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ZeroPhone / ZeroPhone-PCBs

PCB files for ZeroPhone - for gerbers and schematics, go to "Releases" tab!
GNU General Public License v3.0
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UART buffer addition #6

Closed CRImier closed 6 years ago

CRImier commented 7 years ago

Currently, the Pi Zero UART (3.3V) is routed straight to the GSM UART (2.8V). The SIM800 datasheet advises against this, but fixing it would require quite some additional parts, such as a buffer of some kind, a voltage reference for 2.8V (since it's not exposed on the breakout). One more thing is that the same UART is, most likely, going to be used for the debug console - because there is just one UART, and debug console is a vital feature when doing low-level stuff and generally just debugging the reasons why system does not boot. So, theoretically, same buffer could be used for disconnecting the UART from the GSM modem - provided there's a free GPIO that could toggle the buffer, making sure that Linux boot data is not sent to the modem until the buffer is sent, and the boot console doesn't confuse the modem by printing lots of characters that may or may not contain something interpretable as AT commands. Ideally, the buffer could also be controlled by a small slide switch - so that you can signal the system not to turn the UART debug console off.

A suitable (easy to source, soldering-iron-solderable and cheap) buffer IC needs to be chosen, then added into the circuit, possibly using one of the GPIOs that are still free on the MCP23017.

CRImier commented 7 years ago

Update - yes, the modem receives Linux boot data and this is a problem. I'll most likely work around this in software, keeping modem in RESET state (using RESET GPIO on MCP23017) until the GSM software starts.

CRImier commented 6 years ago

Done, should be on the Delta revision boards. Both RXD and TXD of the modem are now controlled by the buffer.