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Note that earlier versions of the AM57x SOM (PCM-057.A5 and earlier) populate C251 with a 1uF capacitor, which is connected between X_PWRON and ground. This capacitor will introduce further delay on the X_PWRON rise/fall time, potentially causing the board not to respond or boot as expected. The design was updated to populate C251 with a smaller 0.1uF capacitor, which results in an X_PWRON response similar to Figure 2. Consider this capacitor value when implementing this design with an older version of the SOM. |
Trouble Shooting
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Please refer to the following timing diagram. Each window of time labeled 1, 2, 3, etc... will be referred to as T1, T2, T3, etc... respectively.
Issue #1 - Booting is inconsistent, unreliable, or even not working at all.
Solution #1 - This may be a result of X_PWRON being toggled low too soon before VCC_3V3 is on and stable. For reliable booting, ensure that T3 is at least ~5.5ms to allow the device to arrive at the 'OFF' state from the 'NO SUPPLY' state (further details available in this application note). Increase the R3 and C2 values to increase the delay time before X_PWRON is toggled low.
Issue #2 - Board begins to boot, but then shuts off after a few seconds.
Solution #1 - This may be a result of X_PWRON being toggled low for too long, triggering a 'long press' at the PMIC (typically a few seconds). When a long press is detected the PMIC will transition the system to the OFF state. The X_PWRON low time, T4, can be adjusted by decreasing the values of R2 and C3.
Circuit Design
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This circuit will be powered by the main 3.3V rail (labeled VCC_3V3_IN on the AM57x SOM). The circuit consists of an RC delay stage, a Schmitt trigger stage, and an RC + FET circuit to generate the X_PWRON pulse.
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