It is a matter of timing.


The SYSEN line goes high before the Gumstix looks for input on UART. At the time when the Gumstix looks for input on the UART, the UART is already pulled up.


The SYSTEN line is active high – which makes sense, because it should be low when there is no power. However the OE lines on the voltage converter chips are all active low. This means you need to add a single inverter chip.




From: Dan Nelson [via Gumstix] [mailto:[hidden email]]
Sent: 04 April 2012 11:37 AM
To: glamprecht
Subject: Re: How to debug hardware problem?


I’ve taken this from the design recommendations for custom expansion boards on the Gumstix site:


“4.  Noise on the UART_RX3 line during boot can interrupt the normal booting sequence. Consider adding a pull-up resistor to a SYSEN-gated 1.8V supply on this line.”


Can anyone tell me what this means?


This isn’t  implemented on the Tobi board, there are no pull-ups.   The Tobi board leaves the UART_RX3 line floating until its gate is enabled by SYSEN.  Is this what’s recommended?  Or do you enable the 1.8 volt supply straight away (and thus the gate) and pull up  UART_RX3 to 1.8 volts?  Which would tend to contradict:


“1.  Use the SYSEN line to protect any IO pins to the OMAP CPU. SYSEN is brought high when the Overo is ready to communicate; driving GPIOs before this point can damage the processor.”






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