oscillating between starting and crashing. Such behavior could result in higher overall power
consumption or even audible noise of power converters. Some LEDs may slightly light up or start
blinking.
Backfeeding can cause latch-up situations. IO blocks can go into unintended states, which might
even cause short-circuits that could lead to further chip damages if countermeasures are not taken.
If the IO rail reaches a certain voltage level, it could mean that some power-on-reset circuits are
not triggering when fully turning on the power rails since the reset was already released. This can
cause devices and peripherals to stay in unintended/unpredictable states and might fail to boot the
system. The system could be locked up by backfeeding.
Whether backfeeding is actually causing issues or not depends on the backfeeding current and its
residual voltage on the IO rail. It also depends on the specific chip design. The lower the current
per backfeeding pin is, the smaller the chance of damage is. The lower the remaining voltage on
the IO rail is, the smaller the chance of unexpected behavior is. As a rule of thumb, a few
milliamperes are unlikely to cause damages to an IO pin. If the resulting IO rail voltage is below
0.5V, issues are often not to be expected. However, these are just rough numbers. The actual limits
are heavily depending on the system and the actual devices in use.
3.7.4 Identify Backfeeding Issues
3.7.4.1 System Design
Ideally, potential backfeeding issues should be identified in the design phase of a carrier board,
not in the prototype phase. A power delivery block diagram can help to identify different power
domains. A power domain is a group of devices or peripherals which are always in the same
power state. Figure 36 shows a simple example of a system block diagram. The Colibri module
and the RS232 transceiver are not in the same power domain. The transceiver uses the 3.3V rail,
which can still be enabled while on the Colibri module, the IO voltage rails are turned off.
However, the behavior of the module IO voltage rail depends on the Colibri module.
In this example, the SD card also has its own power domain since the SODIMM pin 100 might be
used for individually control the card power. Also, external devices like a VGA display or the host
PC must be considered as separate power domains. These peripheral devices are powered
independently from the Colibri carrier board.
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