ELM440
Elm Electronics Circuits for the Hobbyist
< http://www.elmelectronics.com/ >
Connection Diagram
PDIP and SOIC
(top view)
V
DD
V
SS
1
2
3
4
8
7
6
5
60Hz Generator
The ELM440 is an 8 pin digital divider integrated
circuit, that provides both 60Hz and 1Hz outputs
from a common 3.58MHz NTSC colourburst crystal.
Externally, the designer need only provide the
crystal and two appropriate loading capacitors, as
well as a suitably bypassed power supply. Internal
oscillator circuits then use this reference frequency
to precisely derive a stable 60Hz signal. This signal
is then further divided to provide a 1Hz output.
The ELM440 can also be locked in synchronism
to an external 60 or 120 Hz line frequency signal. If
derived from the electrical grid voltage, this results in
excellent long-term stability, as the power generation
frequency is traceable to national standards. The
transition to or from sync-lock with this external
signal occurs automatically.
A reset input is also provided to restart the
internal counters, if desired. An active low signal on
this pin will force both outputs to an open or tristate
condition.
XT1
XT2
Description
Stable 60Hz and 1Hz reference sources
Master oscillator for clock / timekeeping functions
DC powered backup 60Hz reference signal
Applications
Block Diagram
1 of 4
Low power CMOS design
Wide supply range - 3.0 to 5.5 volt operation
Generates both 60Hz and 1Hz references
Automatically synchronizes to the line frequency
for long term stability, when it's available
Uses a standard 3.58MHz crystal for the time
reference if there is no sync input
High current drive outputs - up to 25 mA
Features
ELM440DSB
1Hz
sync
60Hz
2
3
XT1
XT2
5
sync
60Hz
Master
Oscillator
Divider
60
6
V
DD
reset
1Hz
7
4
reset
ELM440
Elm Electronics Circuits for the Hobbyist
< http://www.elmelectronics.com/ >
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............................... ELM440P
200 mil SOIC..................................... ELM440SM
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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.
V
DD
(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.
XT1 (pin 2) and XT2 (pin 3)
A 3.579545MHz NTSC television colourburst
crystal is connected between these two pins.
Crystal loading capacitors (typically 27pF) will
also normally be connected between each of the
pins and Vss.
reset (pin 4)
This pin can optionally be used to reset the circuit
by applying a logic low level to it. While held low,
both outputs are placed in an open or tristate
condition. If unused, this pin should be left open
(as a pullup resistor is provided) or connected to
V
DD
.
sync (pin 5)
The synchronizing reference is (optionally)
applied to this pin. Slowly varying sinusoidal
waveforms may be applied to this input, as the
input circuitry is of the Schmitt trigger type. The
input frequency can be either 60Hz or 120Hz (as
would be obtained from a full wave rectifier), as
internal circuitry will adjust to each. If this input is
not used, the pin should be connected to either
V
DD
or V
SS
.
1Hz (pin 6)
The output on this pin is a 1Hz signal of
nominally 50% duty cycle.
60Hz (pin 7)
The output on this pin is a 60Hz signal of
approximately 50% duty cycle. Actual duty cycle
will vary by up to 2% from ideal due to the type
of algorithm used to maintain sync-lock. This
algorithm also contributes to a slight variation in
period that is automatically corrected for every
four cycles. Typical timekeeping functions will
normally not be affected by either variation.
V
SS
(pin 8)
Circuit common is connected to this pin. This is
the most negative point in the circuit.
ELM440DSB
Elm Electronics Circuits for the Hobbyist
< http://www.elmelectronics.com/ >
ELM440
Electrical Characteristics
Absolute Maximum Ratings
Storage Temperature....................... -65C to +150C
Ambient Temperature with
Power Applied....................................-40C to +85C
Voltage on V
DD
with respect to V
SS
............ 0 to +7.5V
Voltage on any other pin with
respect to V
SS
........................... -0.6V to (V
DD
+ 0.6V)
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.
3 of 4
All values are for operation at 25C and a 5V supply, unless otherwise noted. For further information, refer to note 1 below.
Characteristic
Minimum
Typical
Maximum
Conditions
Units
Supply Voltage, V
DD
3.0
5.0
5.5
V
V
DD
rate of rise
0.05
V/ms
Average Supply Current, I
DD
0.60
2.4
mA
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. The value of the pullup resistance is supply and temperature dependent.
4. This specification represents current flowing through the protection diodes when applying large voltages to
the sync input (pin 5) through a current limiting resistance. Currents quoted are the maximum continuous.
Input low voltage
V
SS
0.15 V
DD
V
Input high voltage
V
DD
V
0.85 V
DD
Output low voltage
0.6
V
Output high voltage
V
V
DD
- 0.7
Current (sink) = 8.7mA
Current (source) = 5.4mA
see note 2
ELM440DSB
Output Duty Cycle
%
50
any output
0.35
2.4
mA
see note 3
Reset pin internal pullup resistance
V
DD
= 5V
V
DD
= 3V
K
470
300
600
Input current
+ 0.5
mA
- 0.5
sync input only, see note 4
Example Application
4 of 4
The ELM440 can be easily configured to
provide a 60Hz output as shown in Figure 1 below.
A 3.579545MHz crystal is connected to the
integrated circuit, along with suitable loading
capacitors, and the 60Hz output is then taken from
pin 7. Pin 5 is grounded, as no reference signal is
supplied. As usual, it is also good practise to place
a bypass capacitor across the power supply as
well.
Figure 2 shows how easily this circuit can be
modified to operate in synchronism with the AC
power line. A reference signal is taken from one
half of a 16Vrms CT transformer winding through a
47K
resistor. Both circuits share a common V
SS
.
The resistor was chosen to limit peak currents in
this circuit to about 0.24 mA, which is well within
the IC's capabilities. The ELM440 will continue to
use this signal as it's reference, as long as it is
available. Upon loss of the AC, the circuit will
automatically switch over to using the crystal as a
reference.
Using a rectifier circuit for the reference signal
can pose some problems if one is not careful. Full-
wave circuits can be particularly troublesome as
all of the diodes may cease to conduct after the
peak of the waveform, and the input voltage level
is then essentially floating. In this case, at
minimum, a large value (470K
) resistor should
also be connected between the sync input and
V
SS
, to provide a ground reference during these
times.
Both circuits shown are simple, leaving little
room for change. One modification that might be
considered for either is to replace one of the fixed
value capacitors with a variable one, in order to be
able to trim the oscillator frequency for greater
accuracy.
ELM440
ELM440DSB
Elm Electronics Circuits for the Hobbyist
< http://www.elmelectronics.com/ >
60Hz Out
0.1F
+5V
1
2
3
4
8
7
6
5
27pF
3.58MHz
27pF
Figure 1. Typical Connections
Figure 2. Using the Sync Input
0.1F
+5V
1
2
3
4
8
7
6
5
60Hz Out
27pF
3.58MHz
27pF
47K
8V
8V
+
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