|Re: [WP34S] HP cable vs USB adapters compatibility question|
Message #3 Posted by E.Lub_EU on 27 Sept 2011, 6:00 a.m.,
in response to message #2 by Marcus von Cube, Germany
Thanks for your elaborate response.
I have come exactly to the same conclusion, and am preparing an order from FTDI on the same line:
- one "plain" USB-RS232 to use with an unmodified HP cable,
- one USB-"3V-TTL-serial" with an open end cable.
I had come to the same implementation idea (cutting the wide trace between the switch and the level shifter), cabling direct from the FTDI cable to the calculator side of the HP cable. Big advantage is that there is no need for another PCB inside the HP "button box".
Some possible caveats could be:
- the 3V3 signals from the FTDI chip will exceed hte Vih spec, being above the Vbat level.
Probably just ok with fresh batteries, unless the 20mA drain causes them to sag significantly. Has this been measured already?
With partly depleted batteries, one would actually offset the SAM7 internal Vdd plane value through the Vddio (bit of speculation here, as the internals of the SAM7 are not that detailed), I believe that keeping this condition for a long time might even damage the chip.
There might also be a problem with the reverse (charging) current spec of the batteries, like when leaving the calc in low-power state with the cable connected (USB powered)
This does leave the calculator running from ts own batteries, and it was mentionned here that they drain fast under these conditions.
So I was toying with the idea of actually powering the calc through the cable (same work on the cable PCB, plus connect Vbat to the 3V3 from FTDI).
This implies some constraints on the user:
- removing both batteries before plugging in the cable, so saving RAM to flash is mandatory befor starting a serial session,
- same before exiting the serial session, as the cable has to be disconnected before any battery can go in,
- a slight risk of losing power to the usb if/when the PC goes into standby. Aprat from changing the PC settings, this can be avoided easily by powering the USB cable through a powered hub.
This solution might appeal more to a developper (long sessions with a PC connection), that an "normal" WP34S user.
Sorry for the lenth of this repliy!
I am still puzzled by the Gnd on pin 9 in the mentionned document though. Will check when I get mine.
Thanks for your help,