Abstract: This article describes both the function of power-on resets and the strategies for choosing their threshold voltages, when used with single-supply and dual-supply processors. It then discusses manual reset and power-fail and low-line signals. The article demonstrates why to avoid discrete PORs and PORs internal to processors. It concludes with explanations of voltage sequencing, voltage tracking, and reset sequencing.
One task of the power-on reset (POR) is ensuring that the
processor starts at a known address when power is first
applied. To accomplish that task, the POR logic output
holds the processor in its reset state when the processor's
power supply is first turned on. The POR's second task is
to keep the processor from starting its operation from that
known address until three events have occurred: the
system power supplies have stabilized at the appropriate
levels; the processor's clock(s) has (have) settled; and the
internal registers have been properly loaded. The POR
accomplishes this second task through an onboard timer,
which continues to hold the processor in its reset state for a
prescribed period of time. That timer triggers after the
processor's power supply reaches a specific voltage
threshold. After a set time elapses, the timer expires,
causing the POR output to become inactive, which in turn
makes the processor come out of reset and begin operation
(Figure 1). The processor's data sheet specifies the
required duration of the timer's delay. The timer, incidentally,
is what differentiates a POR from a voltage detector,
a device that also detects a voltage threshold, but does not
time an event.
Figure 1. A POR holds a processor in its reset state until the supply voltage exceeds the POR threshold and a specific delay period
has elapsed.
The superior noise immunity of a POR, which is necessary
when monitoring a processor, also distinguishes it from a
voltage detector. This is because a POR should not issue a
reset when a small, fast glitch appears on the supply, as the
processor itself does not react to such glitches. However,
both a small glitch of long duration and a large glitch of
short or long duration can cause problems for the
processor. Therefore, the best approach is to use a POR
that examines both the size and duration of disturbances to
the power-supply voltage for determining when to assert a
reset. The intent is to mirror the processor's own behavior
and to assert a reset only when one is needed, as there is no
point in resetting the processor if it is working properly.
Figure 2 is a graph from the MAX6381/MAX6382 data
sheet that details an example of the magnitude/duration of
the supply-voltage disturbance required to trigger a reset.
This graph illustrates that the MAX6381/MAX6382 issues
a reset when the monitored power supply is 100mV below
the specified threshold for at least 10ms.
Figure 2. Whether a POR generates a reset is a function of both the amplitude and duration of the glitch.
Should the supply voltage return above the threshold, the
POR timer allows the reset signal to deassert only after
the prescribed interval.
Some processors provide a bidirectional reset pin—a pin
that can not only receive a reset signal, but can also
transmit one. At first glance, a POR with an open-drain
output would seem to be needed in this situation.
However, other considerations apply, because the
processor must determine whether it or an external device
initiated the reset. A POR specially configured for such a
situation is necessary (refer to the MAX6314 data sheet).
Determining the POR Threshold Voltage -
Single-Supply Processors
Determining the correct POR threshold level and the
required accuracy for that level are both often misunderstood.
To shed light on those tasks, assume a processor is
used that guarantees accurate operation with a 3.3V
±0.3V supply voltage - specifically, from 3.00V to
3.60V. Board designers follow one of two strategies when
choosing the threshold voltage.
One strategy is to ensure that the tolerance of the 3.3V
supply is tight enough so they can use a POR with a
threshold plus tolerance that remains entirely within the
±0.3V range. In that case, the POR threshold lies
between the lower end of the supply's range (±3%) and
the lower end of the processor's allowed voltage range
(Figure 3a). Under this strategy, the POR does not issue
a reset when the supply is within tolerance. However, the
POR does issue a reset when the supply voltage has
dropped below its tolerance level, and remains within the
range where the processor is guaranteed to operate
correctly. This ensures that reset occurs before the
processor can operate erroneously at a voltage below its
guaranteed operational level.
Figure 3. To ensure that the processor gets reset when the supply voltage is below the power supply's specified voltage range and above the bottom of the processor's range of permitted voltages, choose a POR threshold as shown in Figure 3a. However, choosing a POR threshold below the processor's range of permitted voltages (Figure 3b) guarantees that reset will not occur anywhere within that range and allows the use of a power supply with a looser tolerance.
A suitable choice of POR for this strategy is the version of
the MAX6381 with a threshold range of 3.00V to 3.15V
over temperature (Figure 3a). With this POR included, the
processor will reset after the power supply drops below its
specified voltage range, but before the supply drops below
the processor's specified voltage range. Also, given that
the upper end of the threshold's range is 3.15V, a reset
cannot occur when the power supply is within its allowed
range. However, voltage drops through the edge
connector and board trace that connect the supply voltage
to the processor might cause the voltage at the processor
to drop below 3.15V. In this case, a reset could occur
even though the supply voltage is within specifications. A
tighter tolerance supply or tighter tolerance POR
threshold, or both, would be necessary.
This design approach is more susceptible to power-supply
glitches and noise, because the supply voltage can be
fairly close to the POR threshold (depending on where the
POR threshold and supply voltage lie within their tolerances).
Therefore, this approach is appropriate for systems
where glitches and noise are minimized and power-supply
tolerances are tight.
Some board designers adopt a second, different strategy
when choosing a POR threshold level. They employ a POR
with a threshold below the processor's guaranteed
operating voltage (3.00V, in this example). This allows the
processor to operate anywhere within the range of
permitted voltages without encountering a reset. It also
permits a looser tolerance power supply. These designers
are comfortable assuming that, during power-up, the power
supply will continue to rise above the POR threshold level
and settle within its specified range of voltages (3.20V to
3.40V, in this case). This is expected to happen well before
the POR timer times out and the processor begins to
operate. Often, designers use the power-OK signal
provided by some power supplies to ensure that the supply
operates within its specified range.
These same designers are unconcerned about the effect of
a brownout condition. If a brownout occurs, the
processor could encounter a supply voltage that falls
below its minimum guaranteed operating voltage, but
remains briefly above the POR threshold level (beneath
which the POR would generate a reset). While powered
by supply voltages in that range, the processor could
operate erroneously.
Contrary to choosing a threshold within the processor's
permitted supply voltage range, the second approach is
more appropriate for those systems where glitches and
noise tend to be larger, because the POR threshold and the
power-supply voltage are further apart. As mentioned
above, this also permits wider tolerance power supplies.
The version of the MAX6381 that has a threshold range
of 2.85V to 3.0V over temperature is a good choice here,
because the threshold is below the low end of the
processor's permitted range (Figure 3b). One could also
use a power supply with a tolerance wider than that
shown in Figure 3.
Occasionally board designers position their power
supply's nominal voltage closer to the lower end of the
processor's permitted range to reduce power consumption.
Doing this can be quite effective, because power
consumption is proportional to the square of the supply
voltage. Given the 3.0V to 3.6V range of permitted
processor voltages, a 3.15V ±2% supply would be
suitable, provided there is no significant voltage drop
through the edge connector and trace that connects the
supply to the processor. The MAX6381 POR with the
2.85V to 3.0V threshold voltage range would be an appropriate
choice, if noise levels are sufficiently low to
prevent false resets.
Determining the POR Threshold Voltage—Dual-Supply Processors
If a processor requires another supply (e.g., a 1.8V core
supply) in addition to a 3.3V supply, then the design may
call for a POR that monitors two voltages. This type of
POR deasserts its reset only after both supplies are above
the POR's two corresponding thresholds and the required
timeout period has passed. PORs that monitor two, three,
and four voltages are available.
The same choices apply when monitoring multiple
supplies or a single supply. For the dual-supply case (e.g.,
3.3V and 1.8V), one can elect to use a POR with two
thresholds that are both above or below the processor's
minimum guaranteed operating voltages. Also, one could
use a threshold that is below the guaranteed operating
voltage for the 3.3V I/O supply and another threshold that
is above the guaranteed operating voltage for the 1.8V
core supply. Some board designers opt for the latter
strategy, because sometimes the core of the processor is
more sensitive than its I/O to problems caused by a low
supply voltage.
Core supply voltages have consistently dropped over
time, and thus reduced POR threshold voltages have
become necessary. Devices within the MAX6736 family
provide thresholds as low as 788mV without external
resistors, and as low as 488mV with external resistors.
These thresholds are low enough to monitor most modern
core voltages.
For low-cost systems, some circuit designers elect to
monitor only the 3.3V supply if the 1.8V supply is
derived from it. They assume that if the 3.3V supply
reaches its correct voltage, the 1.8V supply will follow.
For systems requiring higher reliability, designers usually
decide to monitor both supplies.
Manual Reset
It is often useful to manually trigger a reset while the
power-supply voltage remains within tolerance. Not only
is this feature used for debugging and final testing, it is
also valuable when the processor locks up—it allows the
processor to restart without turning off the power. This
function is especially useful for those products with
processors that are never powered down. It is common for
an on/off switch to only wake up/suspend the processor
without ever turning off the processor power.
Although a logic signal from an I/O line, a watchdog
timer, or a power-fail output often initiates a manual
reset, a pushbutton switch can also be used. When
pressed, this type of switch usually bounces, opening
and closing several times before settling in the appropriate
state. Therefore, most manual-reset inputs include
debounce circuitry that ignores the ringing caused by the
pushbutton switch.
Discrete PORs and PORs Internal to the
Processor
Using a discrete POR created with a resistor and capacitor
(Figure 4a) is a risky proposition. The longer rise and fall
times at the output of this type of POR can create
problems for some processors—especially those with reset
inputs not including a Schmitt trigger and for those with
bidirectional reset pins. Adding a Schmitt trigger can help
the former case, but can also contribute to cost, space, and
startup issues.
Figure 4. The discrete R/C POR (Figure 4a) is not reliable enough for most applications. In some cases, adding a diode to the circuit
(Figure 4b) corrects quick-supply-cycling problems and
improves the circuit's performance.
Another problem arises when a discrete POR is used
along with a supply that, when powering up, rises slowly
in relation to the POR time constant. The processor can
come out of reset well before it has stabilized. To
prevent this problem, the time constant of the R/C circuit
may need to be increased. Also, some manufacturers
whose processors include an internal POR, recommend
that an R/C (plus a diode described below) be added to
the reset input if the power supply comes up slowly.
If the power supply has a glitch after power up, the R/C
circuit might filter that glitch, thus preventing a reset from
happening. Also, if the supply droops, the voltage at the
processor's reset pin could remain higher than its VIH,
which is too high for a reset to occur. This can transpire
even when the supply has dropped below the processor's
minimum guaranteed operating voltage. This happens
because a reset pin's VIH is often lower than the
processor's minimum guaranteed operating voltage.
Another problem can arise if the power is turned off and
then on again quickly—the capacitor might not have sufficient
time to discharge prior to the power coming back on.
By adding a diode (Figure 4b), the R/C circuit can
respond to glitches, because the diode quickly discharges
the capacitor whenever a glitch appears. The glitch must
be sufficiently large to drop the voltage at the reset pin to
VIL (min). Additionally, the other problems listed previously
for the R/C circuit without the diode can potentially
plague this circuit. However, sometimes the diode does fix
the problem created when the supply is quickly cycled off
and on.
Using an integrated POR makes the most sense for most
equipment, as this device creates none of these problems.
Using a POR internal to a processor can also cause difficulties.
These PORs often suffer from inaccuracy and can
exhibit problems at lower voltages. Furthermore, some
internal PORs are set up to provide a reset during powerup,
but not when the supply voltage dips during a
brownout condition. Some manufacturers suggest adding
a discrete circuit to accommodate that condition.
Finally, a system powered by multiple supplies may pose
another problem for an internal POR. For example, you
could encounter a problem when an internal POR timeout
period is appropriate for its processor, but not for external
circuitry (e.g., memory) whose supply voltage comes up
more slowly. In that case, a solution would be an external
POR with a longer delay time that monitors both the
processor and the external-circuit supplies.
Power-Fail and Low-Line Signals
Supervisor circuits that include power-fail or low-line
signals warn the processor that a brownout or power
failure is imminent. When either of those signals interrupts
the processor, the processor can enter a power-down
routine. This routine causes the processor to cease its
current activities and back up important data prior to the
POR placing the processor in reset.
To create a power-fail signal, the supervisor's power-fail
comparator monitors the unregulated DC voltage (or some
other upstream regulated voltage). This voltage feeds the
regulator, which powers both the processor and supervisor
circuit. The unregulated voltage drops before the
regulator's voltage because the regulator's output
capacitor retains its output voltage (Figure 5). Thus, a
drop in the unregulated voltage indicates a possible drop
in the regulator's voltage. Detecting that drop and interrupting
the processor allow the processor to enter its
power-down routine prior to being reset, if the powersupply
voltage were to drop low enough.
Figure 5. The power-fail comparator within the MAX6342 generates the power-fail signal (PFO-bar) by monitoring whether the unregulated DC supply has dropped.
When there is no access to the unregulated voltage (or an
upstream regulated voltage), the processor can still receive
warning of an imminent power failure. That warning
could come from a supervisor that provides a low-line
signal, which goes active whenever the monitored power
supply drops to a level slightly above the reset threshold
(e.g., 150mV above). Thus, the low-line signal warns the
processor that the power-supply voltage may decrease
enough to cause the POR to issue a reset. Here, as with a
power-fail comparator's signal, the processor backs up
important data in anticipation of the POR generating a
reset due to a brownout or power failure.
Voltage Sequencing and Voltage Tracking
Most data sheets of processors powered by two supplies
specify the order in which the supplies should come up.
Parts such as the MAX6819/MAX6820 can sequence the
supplies in the proper order. If the processor's supplies
are not sequenced properly, the processor can latch up,
initiate incorrectly, or endure long-term reliability degradation.
Sometimes, the various supply voltages are not
locally generated (e.g., they come from a main system
bus, an externally purchased silver box, or supplies that
do not include enable and power-OK pins that facilitate
sequencing). In such cases, power-on and power-off
sequencing can be difficult to control or predict, thus
making a voltage-sequencing IC necessary. This type of
IC is also needed when different resistive and capacitive
loads affect the turn-on and turn-off times of the various
supplies. This makes it difficult to predict the order in
which the supplies power up and down.
A unique method for sequencing two power supplies is
found in the MAX6741/MAX6744. These devices work
by first allowing one supply to power up. Then, after a
delay period, they allow the second power supply to
power up by issuing a power-OK signal, which takes the
supply out of shutdown. After both supplies are up and
another time delay elapses, the MAX6741/MAX6744
reset signal deasserts.
Some processors require that the two supplies track each
other during power up. In that case, the MAX5039/
MAX5040 can achieve tracking by clamping the two
supplies together until the lower-voltage supply reaches its
final voltage. At this point, the higher-voltage supply is
free to continue up to its final voltage.
Reset Sequencing
When a circuit incorporates two processors, often one
processor must come out of reset prior to the second.
Previously, board designers wired two PORs together to
handle this requirement. The output of the first POR both
reset the first processor and controlled the manual-reset
input of the second. The second POR output reset the
second processor (or, in some cases, the memory).
Currently, dual PORs with time-staggered reset outputs are
available for this task (Figure 6). These PORs assert both
reset outputs whenever the master supply voltage (3.3V, in
Figure 6) strays below the POR's internally set threshold.
(The slave POR asserts slightly before the master.) Once
the supply returns above this threshold, one of the two
reset outputs deasserts after its timer has timed out
(active-low RESET1, in Figure 6). For the second POR to initiate its
timer and deassert its output, two conditions must be met:
active-low RESET must be deasserted; and the slave supply voltage,
monitored by the second POR, must be above the threshold
set by external resistors. If the same supply voltage powers
both processors, RSTIN2 can be connected directly to the
supply instead of using a voltage divider.
Figure 6. This circuit allows the master processor to come out of reset prior to the slave by monitoring the supplies that power the two processors.
For the MAX6392 shown in Figure 6, the second POR
output always comes out of reset after the first output. In
fact, the time specified for it to come out of reset is
measured from the time that the first output deasserts.
Thus, the Figure 6 circuit forces the slave processor to
come out of reset after the master processor has begun
operating. The second POR delay time can be increased
by adding a capacitor to the IC.
If three processors need to be sequenced, the DS1830 can
be considered. The three PORs within this device operate
with minimum reset periods of 10ms, 50ms, and 100ms
from the time the power-supply voltage crosses the POR
threshold. A single logic pin allows multiplication of those
reset periods by a factor of two or five.
Conclusion
Although choosing the appropriate microprocessor supervisor
and operating it correctly are often straightforward,
some aspects of that exercise may require careful
planning. Such is the case with power-on resets. Choosing
the correct voltage and tolerance for both the power
supply and the POR threshold requires some thought.
Also, well worth considering are newer devices that
accommodate processor requirements such as multiplevoltage
reset, reset sequencing, power sequencing, and
voltage tracking.
A similar article appeared in the April, 2004 issue of EDN.
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