7.4 I/O Pin Current Injection from Remote Circuits
Problem 4: Intermittent MCU POR, BOR, BOD reset issues occur during start-up, resulting in exception errors or
lock ups.
In interconnected designs, (i.e., cable connected PCB’
s as depicted in the following figure to the left side of the
image), where remote interface circuits have independent power supply sources, injection current is a constant
concern that can cause intermittent start up related issues due to POR, BOR, and BOD anomalies because the
MCU can be powered through the I/O pins internal high side pin protection diodes (i.e., image on the right) rather
than the VDD when MCU power is switched off from the remote still active circuits. In those cases, the VDD never
actually goes to a voltage low enough to enable the MCU internal Power-on-Reset circuits properly. This can usually
be detected if that is the case by taking a scope plot during the power up and power off period and inspect for any
VDD anomalies where the VDD does not start from 0v volts as expected when it is turned off or the VDD appears
to power up to between 0.6v to 1.2v, plateaus for a brief time, then continues upward to its expected nominal value,
similar to the VDD Ramp Rate Violation example. Designs like these are very susceptible to the following:
• Power sequencing in the interconnected PCB network
• Ground loops which are covered later
Figure 7-5. I/O Pin Current Injection
+
-
C
O
N
N
P
W
R
C
O
N
N
P
W
R
C
O
N
N
C
O
N
N
P
W
R
Remote PCB_1
Remote PCB_2
Power
Power
+
-
+
-
Power
MCU
Supply A
Supply B
Supply C
MCU Start-up Problems
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echnology Inc.
and its subsidiaries
Manual
DS70005439B-page 16
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