Re: gEDA-user: Announcement : gTAG schematics released

["Andrew M. Dyer" <adyer@enteract.com>]

I looked over the schematics and I had a few comments in no particular
order.  You may consider it overkill, and that's okay, they're just
suggestions.

1) I would consider opto isolating the jtag from the USB side to keep
ground loops from doing strange things - you could conceivably have a
loop between the gTAG board, target PCB, wall socket, wiring, PC and
back to gTAG spanning many meters that can cause a lot of problems.
This might be especially important in a production line environment.

2) pull-up/pull-down the lines from the micro to the 7414 to keep them
from floating and doing weird stuff before the micro gets around to
initializing them as outputs

3) consider re-ordering the pinout of the JTAG connector to allow use
of the NC pin as a keying pin, so if somebody made a nice cable, you
could be sure the (l)user would plug it in right.

4) USB connector shows no connections for the metal shield

5) you might consider making the connectors into two-row versions to allow
for ribbon cables.

6) maybe add some leds for the programmers on PA4/PA5

7) I see no decoupling for the 7414

8) maybe add a two pin jumper on reset to allow shorting it to ground for
development

9) current limiting resistors on the outputs of the 7414 might be a good
idea and/or protection diodes.  Slew rate limiting caps might be good if you
are using a higher speed variant of the 7414.

10) you have the wakeup# pin on the micro pulled high - if you aren't
planning on using suspend mode the docs say to hold the pin low.

11) what type of 7414 are you planning on using?  If the 7414 is
running off the jtag_power at a +5V rail will the 3.3V logic on the
microcontroller be able to swing to a high enough voltage to trigger
the schmitt trigger inputs?  Is the micro 5V tolerant on the input?  I
haven't looked...  You might also consider using "open collector"
outputs on the micro with pull-ups tied to jtag_power.