
is1C33L17sPeCifiCations:PreCautionsonMounting
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noise-inducederraticoperations
If erratic IC operations appear to be attributable to noise, consider the following five points.
(1) TEST0 pin
If this pin is exposed to high-level noise, the entire IC enters test mode or a high-impedance state and
becomes inoperable. In such cases, the IC will not be restored, even when the pin is returned to a low level.
Therefore, always make sure the TEST0 pin is connected to GND on the circuit board. Although the IC
contains internal pull-down resistors, it is susceptible to noise because these resistors are high impedance
(approximately 50 to 100 k
).
(2) DSIO pin
Exposure of this pin to low-level noise causes the IC to enter debug mode. In debug mode, the clock is
output from the DCLK pin and the DST2 pin is high, indicating that the IC is in debug mode.
In product versions, it is recommended that the DSIO pin be pulled high by connecting it directly to VDD or
through a resistor of 10 k
or less.
Although the IC contains internal pull-up resistors, it is susceptible to noise because these resistors are high
impedance (approximately 50 to 100 k
).
For details, refer to the “S1C33 Family Application Note.”
(3) #RESET pin
Low-level noise on this pin resets the IC. However, the IC may not always be reset normally, depending on
the input waveform.
Due to circuit design, this situation tends to occur when the reset input is in the high state, with high
impedance. For details, refer to the “S1C33 Family Application Note.”
(4) #NMI pin
Low-level noise on this pin causes an NMI interrupt. Due to the circuit design, this situation tends to occur
when the #NMI pin is in the high state, with high impedance. Lower the impedance of #NMI when it is
held high, or incorporate corrective measures into the software to protect against erratic operations.
(5) VDD, VSS, and VDDH power supplies
If noise lower than the rated voltage enters one of these power-supply lines, the IC may operate erratically.
Take corrective measures in board design; for example, by using solid patterns for power supply lines,
adding decoupling capacitors to eliminate noise, or incorporating surge/noise counteracting devices into the
power supply lines.
To confirm the above, use an oscilloscope capable of observing higher-frequency waveforms of 200 MHz. The
generation of fast noise may not be observed with a low-frequency oscilloscope.
If potential noise-induced erratic operations are detected through waveform observations using an oscilloscope,
connect the suspected pin to the GND or power supply with low impedance (1 k
or less) and check once
again. If erratic operations are no longer detected or occur at reduced frequency, or if different symptoms of
erratic operations are observed, said pin may with reasonably certainty be considered to be the source of the
erratic operations.
The TEST0, DSIO, #RESET, and #NMI input circuits described above are designed to detect the edges of
the input signal (#NMI can be changed to level sense mode), so that even spike noise may result in erratic
operations. Among the digital signal circuits, these pins are most susceptible to noise.
In the design of the circuit board, take the following two points into consideration to protect the signal from
noise.
(A) The most important measure is to lower the signal-driving impedance, as described in each item above.
Connect pins to the power supply or GND, with impedance of 1 k
or less, preferably 0 . In addition,
limit the length of the connected signal lines to approximately 5 cm.
(B) Parallel routing of said signal lines with other digital lines on the board is undesirable, since the noise
generated when the signal changes from high to low or vice versa may adversely affect signals. The signal
may be subject to the most noise when signal lines are laid between multiple signal lines whose states
change simultaneously. Take corrective measures by shortening the parallel distance (to several cm) or
separating signal lines (2 mm or more).