A project log for FTDI-be-gone

My version of the ubiquitous USB serial port gizmo

nick-sayerNick Sayer 03/21/2016 at 12:590 Comments

The latest boards for the RS-232 variant have a little problem.

I switched from using !SLEEP to using !POWER for the !FORCEOFF pin. Well, !SLEEP and !POWER sort of have opposite meanings. I had intended to use !POWER because it is an indication of the chip either being in USB suspend OR not yet being successfully enumerated. The trouble is, it's low when that happens. !SLEEP was low when in USB suspend. !POWER is high during USB suspend.

It turns out, though, that I can go back to using !SLEEP for the RS-232 variant, because prior to enumeration, you're allowed to pull up to 100 mA while in suspend you're only allowed 2.5 mA. Using !SLEEP to power off the level converter, therefore, is necessary, but turning off that chip before enumerate isn't.

I also can salvage the boards I have now by remapping the GPIO pins with Cypress' configuration utility.

This won't affect the TTL variant boards that are on the way, because the MOSFET that gates power for the output header really does need an active-low power signal. So there's that.