ELM ELM621P, ELM621SM Datasheet
ELM621
Serial Three Bit Port
Description
1238765
4
5
467
2
3
Features
The ELM621 is a serially controlled, three bit
parallel I/O port in an 8 pin package. The high level
of integration employed means that very few
external components are required in order to
construct a fully functional circuit with this IC.
The ELM621 offers several convenient features
that many users will appreciate. The provision of a
16 byte buffer allows commands to be chained and
executed sequentially with one key stroke. The
controllable pullup resistor on the bit 3 port pin is
particularly helpful when interfacing to mechanical
switches, while the ability to control character echo
reduces overhead while operating under computer
control. Naturally, individual as well as simultaneous
read, set, clear and toggle commands are also
supported.
All communications with this device are via a
standard 9600 baud RS232 connection, using AT
style commands. No special characters are required,
so that virtually any terminal program can be used
with the ELM621. More advanced users will be
interested in operating this device under computer
program control – simply a matter of writing to and
reading from the device.
• Low power CMOS design - typically 1mA
• High speed (9600 baud) RS232 communications
• Standard ASCII commands and responses
• 16 byte receive buffer
• Switchable pullup resistance
• Minimum of external components required
• External reset input
• High current drive outputs - up to 25 mA
Connection Diagram
PDIP and SOIC
(top view)
VDD VSS
B2
B1
B3
Tx
Applications
• Home automation monitoring and control
• Robotics
• Security monitoring
• IC programmers
Block Diagram
Tx
Rx
ELM621DSA
RS232
Interface
reset
Elm Electronics – Circuits for the Hobbyist
reset Rx
Command
Interpreter
VDD
Control
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VDD
B3
Port
B2
B1
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ELM621
Pin Descriptions
VDD (pin 1)
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 30KΩ) 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
resistor is provided) or connected to VDD.
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 47KΩ).
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.
VSS (pin 8)
Circuit common is connected to this pin. This is
the most negative point in the circuit.
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
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.
ELM621DSA
Elm Electronics – Circuits for the Hobbyist
< http://www.elmelectronics.com/ >
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