單片機(jī)畢業(yè)設(shè)計(jì)外文翻譯單片機(jī)at89c51-單片機(jī)-展示頁(yè)
2025-01-31 08:00本頁(yè)面
【正文】 rt 3： Port 3 is an 8bit bidirectional I/O port with internal Port 3 output buffers can sink/source four TTL 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 3 also serves the functions of various special features of the AT89C51 as listed below: Port 3 also receives some control signals for Flash programming and verification. ALE/PROG： Address Latch Enable output pulse for latching the low byte of the address 八路智能搶答器的設(shè)計(jì) 第 4頁(yè) 共 13頁(yè) during accesses to external memory. This pin is also the program pulse input (PROG) during Flash normal operation ALE is emitted at a constant rate of 1/6the oscillator frequency, and may be used for external timing or clocking purposes. Note, however, that one ALE pulse is skipped during each access to external Data desired, ALE operation can be disabled by setting bit 0 of SFR location 8EH. With the bit set, ALE is active only during a MOVX or MOVC instruction. Otherwise, the pin is weakly pulled high. Setting the ALEdisable bit has no effect if the microcontroller is in external execution mode. PSEN： Program Store Enable is the read strobe to external program the AT89C51 is executing code from external programmemory, PSEN is activated twice each machine cycle, except that two PSEN activations are skipped during each access 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 , however, that if lock bit 1 is programmed, EA will be internally latc hed 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 onchip peripherals remain activ
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