ELM ELM627P

ELM627
RS232 Break Detector
Description
Features
The ELM627 is a complete circuit for detecting
break signals on RS232 lines, in an 8 pin package.
This is useful for remotely resetting circuits through a
serial link. For convenience, this circuit also
generates an output pulse on power-up.
The timing threshold for break signal detection is
user selectable, allowing the circuit to respond to
transmitted null characters, <ctrl>@, at various baud
rates. This provides a convenient means for the
remote triggering of this circuit by simply shifting to a
lower baud rate and sending a null byte.
Two complementary high current outputs are
provided by the ELM627, so that further buffering is
not usually required. The output signal can be
selected to be either a continuous one (following the
TxD input line) or else a 100ms pulse output.
No external components are required for circuit
operation other than possibly a current limiting
resistor, as all timing references are generated
internally.
• Low power CMOS design - typically 1mA at 5V
• No external timing elements required
• Complementary outputs
• Digitally controlled threshold timing
• Selectable output duration
• High current drive outputs - up to 25 mA
Connection Diagram
PDIP and SOIC
(top view)
Applications
VDD
1
8
VSS
Q
2
7
T0
Q
3
6
T1
P/C
4
5
TxD
• Remote resetting of RS232 devices
• Edge triggered one-shot circuits
• Digitally controlled sequencing schemes
• Watchdog type signal monitors
Block Diagram
Power on
Detect
TxD
5
Programmable
Timer
ELM627DSA
Output
Logic
2
Q
3
Q
6
7
4
T1
T0
P/C
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ELM627
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.
Q (pin 2)
This signal is driven to a logical high level when a
break signal is detected, and during powerup.
Otherwise, the output will remain active
continuously until pin 5 returns to a logical low
level. The output duration will always be a
minimum of 10ms, and is 100ms on powerup.
TxD (pin 5)
This is the monitored signal input, usually
connected directly to the RS232 Transmit Data
line through a suitable current limiting resistor.
This input employs Schmitt trigger logic so that
input waveshaping circuitry is not normally
required.
Q (pin 3)
This output signal is normally held at a high level,
and is driven low when a break signal is detected,
as well as immediately after powerup.
T1 (pin 6) and T0 (pin 7)
These are the threshold setting inputs. Logic
levels on these pins at the rising edge of TxD are
used to determine the required minimum duration
of the break signal, as shown in Table 1 below.
P/C (pin 4)
This is the pulse/continuous control input pin,
used to modify the behavior of the circuit output.
If at a logic high when a break signal is detected,
the output will consist of a single 100ms pulse.
VSS (pin 8)
Circuit common is connected to this pin. This is
the most negative point in the circuit.
Threshold Setting
T1
T0
L
L
H
H
L
H
L
H
Min Break Signal
(msec)
5.0
11
22
120
Baud rate, ‘00’ character sent
will trigger at
but not at
1200
600
300
-
2400
1200
600
110
Table 1. Threshold Settings
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............................... ELM627P
200 mil SOIC..................................... ELM627SM
All rights reserved. Copyright ©1998 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.
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ELM627
Absolute Maximum Ratings
Storage Temperature....................... -65°C to +150°C
Note:
Stresses beyond those listed here will likely damage
the device. These values are given as a design
guideline only. The ability to operate to these levels
is neither inferred nor recommended.
Ambient Temperature with
Power Applied....................................-40°C to +85°C
Voltage on VDD with respect to VSS............ 0 to +7.5V
Voltage on any other pin with
respect to VSS........................... -0.6V to (VDD + 0.6V)
Electrical Characteristics
All values are for operation at 25°C and a 5V supply, unless otherwise noted. For further information, refer to note 1 below.
Characteristic
Minimum
Typical
Supply Voltage, VDD
4.5
5.0
VDD rate of rise
0.05
Average Supply Current, IDD
Maximum Units
5.5
1.0
Conditions
V
V/ms
see note 2
2.4
mA
see note 3
Input low voltage
VSS
0.15 VDD
V
Input high voltage
0.85 VDD
VDD
V
0.6
V
Current (sink) = 8.7mA
V
Current (source) = 5.4mA
Output low voltage
Output high voltage
VDD - 0.7
Output Pulse Width
95
Maximum Timing Error
Input Current
100
105
5
-0.5
+0.5
msec
see notes 4 and 5
%
see note 4
mA
see note 6
Notes:
1. This integrated circuit is produced with a Microchip Technology Inc.’s PIC12C5XX as the core embedded
microcontroller. For further device specifications, and possibly clarification of those given, please refer to the
appropriate Microchip documentation.
2. This spec must be met in order to ensure that a correct power on reset occurs. It is quite easily achieved
using most common types of supplies, but may be violated if one uses a slowly varying supply voltage, as
may be obtained through direct connection to solar cells, or some charge pump circuits.
3. Device only. Does not include any LED or drive currents.
4. Pulse timing is generated internally, and is affected by both temperature and supply voltage. Although the
limits cannot be absolutely guaranteed, these are generally the widest variation that would normally be
encountered.
5. This pulse is also generated immediately after circuit powerup.
6. This specification represents current flowing through the protection diodes when applying large voltages to
the TxD input (pin 5) through a current limiting resistance. Currents quoted are the maximum continuous.
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ELM627
Example Application
Figure 1 shows the ELM627 installed in a remotely
connected, RS232 linked device. This is typical of many
connections used today - a 9600 baud serial connection
is made between a PC and a remote device. Serial data
is continually sent between the two devices, but control
of the remote system is by software only.
Pins 6 and 7 are shown shorted to VSS, setting the
circuit to trigger on any high level that exceeds 5msec
in duration. This allows for triggering of the circuit on a
null character (‘00’) at 1200 baud, easily accomplished
with most systems.
The final connection to note is that the
pulse/continuous control (pin 4) has been tied to a high
level so that the generated reset pulse will always be of
100msec duration. This pulse width isn’t normally
necessary for the circuitry, but may be desireable if one
wants to connect an LED or a buzzer to the circuit for
feedback.
It is often quite desireable in such a case to be able
to reset the remote system to a known state under
software control. This can be either at startup, or if the
remote system fails to respond to signals for any reason.
The circuit shown below represents all that is
typically required to provide remote as well as power on
resetting capability. The ELM627 contiuously monitors
the transmit data line for a high level (3-25V) through the
100KΩ current limiting resistor. The second 100KΩ
resistor is only used to provide a pull-down to VSS,
should the data link become open circuited. Depending
on what other devices are conected, this resistor may
not be required.
Several variations on this type of circuit are
possible. By noting that the ‘reset’ output can be used
for other control purposes, and possibly adding a toggle
type latch, the circuit can be expanded to remotely turn
on and off devices, leading to several automation
possibilities…
Host Computer
TxD
RxD
SG
Remote System
9600 baud RS232 Link
+5V
0.1µF
1
8
2
7
3
6
4
5
100KΩ
100KΩ
to reset
circuitry
Figure 1. Remote Reset Control
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