Newbie Question : Board level design

[zilinxchip]

Hi,
Upto now I've been building simple little projects on breadboards.
Now I'd like to try my hand at designing a pcb board with an MCU onboard
and all.

I have a couple of questions which have puzzled me.

1) Is the board designed first and then the software written for the MCU
or is it the other way around. I had one old school electical engineer
tell me the board is designed first whereas I thought the software and
circuitry is prototyped in pieces first and then comes the completed
schematic layout.

2) How do board level designers (who may not know much about programming
the MCU) know HOW to layout the board? Do they just look at application
notes from the manufacturer and lay things out and get it right on the
first shot!?

3) Is it possible that a board level designer can layout a board without
knowing anything about programming the chips onboard?

4) For surface mount chips (not in a DIP format where you can plop it into
a breadboard for trial purposes), how do you go about trying them out
before actually committing them to be produced on a PCB?

5) When you want to incorporate a chip into your design and hook it upto
other chips onboard, what is the first thing you go searching for? The
datasheet or...? How can you be sure it will work in harmony with what's
already onboard (i.e. all the pin connections are correct)

I got a few more questions but I'll stop here for now.

Thanks

 

[Gman]

Hi,

I'm a newbie myself, so i'm probably not gonna help much ...but here
goes.

I am designing a fairly passive circuit right now (sensors, diodes,
leds, resistors, etc) and I need to design the circuit to perform and
all of the funtionality is built onto the board, so it only makes sense
that I prototype the logic/schematic and thent he layout of the board.

This changes if you are only going to be using certain pins on an IC
dependent on their funtionality. So you will have to determine in
advance if you want the option of using what pins based on programming
that will be done.

I can;t even begin to imagine the WiFi guys that are actually using the
board as an antenea and how they do that and what the order of
operations is, every component and even the placement of the trace
maybe effecting the outcome on the performance of an antenea
(guessing).

Any way, my .0032 cents worth (after inflationary adjustments)
Gman

 

[Rich Webb]

On Fri, 19 Aug 2005 17:47:16 -0700, zilinxchip
<fakeemailaddress@fakespam.com> wrote:

Hi,
Upto now I've been building simple little projects on breadboards.
Now I'd like to try my hand at designing a pcb board with an MCU onboard
and all.

I have a couple of questions which have puzzled me.

1) Is the board designed first and then the software written for the MCU
or is it the other way around. I had one old school electical engineer
tell me the board is designed first whereas I thought the software and
circuitry is prototyped in pieces first and then comes the completed
schematic layout.

The board *could* be designed first if all of the MCU pins had exactly
one possible function. This is often the case with micro-PROCESSORS;
this is rarely the case with micro-CONTROLLERS. The schematic (not
necessarily the PCB layout) is often a compromise between what's easiest
for the software and what's easiest for the hardware.

The programmer may not be too involved with the translation from
schematic to PCB layout. The design engineer (who may be the layout guy,
as well) must be.

2) How do board level designers (who may not know much about programming
the MCU) know HOW to layout the board? Do they just look at application
notes from the manufacturer and lay things out and get it right on the
first shot!?

There are design rules and thumb rules for power busses, ground planes,
bypass caps, trace widths and shape, etc., etc., etc.

3) Is it possible that a board level designer can layout a board without
knowing anything about programming the chips onboard?

Absolutely, if it is communicated to them what the purpose of each net
and node may be.

4) For surface mount chips (not in a DIP format where you can plop it into
a breadboard for trial purposes), how do you go about trying them out
before actually committing them to be produced on a PCB?

In-system programming is your friend.

5) When you want to incorporate a chip into your design and hook it upto
other chips onboard, what is the first thing you go searching for? The
datasheet

Yes.

or...? How can you be sure it will work in harmony with what's
already onboard (i.e. all the pin connections are correct)

The other datasheet.

--
Rich Webb Norfolk, VA

 

[David L. Jones]

zilinxchip wrote:

Hi,
Upto now I've been building simple little projects on breadboards.
Now I'd like to try my hand at designing a pcb board with an MCU onboard
and all.

I have a couple of questions which have puzzled me.

1) Is the board designed first and then the software written for the MCU
or is it the other way around. I had one old school electical engineer
tell me the board is designed first whereas I thought the software and
circuitry is prototyped in pieces first and then comes the completed
schematic layout.

You dont have to prototype first if you know what you are doing and
your confidence is high that it will work. The PCB can be your
prototype. In this case you have to make sure you use ISP (In-System
Programming) or have a socket for the micro etc.
Often you are forced to do a PCB because the circuit is too complex to
breadboard or some other factor.

2) How do board level designers (who may not know much about programming
the MCU) know HOW to layout the board? Do they just look at application
notes from the manufacturer and lay things out and get it right on the
first shot!?

Experienced board designers must know all about signal integrity issues
and are often intimitely involved in the design process. In many cases
the design engineer will do the PCB layout.

3) Is it possible that a board level designer can layout a board without
knowing anything about programming the chips onboard?

Yes. But the PCB designer will need a complete schematic to work with,
developed by the design engineer who knows all about what pins to use
on the micro.

4) For surface mount chips (not in a DIP format where you can plop it into
a breadboard for trial purposes), how do you go about trying them out
before actually committing them to be produced on a PCB?

You use ISP In-System-Programming. Where this is not possible you might
use a surface mount to DIP adaptor board so you can unplug the chip.

5) When you want to incorporate a chip into your design and hook it upto
other chips onboard, what is the first thing you go searching for? The
datasheet or...? How can you be sure it will work in harmony with what's
already onboard (i.e. all the pin connections are correct)

Yep, the datasheet.

You might want to have a look at my PCB Design tutorial:
http://www.alternatezone.com/electronics/pcbdesign.htm

Dave

 

[Peter Bennett]

On Fri, 19 Aug 2005 17:47:16 -0700, zilinxchip
<fakeemailaddress@fakespam.com> wrote:

Hi,
Upto now I've been building simple little projects on breadboards.
Now I'd like to try my hand at designing a pcb board with an MCU onboard
and all.

I have a couple of questions which have puzzled me.

1) Is the board designed first and then the software written for the MCU
or is it the other way around. I had one old school electical engineer
tell me the board is designed first whereas I thought the software and
circuitry is prototyped in pieces first and then comes the completed
schematic layout.

The system engineer has to have some knowledge of both the
microcontroller architecture and electrical requirements, so he can
assign inputs and outputs to the microcontroller pins in a way that
won't unnecessarily complicate the programmer's task.

As programming proceeds, it is quite possible that there will be
changes to the schematic to accommodate the programmer's requirements.
The actual PC board layout may not happen till fairly well into the
project.

In most of my projects at work, I'm the project engineer, programmer,
electronic designer, and PC layouter, so I get to argue with myself
over all aspects of the job, and will often change pin assignments on
the schematic to make the software easier.

2) How do board level designers (who may not know much about programming
the MCU) know HOW to layout the board? Do they just look at application
notes from the manufacturer and lay things out and get it right on the
first shot!?

App notes help, but with microcontrollers the programmer or project
manager has to provide pinout assignments - which pins are used for
which input or output signal.

3) Is it possible that a board level designer can layout a board without
knowing anything about programming the chips onboard?

Yes. In many companies, the board layout person just does board
layout, and needn't know much about the function of the board - he
just gets a schematic from the designer, and makes sure the board
matches it.

4) For surface mount chips (not in a DIP format where you can plop it into
a breadboard for trial purposes), how do you go about trying them out
before actually committing them to be produced on a PCB?

Try to get a DIP version, even if you will use SM in production. For
commercial products, it is common to make a few prototype boards
before getting to the "real" board. Test areas, where SM parts can be
mounted, can be put on these boards.

5) When you want to incorporate a chip into your design and hook it upto
other chips onboard, what is the first thing you go searching for? The
datasheet or...? How can you be sure it will work in harmony with what's
already onboard (i.e. all the pin connections are correct)

You need data sheets for all parts you will be using. Careful study
of the datasheets should tell you how to interconnect the parts, and
if the voltage levels and timing requirements are suitable.



--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
new newsgroup users info : http://vancouver-webpages.com/nnq
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca

 

[John Larkin]

On Fri, 19 Aug 2005 17:47:16 -0700, zilinxchip
<fakeemailaddress@fakespam.com> wrote:

Hi,
Upto now I've been building simple little projects on breadboards.
Now I'd like to try my hand at designing a pcb board with an MCU onboard
and all.

I have a couple of questions which have puzzled me.

1) Is the board designed first and then the software written for the MCU
or is it the other way around. I had one old school electical engineer
tell me the board is designed first whereas I thought the software and
circuitry is prototyped in pieces first and then comes the completed
schematic layout.

Usually you design the board, and after you order the bare board and
the parts, while you're waiting for that stuff to arrive, you write
the code. You *think* about the code before you order boards!

2) How do board level designers (who may not know much about programming
the MCU) know HOW to layout the board?

The same way tennis players know how to play tennis; lots of practice.
But board designers usually do understand the CPU and the application
pretty well; some of us write the code, too.

Do they just look at application
notes from the manufacturer and lay things out and get it right on the
first shot!?

Not "just"; designers design, they don't copy.

First shot is good, and possible if you're careful.

3) Is it possible that a board level designer can layout a board without
knowing anything about programming the chips onboard?

Not recommended.

4) For surface mount chips (not in a DIP format where you can plop it into
a breadboard for trial purposes), how do you go about trying them out
before actually committing them to be produced on a PCB?

You don't. You read the datasheets carefully and lay out the real
thing.

5) When you want to incorporate a chip into your design and hook it upto
other chips onboard, what is the first thing you go searching for? The
datasheet or...?

Datasheet and app notes. The gotchas are usually hidden in the
appnotes.

How can you be sure it will work in harmony with what's
already onboard (i.e. all the pin connections are correct)

Well, that's the job.


John

 

[PeteS]

There have already been lots of replies, but I'll throw in my few
comments...

::
Hi,
Upto now I've been building simple little projects on breadboards.
Now I'd like to try my hand at designing a pcb board with an MCU
onboard
and all.
::

Do you have any actual layout tools in mind? Although they all require
the same basic skills, they all differ in implementation and difficulty

::
I have a couple of questions which have puzzled me.

1) Is the board designed first and then the software written for the
MCU
or is it the other way around. I had one old school electical engineer
tell me the board is designed first whereas I thought the software and
circuitry is prototyped in pieces first and then comes the completed
schematic layout.
::
In a perfect world (rarely realised) the board requirements are stated
first (Someone mentioned a project manager - that's one option). If the
requirements of the board are clear, the electrical design requirements
are at least clearer. As noted, unless the board is for a single
purpose or perhaps just a general purpose (with I/O connector
positions, for example) it is *never* a good idea to try and design the
board without knowing what the software must do.
Often, all the design functions (electrical, firmware software, layout)
reside in one person. I don't know many electrical engineers who have
not written significant amounts of code - they *want* to know what the
code must do.

::
2) How do board level designers (who may not know much about
programming
the MCU) know HOW to layout the board? Do they just look at
application
notes from the manufacturer and lay things out and get it right on the
first shot!?
::
It is the task of the electrical engineer to guide the layout person
(who may in fact be that same EE). We do schematic capture (with lots
of notes), generate netlists and footprint requirements and pass them
to the layout person with our notes. It is not unusual for the EE to
sit with the layout person to deal with 'special' areas of the board.
As to HOW to lay out the board - practise, practise, practise. A
healthy does of aptitude helps, though.

We don't just 'look at app notes', although that forms part of the
design exercise. App notes live in a perfect, isolated world. We design
their parts into a larger scheme, which requires us to know how to
adapt the information in the app note to our current requirements.

As to getting it right first shot, that takes a lot of practise (luck
helps) and is usually a function of the complexity of the board,
although classic neophyte problems abound for even the simplest of
units.
I had 5 in a row completely correct (i.e. the prototype is the shipping
unit) for very complex boards (varied between really small and tight to
big and hairy). That's the exception although we always try to get it
right. It's the gotchas (which is why you should read the datasheets
and app notes thoroughly) that catch you.
::

3) Is it possible that a board level designer can layout a board
without
knowing anything about programming the chips onboard?
::
If you are talking about the electrical designer, I would say *no*
except for the simplest of devices. All newer processors and
controllers have multi-use pins in these days, and a thorough knowledge
of what the code/system requires is necessary to assign the correct
pins to the correct board functions.
If you are referring to the PCB layout person, then they don't
necessarily have to know details, although we have to convey rules
about the circuitry (which could be high currents, fast transients,
high speed systems etc).

::
4) For surface mount chips (not in a DIP format where you can plop it
into
a breadboard for trial purposes), how do you go about trying them out
before actually committing them to be produced on a PCB?
::
Others mentioned making a prototype board first - I have done that
myself. On other cases, the SMD board *is* the prototype. For complex
boards (or for analog sensitive boards) two different layouts will
yield two different results, for an identical netlist.

::
5) When you want to incorporate a chip into your design and hook it
upto
other chips onboard, what is the first thing you go searching for? The
datasheet or...? How can you be sure it will work in harmony with
what's
already onboard (i.e. all the pin connections are correct)
::
The datasheets, application notes, tecchnical articles from the device
manufacturer. I also call the local FAEs to see if they have reference
designs.

::
I got a few more questions but I'll stop here for now.

Thanks
::

No problem

Cheers

PeteS

 

[John Larkin]

On 20 Aug 2005 03:12:36 -0700, "PeteS" <ps@fleetwoodmobile.com> wrote:

There have already been lots of replies, but I'll throw in my few
comments...

::
Hi,
Upto now I've been building simple little projects on breadboards.
Now I'd like to try my hand at designing a pcb board with an MCU
onboard
and all.
::

Do you have any actual layout tools in mind? Although they all require
the same basic skills, they all differ in implementation and difficulty

::
I have a couple of questions which have puzzled me.

1) Is the board designed first and then the software written for the
MCU
or is it the other way around. I had one old school electical engineer
tell me the board is designed first whereas I thought the software and
circuitry is prototyped in pieces first and then comes the completed
schematic layout.
::
In a perfect world (rarely realised) the board requirements are stated
first (Someone mentioned a project manager - that's one option). If the
requirements of the board are clear, the electrical design requirements
are at least clearer. As noted, unless the board is for a single
purpose or perhaps just a general purpose (with I/O connector
positions, for example) it is *never* a good idea to try and design the
board without knowing what the software must do.
Often, all the design functions (electrical, firmware software, layout)
reside in one person. I don't know many electrical engineers who have
not written significant amounts of code - they *want* to know what the
code must do.

The first thing I do is write the manual. Then board layout, parts
list, FPGA design, embedded software, test software.


John

 

[PeteS]

The esteemed John Larkin wrote:
"The first thing I do is write the manual. Then board layout, parts
list, FPGA design, embedded software, test software. "

I completely agree - I wish more of my 'engineering managers' did.

For the elucidation of the original askee:

This is not merely a good idea, it's almost a requirement for any
design if one wants to get it right. There have been many times I have
heard (or have been known to comment / asked) "...but we thought the
functionality was x". Alternatively "but we wanted to be able to use
that pin functionality in a bidirectional way". More such comments
abound.

A written spec is more than just guidance, it makes sure everyone
involved understands what the unit will do, and just as importantly,
what it will *not* do.

It takes a little time, but it takes more time to clear up
misunderstandings later. The old saw that the further into the
design/product process you are, the more expensive (in time and money)
any change becomes is no more true than for this.

A week (or maybe more - a lot more, depending on the product) up front
saves a lot of time in the back end.

Cheers

PeteS

 

[Rich Webb]

On 20 Aug 2005 03:12:36 -0700, "PeteS" <ps@fleetwoodmobile.com> wrote:

There have already been lots of replies, but I'll throw in my few
comments...

::
Hi,
Upto now I've been building simple little projects on breadboards.
Now I'd like to try my hand at designing a pcb board with an MCU
onboard
and all.
::

Do you have any actual layout tools in mind? Although they all require
the same basic skills, they all differ in implementation and difficulty
[snip...snip...]

If I may make a suggestion that it would be a heck of a lot easier
to follow your replies if you would employ the usual convention of
prefixing each line of the embedded quote with a single character
(usually a '>') in the first column.

I understand that you're posting through Google and that the Google
usenet interface is egregiously bad. "Do no evil." Feh!

--
Rich Webb Norfolk, VA

 

[JeffM]

PeteS wrote [with hand-generated context]:
..
The esteemed John Larkin wrote:
"The first thing I do is write the manual. Then board layout, parts
list, FPGA design, embedded software, test software. "
------------
There have already been lots of replies,
but I'll throw in my few comments...
------------
:
:If I may make a suggestion that it would be a heck of a lot easier
:to follow your replies if you would employ the usual convention of
refixing each line of the embedded quote with a single character
usually a '>') in the first column.
:
:I understand that you're posting through Google and that the Google
:usenet interface is egregiously bad. "Do no evil." Feh!
: Rich Webb

Getting blockquotes on Google Groups (The Easy Way):
http://groups-beta.google.com/group/sci.electronics.design/browse_frm/thread/1e72eb22780bfcee/6939ac83d77b8ded?q=don't-click-the-reply-link-that-is-in-plain-sight+To-get-context-the-easy-way+on-Google+zzz+show-options+click-THAT-Reply-link

 

[Jasen Betts]

In article <pan.2005.08.20.00.47.15.506641@fakespam.com>, zilinxchip wrote:

Hi,
Upto now I've been building simple little projects on breadboards.
Now I'd like to try my hand at designing a pcb board with an MCU onboard
and all.

I have a couple of questions which have puzzled me.

1) Is the board designed first and then the software written for the MCU
or is it the other way around. I had one old school electical engineer
tell me the board is designed first whereas I thought the software and
circuitry is prototyped in pieces first and then comes the completed
schematic layout.

earlier this year I made my first MCU design, I used an off-the shelf
stripboard designed for DIP ICs I first loaded the MCU with a simple program
so I could drive the outputs and verify that the inputs functioned correctly
and then after getting those parts working satisfactorilly I started on the
software, because I kept adding features (and bugs) I exceeded the 1000
write cycles the chips flash could handle and then I was glad I had used a
socket for it,

I think it would be hard to test the software without having a bouard to run
it on.

2) How do board level designers (who may not know much about programming
the MCU) know HOW to layout the board? Do they just look at application
notes from the manufacturer and lay things out and get it right on the
first shot!?

once the designer knows which pins will connect to which board features the
layoy task would be similar to any other,

3) Is it possible that a board level designer can layout a board without
knowing anything about programming the chips onboard?

as far as I can see, he need only know what the chip does, not how it has
been implemented.

4) For surface mount chips (not in a DIP format where you can plop it into
a breadboard for trial purposes), how do you go about trying them out
before actually committing them to be produced on a PCB?

my brother solderes them to a dip-shaped carrier using 10A fusewire,

10A fuse wire is about the right gague for soldering to the terminals of
surface mount ICs...

5) When you want to incorporate a chip into your design and hook it upto
other chips onboard, what is the first thing you go searching for? The
datasheet or...? How can you be sure it will work in harmony with what's
already onboard (i.e. all the pin connections are correct)

look at the chips data sheet see which pins can do what.... the 90s2313 I
used had one pin with a PWM output, I used this to drive a loudspeaker for
simple polyphonic sound. another pin had an async serial output capability
I reserved this function for debuggging output...

Bye.
Jasen

 

John Larkin wrote:

Usually you design the board, and after you order the bare board and
the parts, while you're waiting for that stuff to arrive, you write
the code. You *think* about the code before you order boards!

I find the board arrives entirely too fast. I haven't even ordered
*all* the specialized parts yet (the assumption that everything's
in stock fails with SMT parts), and the time was spent putting out
the fires that began burning while the PCB-design push was underway.