Issue
155 June 2003
E-Field
Evalulation Module
A
Z8-BASED E-FIELD EVM
Armed
with a hardware reference design and four brand new
MC33794 devices, I decided to build my own e-field EVM.
Recently, I worked with the Zilog Z8 Encore! development
kit and was impressed with the capabilities of the new
microcontrollers (“Test-Driving the Z8,” Circuit Cellar
153). I decided to do an encore (pun intended) performance
and bring in a Z8F6403 as the MC33794’s companion microcontroller.
My
first hurdle to create my Z8 E-Field EVM was to develop
a plan to successfully integrate the 5-VDC logic of
the MC33794 with the 3.3-VDC logic of the Z8F6403. The
Z8’s I/O is 5-V tolerant, which alleviates any problems
with interfacing to the MC33794’s 5-V-based I/O. However,
some of the MC33794’s analog outputs exceed the Z8’s
3-V ceiling for its ADC.
The
PWR IN MON-to-PWR IN ratio is a nominal 0.2444 V/V and
can range from a minimum of 0.2200 V/V to a maximum
of 0.2688 V/V. My Z8-based EVM is powered by the 12-VDC
portion of my standard PC power supply. Measuring the
12-VDC leg of my power supply at the input of the Z8-based
EVM’s LM340 5-V regulator yielded 11.2 VDC. My 10-bit
Z8 ADC registered 863 counts (2.53 V) with a 3.000-VDC
reference. Using the Z8 ADC as the absolute reference
puts the PWR IN MON-to-PWR IN ratio at 0.2257 V/V, which
falls within the MC33794’s PWR IN MON-to-PWR IN V/V
ratio window. That’s all good considering the possibility
of error introduced by circuit loading from my bench
equipment. The last time I checked my mailbox, I hadn’t
received a multimillion dollar grant for lab-grade equipment
from the National Science Foundation, but my measurements
will stand considering I don’t use cheap test equipment.
The
VDDMON-to-VDD ratio ranges from 0.45 to 0.55 V/V. You
already know that the MC33794 supplies 8.5 VDC at the
VDD output. With a typical 0.5-V/V ratio, that puts
the VDDMON voltage above the 3-V range of the ADC. Because
the MC33794’s VDD output voltage should remain fairly
constant, I added a simple voltage divider to bring
the voltage into the Z8 ADC’s measurement window. With
the voltage divider active, my Z8-based EVM measured
the VDDMON voltage at 748 clicks, or 2.19 V. The voltage
divider accounted for half the voltage drop. So, 4.38
V should be present at the MC33794’s VDDMON pin. Again,
without any equipment support from the National Science
Foundation, I measured the voltage at the MC33794’s
VDDMON pin at 4.30 V. A quick calculation put the VDDMON-to-VDD
V/V ratio at 0.51, which is in line with the nominal
VDDMON V/V measurement.
You’re
probably wondering how I came about that ADC data. The
Z8 development kit comes with tons of example code.
One of the neat features of the Z8F6403 is its on-IC
DMA engine. Using the DMA subsystem allowed me to do
a continuous ADC conversion and, under interrupt control,
put that ADC data into RAM for later retrieval by the
main application. I simply used the DMA-ADC sample code
that comes with the development kit as a basis for getting
my Z8-based EVM readings out to a Tera Term Pro terminal
session. The coding was dead easy because all I had
to do was add a few lines of code to define the MC33794’s
ABCD input-to-electrode mapping. Then, I added some
VT-100 terminal commands to manipulate the Tera Term
session, which was emulating a VT-100 terminal, and
did some minor formatting using printf statements. I
made all of my changes in the main.c module (see Listing
1).
| Listing
1—Don’t try to run this code: it is a partial
listing that should be regarded for example only.
You can get the real thing from the Circuit Cellar
ftp site. This example is intended to show you how
little I had to do to get ADC clicks from the Z8-based
EVM to the Tera Term Pro terminal session. |
|
a)
b)
(Click
here to enlarge)
|
Photo
5a—As you look at the top of my Z8 E-Field EVM,
you can see the top jumper connects the external
VREF to the Z8. The watchdog signal select jumper
is just below the VREF jumper. Note the floppy drive
power connector. b—You’ll find nothing unusual in
my Z8-based EVM. I decided to mount the support
components on this side of the board to free up
as much space as possible for extending the Z8’s
I/O and to provide as much heatsink and ground plane
as I could on the other side of the board. Note
the Ref A and Ref B capacitor positions are just
to the upper right of the header. |
Photo
5 shows the hardware I used to get the Z8’s ADC counts
to the PC’s serial port. The power supply chain on my
Z8-based EVM begins with an LM340S 5-V regulator, which
feeds a Linear LT1086 3.3-V regulator. The National
Semiconductor LM340S also feeds a stock National LM317LZ
adjustable regulator circuit that is used to provide
the 3.000-V reference for the Z8’s ADC subsystem. I
opted not to use the MC33794’s on-IC 5-V regulator to
supply the LT1086 because it can only supply a maximum
of 75 mA. The Z8 runs at 18.432 MHz and can possibly
draw a bit more than 75 mA by itself. Also, the LT1086’s
heatsink tab is not connected to ground and is instead
coupled to 3.3 VDC. I wanted to have plenty of ground
plane area, which does double duty as a heatsink for
the voltage regulators and the MC33794. To keep the
Z8-based EVM as cool as possible and get as much ground
plane area as I could, I decided to include the LM340S
ahead of the LT1086.
The
X-Tools package that comes with the development kit
includes a good C compiler and an excellent debugger.
Taking advantage of the IDE toolset required the addition
of a 74LVTH125 tri-state buffer and a Sipex SP3243ECA
RS-232 converter. The 74LVTH125 is used to implement
the two-wire to one-wire debug interface between the
C compiler/debugger and the Z8-based EVM. The RS-232
component of the debug interface is contained within
the SP3243ECA. The SP3243ECA contains enough drivers
and receivers to support both the Z8 debug interface
and a standard three-wire RS-232 interface, which I
used as the console port of the EVM. The inclusion of
the debug and console interface hardware allowed me
to use the C compiler and debugger without writing a
single byte of Z8, debug, or RS-232 interface code.
I now have the ability to access the Z8-based EVM hardware
from the X-Tools IDE in the exact same manner as I access
the Z8 development board. Note that the unused I/O pins
are available at standard 0.1” header points on
the EVM.
My
finishing touch was adding a MC33794. The EVM schematic
(see Figure 1) shows how to connect the MC33794’s analog
outputs to the Z8’s ADC inputs. The necessary filter
capacitors have been put into place per the schematic
as well. The ground, shield driver, and MC33794 electrode
access points are pinned out to a standard 0.1”
header. Photo 5b shows the other side of the Z8-based
EVM board and helps to associate the remainder of the
physical components with their schematic counterparts.
|
(Click
here to enlarge)
|
Figure
1—The hardware design allows you to purchase the
inexpensive Z8 Encore! development kit and use the
X-Tools Z8 C compiler and debugger to manipulate
the resources on my Z8-based EVM. |
