DS87C520 datasheet(6/45 Pages) DALLAS

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DS87C520/DS83C520 EPROM/ROM High-Speed Microcontrollers

6 of 45

PIN DESCRIPTION (continued)

DIP

PLCC

TQFP

NAME

FUNCTION

P2.0 (A8)

P2.1 (A9)

P2.2 (A10)

P2.3 (A11)

P2.4 (A12)

P2.5 (A13)

P2.6 (A14)

P2.7 (A15)

Port 2 (A8–15), I/O. Port 2 is a bidirectional I/O port. The reset

condition of Port 2 is logic high. In this state, a weak pullup holds

the port high. This condition also serves as an input mode, since

any external circuit that writes to the port will overcome the weak

pullup. When software writes a 0 to any port pin, the

DS87C520/DS83C520 will activate a strong pulldown that

remains on until either a 1 is written or a reset occurs. Writing a 1

after the port has been at 0 will cause a strong transition driver to

turn on, followed by a weaker sustaining pullup. Once the

momentary strong driver turns off, the port again becomes both the

output high and input state. As an alternate function Port 2 can

function as MSB of the external address bus. This bus can be used

to read external ROM and read/write external RAM memory or

peripherals.

P3.0

P3.1

P3.2

P3.3

P3.4

P3.5

P3.6

P3.7

Port 3, I/O. Port 3 functions as both an 8-bit, bidirectional I/O

port and an alternate functional interface for External Interrupts,

Serial Port 0, Timer 0 and 1 Inputs, and

RD and WR strobes. The

reset condition of Port 3 is with all bits at a logic 1. In this state, a

weak pullup holds the port high. This condition also serves as an

input mode, since any external circuit that writes to the port will

overcome the weak pullup. When software writes a 0 to any port

pin, the DS87C520/DS83C520 will activate a strong pulldown that

remains on until either a 1 is written or a reset occurs. Writing a 1

after the port has been at 0 will cause a strong transition driver to

turn on, followed by a weaker sustaining pullup. Once the

momentary strong driver turns off, the port again becomes both the

output high and input state. The alternate modes of Port 3 are

outlined below.

Port

Alternate Mode

P3.0

RXD0

Serial Port 0 Input

P3.1

TXD0

Serial Port 0 Output

P3.2

INT0

External Interrupt 0

P3.3

INT1

External Interrupt 1

P3.4

Timer 0 External Input

P3.5

Timer 1 External Input

P3.6

External Data Memory Write Strobe

P3.7

External Data Memory Read Strobe

External Access Input, Active Low. Connect to ground to force

the DS87C520/DS83C520 to use an external ROM. The internal

RAM is still accessible as determined by register settings. Connect

EA to VCC to use internal ROM.

—

12, 34

6, 28

N.C.

Not Connected. These pins should not be connected. They are

reserved for use with future devices in this family.

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