Figure 36: Power Domain Example
Each time a signal crosses the boundaries of power domains, you need to check whether
backfeeding could be an issue. For each signal, check whether the output is driven high while the
other side's power domain is turned off. In the example above, we need to check whether the
software drives and SD card signal driven high or pulled-up while the SD card power rail is turned
off. Otherwise, the module can back feed to the card. The signal states of the external devices like
the RS232 signals of the host PC or the VGA display's sync signals are hard to control by the Colibri
module and the carrier board. Therefore, it might be required to take other countermeasures for
preventing the backfeeding of these interfaces.
3.7.4.2 Prototype Testing
For identifying backfeeding on a system with a Colibri module, it is recommended to measure the
IO rails of the different power domains in different scenarios. Different scenarios mean testing
different power states of the systems with different types of peripheral devices plugged in and
turned on. If you can measure any significant residual voltage on an IO rail, further investigations
are required for identifying the source of the backfeeding.
The first option is to unplug external peripherals and observe the residual voltage. If the voltage
drops, the peripheral signals are likely a source of backfeeding. If you measure residual voltage in
one power domain, all the input signals crossing this domain should be checked. Measure the
voltage levels on these inputs. A pin that is the source for backfeeding has a higher voltage than
the residual voltage on its IO rail. Typically, due to the protection diode in the backfeeding path,
the voltage at the input pin is around 200mV to 300mV higher than the residual voltage (see
Figure 37)
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