DS2250(T)

121395 4/19

PIN DESCRIPTION

PIN

DESCRIPTION

1, 3, 5, 7, 9,

P1.0 – P1.7. General purpose I/O Port 1

,,,,,

11, 13, 15

pp

17

RST – Active high reset input. A logic 1 applied to this pin will activate a reset state. This pin

is pulled down internally so this pin can be left unconnected if not used. An RC power–on

reset circuit is not needed and is not recommended.

19

P3.0 RXD. General purpose I/O port pin 3.0. Also serves as the receive signal for the on

board UART. This pin should not be connected directly to a PC COM port.

21

P3.1 TXD. General purpose I/O port pin 3.1. Also serves as the transmit signal for the on

board UART. This pin should not be connected directly to a PC COM port.

23

P3.2 INT0. General purpose I/O port pin 3.2. Also serves as the active low External

Interrupt 0.

25

P3.3 INT1. General purpose I/O port pin 3.3. Also serves as the active low External

Interrupt 1.

27

P3.4 T0. General purpose I/O port pin 3.4. Also serves as the Timer 0 input.

29

P3.5 T1. General purpose I/O port pin 3.5. Also serves as the Timer 1 input.

31

P3.6 WR. General purpose I/O port pin. Also serves as the write strobe for Expanded bus

operation.

33

P3.7 RD. General purpose I/O port pin. Also serves as the read strobe for Expanded bus

operation.

35, 37

XTAL2, XTAL1. Used to connect an external crystal to the internal oscillator. XTAL1 is the

input to an inverting amplifier and XTAL2 is the output.

39

GND – Logic ground.

26, 28, 30,

32, 34, 36,

38, 40

P2.7–P2.0. General purpose I/O Port 2. Also serves as the MSB of the Expanded Address

bus.

24

PSEN – Program Store Enable. This active low signal is used to enable an external program

memory when using the Expanded bus. It is normally an output and should be unconnected if

not used. PSEN also is used to invoke the Bootstrap Loader. At this time, PSEN will be pulled

down externally. This should only be done once the DS2250(T) is already in a reset state.

The device that pulls down should be open drain since it must not interfere with PSEN under

normal operation.

22

ALE – Address Latch Enable. Used to de–multiplex the multiplexed Expanded Address/Data

bus on Port 0. This pin is normally connected to the clock input on a ’373 type transparent

latch. When using a parallel programmer, this pin also assumes the PROG function for pro-

gramming pulses.

20

EA – External Access. This pin forces the DS2250(T) to behave like an 8031. No internal

memory (or clock) will be available when this pin is at a logic low. Since this pin is pulled

down internally, it should be connected to +5V to use NVRAM. In an parallel programmer, this