DESIGNCYCLETHE
ADVANCED TECHNIQUES FOR DESIGN ENGINEERS
GETTIN’ JIGGY WITH THE
VINCULUM-II HARDWARE DESIGN
■ BY FRED EADY
Heat up your soldering iron. It’s time to forge some solder, silicon, ceramics,
The first order of business is to pick up where we left off last month.
A MINOR DESIGN CHANGE
If you compare Schematic 1 with last month’s EDTP
Vinculum-II debug module schematic, you’ll see that the
fuse component has been replaced. If you are ahead of
the game and have already built up a version of the EDTP
Vinculum-II debug module, no worries as the new fuse
component has the same footprint as the fuse unit it
replaces. The fuse that has been replaced was not rated to
pass the maximum amount of current that a USB portal
can provide. The PSMF020X is designed to hold at 200
mA and trip at 500 mA. The PSMF020X’s amperage range
is just fine for most low power circuitry. However, the
replacement fuse — a PSMFX050X — holds at 500 mA and
trips at 1,000 mA. Thus, if your external circuitry draws
more than 200 mA and teeters at the maximum current
drain of 500 mA, you won’t be as likely to inadvertently
trip the replacement resettable fuse. The host USB portal
contains a similar over-current protection circuit and
should not allow more than 500 mA of current flow for an
extended period of time. However, my advice is to use
your good judgment as to how not to invoke your circuit’s
magic smoke.
THE PRINTED CIRCUIT BOARD DESIGN
I remember the very first time I designed a circuit
using 1/8 watt through-hole resistors. I recall worrying
about how I was going to hand-mount those — at that
time — tiny parts. Well, I did and the parts in my through-hole designs got progressively smaller. A similar evolution
has taken place with our SMT designs. Our Vinculum II
design here rests on a 1.975 x 0.85 piece of copper-laminated fiberglass and is full of 0603 SMT parts.
The debug module is constructed on a standard
double-sided printed circuit board (PCB). From a purely
trace layout point of view, the physical PCB design could
have been simplified if we placed components on both
sides of it. However, using the power of the PCB via our
21
AGND
GND
GND
GND
7
18
VCCIO
U2
8
6
VBUS
U1
5
20
7
4
RI
DCD
CTS
RTS
DSR
DTR
RXD
TXD
VCC
6
10
11
3
9
2
5
1
GND
VCC
1OE
1
2Y
1A
2A
1Y
3
2
2OE
4
C4
4.7uF
C5
100nF
17
GND
ID
VBUS
FB1
3V3OUT
74LVC2G241
C3
47pF
J1
8
24
NC
NC
5
4
1
MINI-B USB RECP T
6
3
2
SHIELD
D+
D-
16
15
USBDM
USBDP
C1
C2
CBUS0
10nF
47pF
19
CBUS1
26
RESET
OSCI
OSCO
CBUS2
27
CBUS3
TXEN
TXRXLED#
RESET#
PROG#
PWREN#
CBUS4
12
14
13
22
23
28
■ SCHEMATIC 1. The F1
resettable fuse part was
changed to allow the maximum amount of current to
be drawn from the USB host
portal.
TEST
25
U1 VCC
U1 VCCIO
U2 VCC
C6
100nF
C7
100nF
C8
100nF
R3
LED1
TX/RX
Q1
DMP2123L- 7
ds
g
R8
NOTES:
1. FB1 - MOUSER MI0805K400R- 10
2. U1 - FT232RL
3. F1 - MOUSER 652-MF-PSMF050X- 2
4. J1 - MOUSER 538-67503-1020
5. ALL PARTS 0603 SMTUNLESS OTHERWISE NOTED
September 2010 61
