ELM621

Elm Electronics – Circuits for the Hobbyist

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Pin Descriptions

Ordering Information

These integrated circuits are available in either the 300 mil plastic DIP format, or in the 200 mil SOIC surface

mount type of package. To order, add the appropriate suffix to the part number:

300 mil Plastic DIP............................... ELM621P

200 mil SOIC..................................... ELM621SM

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All rights reserved. Copyright ©1999 Elm Electronics.

Every effort is made to verify the accuracy of information provided in this document, but no representation or warranty can be

given and no liability assumed by Elm Electronics with respect to the accuracy and/or use of any products or information

described in this document. Elm Electronics will not be responsible for any patent infringements arising from the use of these

products or information, and does not authorize or warrant the use of any Elm Electronics product in life support devices and/or

systems. Elm Electronics reserves the right to make changes to the device(s) described in this document in order to improve

reliability, function, or design.

This pin is the positive supply pin, and should

always be the most positive point in the circuit.

Internal circuitry connected to this pin is used to

provide power on reset of the microprocessor, so

an external reset signal is not required. Refer to

the Electrical Characteristics section for further

information.

B1 (pin 3), B2 (pin 2) and B3 (pin 7)

These are the three port pins that are software

configurable. At power-up, and after resetting,

these pins are inputs but can be individually set

as either input or output. The B3 pin has an

internal (nominally 30K

Ω) pullup resistor that can

be enabled or disabled as required.

reset (pin 4)

This pin can be used to reset the circuit by

applying a momentary logic low level to it. If

unused, this pin should be left open (as a pullup

Rx (pin 5)

This is the RS232 receive input. Internal inversion

of the logic level allows the pin to be directly

connected to the DTE’s TxD line through a single

current limiting resistor (typically about 47K

Ω).

Precautions should be taken in the circuit design

to allow for the possibility that this input may be

left floating, due to a disconnected serial cable.

Typically,

this

only

requires

a

large-valued

resistor between the RS232 TxD pin and Vss.

Tx (pin 6)

The RS232 data output pin. Signal level is

compatible with most inverting interface ICs, and

drive is sufficient to allow interfacing using only a

single

PNP

transistor.

See

the

Example

Application section for more details.

Circuit common is connected to this pin. This is

the most negative point in the circuit.

ELM621DSA