8XC51GB

ALTERNATE PORT FUNCTIONS

Ports 0 1 2 3 4 and 5 have alternate functions as well as their IO function as described below

Port Pin

Alternate Function

P00ADO – P07AD7

Multiplexed AddressData for External Memory

P10T2

Timer 2 External Clock InputClock-Out

P11T2EX

Timer 2 ReloadCaptureDirection Control

P12ECI

PCA External Clock Input

P13CEXO – P17CEX4

PCA Capture Input ComparePWM Output

P20A8 – P27A15

High Byte of Address for External Memory

P30RXD

Serial Port Input

P31TXD

Serial Port Output

P32INT0

External Interrupt 0

P33INT1

External Interrupt 1

P34T0

Timer 0 External Clock Input

P35T1

Timer 1 External Clock Input

P36WR

Write Strobe for External Memory

P37RD

Read Strobe for External Memory

P40SEPCLK

Clock Source for Serial Expansion Port

P41SEPDAT

Data IO for the Serial Expansion Port

P42ECI1

PCA1 External Clock Input

P43C1EX0 – P47C1EX4

PCA1 Capture Input ComparePWM Output

P52INT2 – P56INT6

External Interrupt INT2 – INT6

RST

Reset input A low on this pin for two machine

cycles while the oscillator is running resets the de-

vice The port pins will be driven to their reset condi-

plied whether the oscillator is running or not An

internal pullup resistor permits a power-on reset with

ALEPROG

Address Latch Enable output pulse for

latching the low byte of the address during accesses

to external memory This pin (ALEPROG) is also

the program pulse input during programming of the

87C51GB

In normal operation ALE is emitted at a constant

rate of

the oscillator frequency and may be used

for external timing or clocking purposes Note how-

ever that one ALE pulse is skipped during each ac-

cess to external Data Memory

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 fea-

ture will be suspended during a MOVX or MOVC in-

struction 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 affect if

the microcontroller is in external execution mode

Throughout the remainder of this data sheet ALE

will refer to the signal coming out of the ALEPROG

pin and the pin will be referred to as the ALEPROG

pin

PSEN

Program Store Enable is the read strobe to

external Program Memory

When the 8XC51GB is executing code from external

Program Memory PSEN is activated twice each ma-

chine cycle except that two PSEN activations are

skipped during each access to external Data Memo-

ry

fetch code from external Program Memory locations

0000H to 1FFFH Note however that if either of the

Program Lock bits are programmed EA will be inter-

nally latched on reset

executions

4