively, of an inverting amplifier which can be configured for use as an onchip oscillator, as shown in Figure 1. Either a quartz crystal or ceramic resonator may be used. To drive the device from an external clock source, XTAL2 should be
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