README.md
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Imperial Debugger

USB Based multi serial port adaptor

Overview

Based on an FTDI FT4232HL USB to serial chip, with 4 ports.

The plan is to bring all points to bring them out as different standards.


1x RS232 with hardware flow control
1x RS422/485 with configuration to switch between the two, wither in software or hardware.
1x RS232/422 selectable
1x Logic level, with external level shifter (selectable to internal 3v3)


Ideas


Onboard 5V to 3v3 voltage converter.
All ports should have logic level headers, and the xcvr based ports should have
jumpers to enable/disable those  from the outputs. That makes it easier to convert to and from the modes.


Power Supply


XCVRs need 3v3 supply. So need to convert down from the USB ~5V. We have ~500mA available, so easily enough
Needs overload protection?
Local ground connections for the Logic level inputs.
Supply 3v3 out? Maybe, with 100mA limit?


Potential XCVR chips

RS232


TI MAX3222E -- Looks good. Has protection in it. 250k min speeds.


MAX3222EEWN is WideSOIC


Also available in SSOP 20 pin, which is harder to solder.

4 signals, so TX/RX and CTS/RTS
3222 has a shutdown pin, Active Low.



RS422/485


MAX3491E
MAX22503
MAX3070-3073-3076 (3076 is fastest, all pin compatible)
SP330 (can do 232 as well, so could use just 3 of these?)


Logic level


Ideally we want to go from 3v3 ->5V but also 3v3 > lower (2.5, 1.8)
LSF0204, can do 3v3 down to Ref_A. Ref_B Is always the higher
To generate 3v3 > 5V need a different route, where the signals are reversed
Such that the input is on the Ref_B side, so could use 2 sets and jumper it.


Could use 3 pin jumpers, to direct the signal to one of two, or maybe use
bus selector chip to auto do it.

Most ttl/cmos all work the same.
LSF0102 - nexperia
NCA9306DC - nexperia


Potential PSU design


LM2594M


In stock, cheap.
~500mA Output
-- Initial choice

MIC4680


In stock.
~1A output

TPS5403


Hard to get hold of, but in stock.

LT1506


Expensive. Hard to get.



Notes


Use externnal eeprom (SPI based) this will allow the usage of the nRI as a TXEN line for RS422/485 usage


93LC56B is recommended. 16bit wide 2Kb eeprom.

Going to use Bus Powered, into 3v3 Reg, then core will generate it's own 1v8.
Use the 3v3 to power the xcvrs