Verdin Carrier Board Design Guide
Preliminary
– Subject to Change
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cause devices and peripherals to stay in unintended/unpredictable states, and might result in a
failure 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 unexptected behavior is. As a rule of thumb, a few
milliamperes is 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.
3.5.4
Identify Backfeeding Issues
3.5.4.1
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 69 shows a simple example of a system block diagram. The Verdin module and
the RS232 transceiver are in the same power domain. The transceiver uses the PWR_1V8_MOCI,
which switches on together with the IO rails of the module.
On the other hand, the camera is in a different power domain since its power gets enabled by the
CTRL_SLEEP_MOCI# signal. Therefore, the power rails of the camera are switched off during sleep
states. In this example, the SD card is also in its own power domain since the SD_1_PWR_EN signal
individually controls the card power. Also, external devices like the HDMI display or the host PC
must be considered as separate power domains. These peripheral devices are powered
independently from the Verdin carrier board.
Figure 69: 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 a camera control signal high while the system is in sleep mode
(CTRL_SLEEP_MOCI# low). In the Verdin platform, the pull-up resistors of the SD card interface are
