Datasheet

DS87C520/DS83C520 EPROM/ROM High-Speed Microcontrollers
6 of 43
PIN DESCRIPTION (continued)
PIN
DIP PLCC TQFP
NAME FUNCTION
21 24 18 P2.0 (A8)
22 25 19 P2.1 (A9)
23 26 20 P2.2 (A10)
24 27 21 P2.3 (A11)
25 28 22 P2.4 (A12)
26 29 23 P2.5 (A13)
27 30 24 P2.6 (A14)
28 31 25 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.
10 11 5 P3.0
11 13 7 P3.1
12 14 8 P3.2
13 15 9 P3.3
14 16 10 P3.4
15 17 11 P3.5
16 18 12 P3.6
17 19 13 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 T0 Timer 0 External Input
P3.5 T1 Timer 1 External Input
P3.6 WR External Data Memory Write Strobe
P3.7 RD External Data Memory Read Strobe
31 35 29 EA
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 V
CC
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.