【正文】 ss to external data memory. EA/VPP:External Access Enable. EA must be strapped to GND in order to enable the device to fetch code from external program memory locations starting at 0000H up to FFFFH. Note, however, that if lock bit 1 is programmed, EA will be internally latched on reset. EA should be strapped to VCC for internal program executions. This pin also receives the 12volt programming enable voltage(VPP) during Flash programming, for parts that require 12volt VPP. 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 left unconnected while XTAL1 is driven as shown in Figure 2. There are no requirements on the duty cycle of the external clock signal, since the input to the internal clocking circuitry is through a dividebytwo flipflop, but minimum and maximum voltage high and low time specifications must be observed. Idle Mode In idle mode, the CPU puts itself to sleep while all the on chip peripherals remain active. The mode is invoked by software. The content of the onchip RAM and all the special functions registers remain unchanged during this mode. The idle mode can be terminated by any enabled interrupt or by a hardware reset. AT89C51 MCU Introduction 第 12 頁(yè)（共 15 頁(yè)） It should be noted that when idle is terminated by a hard ware reset, the device normally resumes program execution, from where it left off, up to two machine cycles before the internal reset algorithm takes control. Onchip hardware inhibits access to internal RAM in this event, but access to the port pins is not inhibited. To eliminate the possibility of an unexpected write to a port pin when Idle is terminated by reset, the instruction following the one that invokes Idle should not be one that writes to a port pin or to external memory. Status of External Pins During Idle and Power Down Modes mode Program memory ALE ^psen Port0 Port1 Port2 Port3 idle internal 1 1 data data data Data Idle External 1 1 float Data data Data Power down Internal 0 0 Data Data Data Data Power down External 0 0 float data Data data Power Down Mode In the power down mode the oscillator is stopped, and the instruction that invokes power down is the last instruction executed. The onchip RAM and Special Function Registers retain their values until the power down mode is terminated. The only exit from power down is a hardware reset. Reset redefines the SFRS 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 On the chip are three lock bits which can be left un programmed (U) or can be programmed (P) to obtain the additional features listed in the table below: Lock Bit Protection Modes Program lock bits Protection type Lb1 Lb2 Lb3 1 U U U No program lock features 2 P U U Movc instructions executed from external program memory are disable from fetching code bytes from internal memory, ^ea is sampled and latched on reset, and further programming of the flash disabled AT89C51 MCU Introduction 第 13 頁(yè)（共 15 頁(yè)） 3 P P U Same as mode 2, also verify is disable. 4 P P P Same as mode 3, also external execution is disabled. 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 wit