I'm designing a microprocessor board intended to be built by students so that they can learn the basic principles of building such a system.
The existing board that we use is based on a 68008 CPU (a description of this can be found here:
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Basically this board consists of a 68008 CPU, a ROM, two RAM ics, two UARTS and a CPLD which implements the Chip Select/ Memory map.
We have decided to upgrade the board using an ARM processor. However, I am new to microcontrollers (as opposed to CPUs) and have only found a handful of ARM processors which feature an external bus (i.e. address and data pins, like on a 68k for example), namely the Atmel
91M series. As this board is intended as an educational tool, it is important that we have these lines to allow the student to manually implement chip select signals for the various peripherals.
I was wondering if anyone has any suggestions or knows of other ARM chips which feature such external address/data lines. Also, could I use the General Purpose I/O lines which feature on ARM chips such as the Philips LPC210x series
So you expect the students to SOLDER this device, as they build the boards ? EBI ARMs have fine-pitch packages!
Depends on what aspects you want to teach them ?
For hands-on assembly, and memory selects, which seems to be your focus, why choose a wide-bus-interface like the ARM ? EBI devices tend to be TQFP144, or worse, BGA :)
The 68008 was 8 bits IIRC, so why not something like an 80C51 ?
That's still in PLCC and DIP, and has internal/external CODE and can PLD select, if you want.
Ideally you also want on-chip DEBUG, and a Socketable package. That narrows things down quite a bit.
The new AT89C51RE2-SLSUM, is PLCC44 and has On chip debug.
Zilog do a Z16F that is socketable, but that package variant lacks a EBI.
Or, you choose pre-built SMD modules (or even USB Sticks), and get the student to make other parts of the system up.
There are a huge number of ARM devices that use an external memory bus. Most of the ARM7 devices use internal memory, but there are also many, many that use external memory. Check out Cirrus, Sharp (did they sell off their entire ARM line?) Philips and OKI.
Then there are the ARM9 devices that nearly all have external memory buses since internal Flash memory can't keep up with the maximum execution rate of most ARM9s. They typically use internal RAM and cache with a large external address space for Flash, RAM and I/O. Many also have general I/O pins as well.
Teaching the ARM instruction set is likely to be useful to students for many years to come, but having them assemble wire wrap boards stopped being useful 15 years ago. I suggest that you find another way to accomplish your goals than to have your students learn a very obsolete skill. I don't know exactly what skills you are trying to teach, but I bet if you give me the list, I can tell you how to accomplish that with PCBs instead of wire wrap.
For example, if you want the students to have to perform debugging, you can provide features on the board to allow you to add problems to the board. You might add a near short between two chip select lines. Or you might open a chip select line. Either way, the student that debugs this will certainly learn what a chip select line does.
Even if you are having the students design their own schematics, you can have multiple different designs produced on a single panel at a price competitive with wire wrapping. But then I guess you might be getting all the wire wrapping materials and equipment for free since they have certainly been tossed into somebody's scrap bin once they dig them out of the back of the storage room.
Yikes. Building reliable wire-wrap stuff isn't as easy as it first appears. I would think it a very arcane skill to be teaching students. I haven't seen anybody do wire-wrap prototypes for about 20 years now. Making a prototype PCB is faster/simpler/cheaper.
Samsung also has some ARM7 parts with an external memory bus.
When I first started working with uControllers, we used to do wire-wrap prototypes. Wire-wrapping something of any complexity wasn't really something a beginner could be expected to do (and have it work).
I'd second the suggestion of making eval/proto PCBs with breadboard areas on them.
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It looks as though the course has been around for some time, so best to assume that the curriculum is well worked out and you know why you are making this choice. I would guess because this provides access to all the bus signals with an oscilliscope, allowing them to "debug" their decisions about the signals they wired for some memory map arrangement. The use of a CPLD seems to merely be to convert portions of a binary address into chip-selects, so that is a given for the course.
So is this the reasoning why wire-wrap?
(My experience with wire-wrapping was mainly with roughly 1-2MHz buses. Never had a problem, there, as a hobbyist in electronics. But I haven't tried to wire-wrap fast buses, though I've seen such jobs done by others, carefully laid out.)
Okay. It's a decision, then. Is this about software tools as well as market changes?
Are you willing to switch to a wider data bus for wire-wrapping, as well? How fast of a bus is appropriate for the coursework that uses wire-wrapping by your students?
That opens up a wide choice of devices, but taking this flow, you would surely be better using a small commercial module ?
Those come with full documentation, and are populated, and decoupled.
ST do a nice USB key, with an ARM9 core
Unlike the 68008, the peripherals are now mostly on-chip, so 'chip selects' are not so relevant.
You CAN setup external device interfaces using SPI, and that is a mor eup to date design approach.
The 680008 HAD no choice but external memory, today's uC have the CODE and DATA on-chip, many peripherals, so it's only 'special' case peripherals that need external access, and for those, a 10+MHz SPI link is usually fine.
A keypad/LCD daughter card, that used a CPLD (or Logic), would give the students something to assemble, and you would interface that via the SPI port.
Not if it the main cost is the student's time. Getting a fast ARM external bus to work with wire- wrap might be a challenge though. While I can see where the idea comes from- they want to see them learn how to select the right lines to use to interface to an external device- they'd be much better off doing this as an exercise in PCB layout. Get Eagle or Kicad something else free, one for each student, and give them devices to interface. There are lots of devices available, so plagiarism should be difficult. With a suitably designed base development board, a simple connector for the bus and a small PCB made for those who get that far should be feasible.
This one has a similar BUS to the 68008, and is typical of the Modules you can get. [I know it does not have an ARM core, but this can use any 8 bit peripherals you already have ] Connections are on 0.1" headers. Looks to be about the same area as a business card.
One reason to *not* use wire wrap is the lack of control it gives
*YOU* over the problems the students have to deal with. I remember well using those white plastic plug in strips in school. They were unreliable, easy to goof up and very hard to debug. It would just be random luck as to the difficulty of the problem any given student would be debugging or even multiple problems simultaneously. Sometimes the difficulty debugging the problem would be too much even for the instructor and the student would have to start all over again. There were times that this led to complete frustration (and very little learning) on the part of the student. Wouldn't it be better if you had control over the issues the student were to debug so they could deal with problems that they can learn from?
But letting them use a PCB with high speed signals, they will learn
*real* debugging techniques like how to attach a probe in a high speed circuit so it does not distort the signal rather than obsolete skills of attaching probes to wire wrap posts. Or with a short between power and ground, they would learn how to use a micro-volt meter or even an ad-hoc approach.
Like I said in my other post, you can find ways of adding problems to their boards to be debugged as part of the course rather than letting random chance select the nature and difficulty of their problems.
The clock speed (or bus speed) is not the issue with wire wrap (or any other interconnect not suited for high speed signals). The problem is the edge rate causing ground bounce and reflections on the signal line. Wire wrap has virtually no control over the impedance of the wires and so it becomes impossible to design a good high speed signal path. With many of the newer chips, the edge rates are very fast to allow high speed signals and even if you slow the cycle rate, you still have the fast edges which cause the problems. For example, on static RAMs and similar logic, the chip enable is a clock that strobes the data into the chip on write. If the rising or falling edges bounce (or ring) you get double clocking, potentially of the wrong data. Likewise, if the data or worse, address lines bounce, you get the wrong data or the wrong address.
The 68008 was in use at a time when edge rates were very suited for wire wrap and signal integrity was not a common term. Now nearly every MCU memory bus has fast edge rates and requires careful control of impedance and length.
In fact, last summer I took a course in high speed signal design and I learned more useful info than the entire time I was in college. Lee Ritchey is a true expert on the topic and explores the subject in theory, simulation and practice. I have never attended a class that covered the subject matter so thoroughly and was more convincing in the results, not to mention the utility of the information. Anything you can do to prepare your students for the practicality of designing high speed signals would be remembered (and used) by them for their entire careers.
Only if the user doesn't follow the instructions. Which is, of course, pretty much a given in a school. In your own lab, they can be quite reliable.
The main way to ruin a protoboard is to use a component lead or wire that is bigger than the makimum allowed. This spreads the spring contacts apart. Also bad is trimming the ends of stranded wire so the little pieces get in the holes of the protoboard. Avoid those abuses and run your wires cleanly and you will find protobards to be quite reliable and a useful tool.
I think you could develop a nice small pcb based on one of the AT91 chip that had several peripherals with the data bus already connected by PCB traces. You could then have chip selects and address lines on two parallel rows of wire wrap pins---one row from the MCU, one row from the peripherals. The students would then have to connect address lines and CS lines to implement the memory map they have set up in the EBI registers.
Just make sure that there are appropriate weak pullups on all the peripheral chip selects so that unused peripheral devices won't clog the data bus.
Were I designing the board for a 10-week course, I would have the following peripherals and excercises:
A. Single or multi-channel UART with a FIFO and interrrupt- driven queue management B. 8-bit parallel DAC and code to output sine wave of pre-determined frequency based on a timer interrupt C. 16-bit SPI ADC connected to GPIO pins where the student has to write the bit-banged SPI driver. A multi-channel ADC that requires SPI input to select the channel would add a bit of spice.
If you used the Atmel AT91R40008, with 256K of RAM, you might even be able to have all the student programs run from RAM and not even use a boot flash. The JTAG interface would remap the RAM and execute all the code from the RAM. Eliminating the flash burning step will speed up turnaround for the students.
If I were designing a board like this for student use, I would probably add pads for an external optional boot flash, pads for some external power mosfets, an SD card connector, etc., etc. so that the board could be used to do a simple process controller or data logger.
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You are probably better off lookin at the newer ARM9 circuits. AT91SAM9260 is available in TQFP. You might be better off using an existing CPU module with the external bus coming out.
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