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How to speed up development and avoid reliability trouble in hardware?

Latest Updated:07/01/2009

Question:

Hardware development (to speed up development and avoid reliability problems)

Answer:

(1)Specifications

• Do not design or use semiconductor devices exceeding the specification range.
Exceeding the specification range may cause malfunction, quality degradation or irreparable damage, leading to a defective end product.
Moreover, the performance of devices differs depending on the device, so it is not possible to use a device's "potential" ability when designing your system. Devices must be designed considering the maximum and minimum values to be "worst case" values.
Note also that Zener diodes must be inserted for protection in places where static electricity or surge can occur.
When connecting devices to each other, the absolute maximum ratings, AC characteristics and DC characteristics of both devices must be compared. Note that the voltage specified in the DC characteristics is the steady-state voltage. Factors such as transient properties on the output side or the wiring inductance may cause an undershoot or overshoot, which may cause the absolute maximum ratings on the input side to be exceeded. In this case, measures such as inserting a noise-clipping diode will be required to avoid problems such as latch-up on the input side or degradation of characteristics.


(2)Pin handling

• Do not leave the input pins of CMOS devices unconnected (except when they are pulled up internally).
Unconnected input pins may lead to the occurrence of noise-attributable through-current, which may result in malfunction or quality degradation.
Note also that for ports that can be switched between input and output and that are connected to the gate of a device such as a FET, while the port is in the input state (and when the input is Hi-Z), the port is in the same state as when it is not connected. In this case, because there is a danger of both connected devices operating unexpectedly, if the level of voltage applied to the gate of the external FET must be fixed until the state of the port is changed to output following reset, that port must be pulled up or down externally.

• Do not connect output pins directly to each other (except open-drain or open-collector pins).
If signals with different polarities collide, an overcurrent exceeding the DC characteristics may flow, causing quality degradation or irreparable damage to the device.

• Do not connect output pins directly to power supply or ground pins.
In the same way as described above with regard to connecting output pins to each other, this may lead to quality degradation or irreparable damage to the device.
Even if the level is fixed when using I/O pins as input pins, the best method is to insert a resistor, because the pin may inadvertently be changed into an output by the effect of noise.

• When inserting a capacitor between the signal lines and ground lines in a MOS device, take measures to prevent inrush current or overvoltage.
Noise prevention measures have been well considered, but simply inserting a capacitor can also cause quality degradation or irreparable damage to the device. This is because an overcurrent exceeding the DC characteristics flows when the signal changes, and also an overvoltage exceeding the absolute maximum ratings is applied when the power supply is cut off (0 V).
It is also necessary to confirm that a change in the time constant will not cause problems with the AC characteristics on the input side.

• When using a motor or coil with an inductive load, be sure to make allowances for surge.
If the drive current of the inductive load changes, the flux around the current also changes and a back electromotive force is generated. Consequently, a large current is made to flow in the direction in which the inductive load prevents this change. This instantaneous large current, or surge, can degrade or damage the drive elements. If a surge exceeding the drive element rating might occur, take measures such as inserting a diode or snubber circuit. Using a transistor or FET that incorporates a diode is also effective.

• When using a power sequence in which the power is cut off, measures may be needed to prevent overvoltage of an input signal from an external device, if the inputs are MOS inputs.
If the power supply voltage is specified as "VDD + a" in accordance with the absolute maximum ratings of the input voltage and if a signal is input when the power supply is cut off (0 V), it may cause quality degradation or irreparable damage to the device.
Moreover, if the power is then reapplied in this state, latch-up can easily occur.
Note that for pins described as having an absolute value of "+6.5 V", etc., inputting a voltage up to this value does not cause a problem.

• Do not generate short pulses that do not satisfy the specifications by switching the external input clock frequency or supply source.
As is described in (1), this may lead to malfunction.
Care is especially required when asynchronously switching the signals using a manual switch.
Moreover, if spike noise occurs during switching, it may cause the voltage to exceed the DC characteristics, leading to quality degradation or irreparable damage to the device.

• Take measures to prevent chattering on the input by manual switches, etc., in a digital circuit.
The operation of mechanical devices such as switches, keys and relays may cause chattering, which can lead to the generation of short pulses. Depending on the circuit, these short pulses may cause the device to malfunction.
When these kinds of mechanical inputs are correctly recognized by the microcontroller, chattering can be prevented by inserting a sampling wait using a soft timer (program processing).
Note that if you are using a sampling circuit in which chattering does not cause a problem because the input is from a toggle switch or circuit switch (i.e., because the sampling interval is longer than the length of chattering), there is no need to take these preventive measures.

• When using a microcontroller in which the analog input pins function alternately as port pins and the AVREF pin is the power supply of the port circuits, power must be supplied to the AVREF pin.
If AVREF is connected to GND, the ports will not function or an abnormal current will flow internally, affecting the device's reliability.
Check the pin I/O circuits to see whether the power supply of the port circuits is AVREF or VDD in the user's manual. (If the power supply is VDD, AVREF can be connected to GND.)

• A design that includes a buffer with the appropriate tolerance is required when connecting devices with different power supplies.
For example, when connecting two CMOS devices with different power supplies—one that operates on 3.3 V and one that operates on 5 V—an open-drain buffer with a tolerance of at least 5 V must be used at the output of the 3.3 V device, and that output must be pulled up to 5 V by using a pull-up resistor. A buffer with a tolerance of at least 5 V is also required at the input of the 3.3 V device. If a normal buffer without a tolerance of at least 5 V is used, the absolute maximum ratings will be exceeded, causing problems with reliability.


(3)Startup

• Do not start up a semiconductor device that has an on-chip oscillator before oscillation has stabilized.
It is impossible to predict what kind of operation will occur if the device operates on an unstable clock.
If the clock is being supplied to the device from an external oscillator, release the power-on reset signal after the clock has stabilized.
Note that some microcontrollers with on-chip oscillators allow an oscillation stabilization wait time to be set via an internal timer, and the oscillation status to be checked via a flag. It may also be possible to set an oscillation stabilization wait time at startup by setting an option byte, or to program the wait time to be set following startup using software.

(4)Signal changes

• Do not make the rise and fall of the input signals too sluggish.
The transition time of the signals is generally prescribed and must be observed.
However, since the port statuses in a microcontroller are read by program, which takes longer than when the statuses are read by random logic, the rise and fall times of the ports are not prescribed. However, problems may occur if the transition states are too long. It is therefore necessary to avoid situations such as the signal edges being made to sluggish by a large-capacity capacitor.
Note that a current or capacitance overload (causing inrush current) may affect not only the shape of the waveforms, but also the reliability of the device.

(5)Supplying power

• Do not change the power supply voltage too rapidly.
Products with an operating power supply voltage of "1.8 to 5.5 V", etc., can still operate even if the voltage changes, as long as it is within this range. However, make sure that the voltage change is within 0.1 V/ms. Also be sure to check any changes in operating frequency range, DC characteristics or absolute maximum ratings that may depend on the power supply voltage.

• When connecting signals between devices that operate on different power supplies, the ground potential of both devices must be the same.
If there is a difference in the ground potential of the two devices, a relative shift will occur in the input and output voltages, causing malfunction or irreparable damage to the devices. If the ground potentials do differ, either connect the grounds to each other, or if this is not possible, take measures such as electrically isolating the signal lines by using photocouplers.

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