Port 3: Port 3 is an 8-bit bidirectional IO port with internal pullups. The Port 3 output buffers can drive LS TTL inputs. Port 3 pins that have 1’s written to them are pulled high by the internal pullups and in that state can be used as inputs. As inputs, Port 3 pins that are externally pulled low will source current(I on the data sheet) because of the pullups. Port 3 also serves the functions of various special features of the MCS-51 Family, as listed below:
Port 3 also receives some control signals for EPROM programming and program verification.
RST: Reset input. A high on this pin for two machine cycles while the oscillator is running resets the device. The port pins will be driven to their reset condition when a minimum VIH1 voltage is applied whether the oscillator is running or not. An internal pulldown resistor permits a power-on reset with only a capacitor connected to Vcc.
ALEPROG: Address Latch Enable output signal for latching the low byte of the address during accesses to external memory. This pin is also the program pulse input (PROG) during EPROM programming for the 87C51. If desired, ALE operation can be disabled by setting bit 0 of SFR location 8EH. With this bit set the pin is weakly pulled high. However the ALE disable feature will be suspended during a MOVX or MOVC instruction, idle mode power down mode and ICE mode The ALE disable feature will be terminated by reset.
When the ALE disable feature is suspended or terminated, the ALE pin will no longer be pulled up weakly. Setting the ALE-disable bit has no effect if the microcontroller is in external execution mode.
In normal operation ALE is emitted at a constant rate of 16 the 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 Memory.
PSEN: Program Store Enable is the Read strobe to External Program Memory. When the 87C51BH is executing from Internal Program Memory, PSEN is inactive (high). When the device is executing code from External Program Memory, 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 VSS in order to enable the 87C51BH to
fetch code from External Program Memory locations starting at 0000H up to FFFFH. Note, however, that if either of the Lock Bits is programmed, the logic level at EA is internally latched during reset. EA must be strapped to Vcc for internal program execution. This pin also receives the programming supply volt age (Vpp) during EPROM programming.
XTAL1: Input to the inverting oscillator amplifier.
XTAL2: Output from the inverting oscillator amplifier.
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