It’s time to try out that “bussed SPI” idea, which could turn the 2×4-pin SPI/ISP connector into a real bus for up to 8 high-speed devices. What I came up with as test case, is to hook up two SPI RAM chips, write different values to each of them, and then read them back to check that the values are correct.

Let’s dive in, which in this electro-world means: grab a breadboard and a bundle of wire jumpers!

At the top, a red USB BUB (v1), and a JeeNode underneath a Bridge Board to bring all the necessary connections onto the breadboard. On the breadboard, from left to right: two 23K256 static RAM chips in 8-pins DIP, and a 74AHC595 shift register as 16-pin DIP package. Apart from the jumper wires, some pull-up resistors, and the scope probes, there are also lots of 0.1 µF decoupling caps. At 8 MHz, they are crucial (as I found out, eventually).

This was (approximately) the code I used for testing (combined with the code in this post):

Here is a scope trace of the result (yes, it’s the same screenshot I posted a few days ago):

There are two decoded SPI signals: MOSI at the top and MISO at the bottom.

What you can see, is that the bytes 0x38 and 0x39 are being read back from one SRAM, immediately followed by the command to write 0x3C and 0x3D – these values then get read back on the next iteration. The values 0x3A and 0x3B end up in the other SRAM. You can also see the slave select setup via the B0 pin for SRAM #1 (0xFD), and a different value for SRAM #2 (0xFB). Note how this needs a mere 2 µs extra to switch SPI slaves.

In short: it all seems to work!

I’ll have to do a lot more testing of course, to find out how reliable this is at this high 8 MHz speed, and how this works out across multiple boards, all stacked up together. But if it does, then this might become a very handy new convention for little “stacker” boards on top of a JeeNode. Look Ma, no extra pins!

The SPI bus: tamed, at last?

Yesterday’s experiment was done with a very specific purpose (yeah, sometimes I do things for a purpose…).

I’m trying to overcome a major drawback of the SPI bus. But to explain this, allow me to go on a little detour first. The I2C bus is an extremely convenient design, because it only needs 4 wires to connect several chips together:

Each chip sees the same information, but it works because each chip has been set up to respond only to a specific address on the bus. And that address happens to be conveniently located at the start of each I2C message.

But although the I2C bus is slow, usability is where the SPI “bus” suffers in comparison:

With one chip, there’s no problem: one “slave select” (SS), a “serial clock” (SCLK), a “master-out slave-in” (MOSI) pin for outgoing data, and a “master-in slave-out” (MISO) for incoming data (brilliant naming – no confusion!).

The trouble is with multiple chips, i.e. slaves. The SCLK, MOSI, and MISO pins can be shared, but not the “slave select”. Each slave will need a separate pin to select it. And that makes it impossible to use it as a real bus…

If only there were a way to get multiple slave selects onto a single pin…

And that’s where yesterday’s post comes in. Suppose we had all the SPI signals, as well as power, as well as two chip selects. Like, eh… the SPI/ISP connector, using B0 and B1 as the two select signals – how convenient!

So here’s the idea: first we send a byte over SPI with B0 set low and then we send the actual data with B1 set low. The B0 transfer is used to select one specific slave, and the B1 transfer then takes place only with that slave.

The good news is that a 74HC595 shift register has all the functionality needed to perform this task, by using the three-state enable pin as a gate to turn the shift register outputs on or off. Here’s the schematic for it all:

The idea is to put this simple circuit on each slave board. On each one, a different output from the shift register gets jumpered. In the above schematic, for example, QD (the 4th bit) has been wired up as slave select.

Here’s code for trying out this idea, optimized to only send data on B0 when a different slave is being selected:

The following code also needs to be executed to set things up properly on power-up:

    // init PB0 and PB1 as outputs, set high
    PORTB |= bit(1) | bit(0);
    DDRB |= bit(1) | bit(0);

Anyway. So much for theory… stay tuned!

PS. Season’s greetings to everyone. This weblog will continue on auto-pilot for the next two weeks. Enjoy!

Here’s a fun experiment – adding 8 very fast output pins via SPI, using just a 74AHC595 shift register:

This is the “spiCycle” test sketch I used to try out the above hardware:

It sends bytes to the shift register, with only bit 0 set, then only bit 1, etc. At the end, a zero byte is sent to clear all bits again, which is why the loop runs to 9 i.s.o. 8. Here’s the output, as seen on the scope via a logic probe:

The yellow trace is SCK. The blue trace is PB0 (Arduino digital 8), which is used as chip select for the shift register. The purple lines are the 8 outputs of the shift register. You can see the “1” bit rippling across all 8 pins.

As you can see, there’s plenty of “ringing” on these signals, which is not surprising, given that it’s a bus running at 8 MHz (and it was all set up on a breadboard using jumper wires). Setting these 8 bits takes just 2 µS!

Note that the number of output bits can easily be extended by “cascading” multiple shift registers, so this is in fact a way to add lots of output pins to an ATmega which can be set very, very quickly.

There’s one oddity in there which I can’t explain – for some reason, the PB0 select pin is being pulled low longer and longer as the loop repeats (see the blue line “low” states).

Tomorrow, I’ll explain the point of this exercise…

Digital circuits work with 0’s and 1’s, right?

Well, yes, but that doesn’t mean the analog voltages and currents are necessarily very “clean”. To fabricate a somewhat extreme example, I connected a JeeNode without regulator and without 10 µF capacitor to a 3x AA battery pack, and made it run this simple loop:

Sleep for 3 seconds, then send an SPI command to the RFM12B wireless module. Note that the RFM12B is not set to receive or transmit mode – the ATmega just sends it 2 bytes over SPI.

Let’s look at the variation in voltage and the current consumption (this shows the benefit of an MSO, BTW):

The ATmega wakes up, sends four 16-bit commands over SPI (the compressed timeline is a bit misleading) and powers off again. The whole process takes less than 200 µs. The four SPI transfers are: wake up the RFM12B, send it the 0xFE00 soft reset, and then two more to send the RFM12B back to sleep. You can even see the ≈ 0.6 mA baseline increase while the RFM12B is awake and idling. The SPI bus runs at 1 MHz in this example i.s.o. 2 MHz, because the ATmega is running off its 8 MHz internal RC oscillator, but the sketch was compiled for 16 MHz.

The current spikes are not so important. It’s normal for switching signals to consume relatively much energy (that’s why turning off the clock saves so much power). The problem here, is that these current fluctuations have such a large impact on the supply voltage – one of the spikes causes the supply to briefly drop more than 0.25V!

This is why “decoupling” capacitors are used around digital chips, even a lowly ATmega consuming just a few milliamps (it’s running at 8 MHz here, BTW). There is a 0.1 µF cap on the JeeNode board, but it’s not enough.

Here’s the same circuit, with both signals in close up:

Nasty stuff. I’m not 100% convinced that the real waveforms look exactly like this (the scope and probe might be distorting it a bit), but there’s no question that each SPI pin change has a substantial impact on the supply rail.

Here’s the same, with a 0.1 µF capacitor added near the battery pack:

And with a 470 µF electrolytic cap (both showing just the scope’s measurement results):

Note that the 0.1 µF cap has much more effect, relatively, than the 470 µF one. It’s better for HF noise reduction.

Does it matter? Yes, probably. Although all these setups work fine, the variation in voltage is fairly large, and could cause problems when operating at lower voltage levels, nearer the specified limits. Also, such currents might generate a fair bit of Electromagnetic Interference (EMI).

By adding more capacitors very near to the power consumers, i.e. the ATmega and RFM12B, this can be reduced. Such decoupling capacitors will act like little charge buffers, helping the supply cope with such sudden changes.

There’s much more to it than that (there always is). At switching frequencies of 1 MHz and above, the impedance of a wire starts to matter a lot. In fact, it’s amazing that digital circuits work at all – even without any HF design!

I’ll investigate further, but for now just remember: when in doubt, add caps … everywhere.

Finally got around to finishing the low-power steppable AC mains transformer – yeay!

The box I had picked was almost too small, so it’s a pretty tight mess in there:

Main parts used (and yes, that board is kept in place with hot glue!):

• 2×11 rotary switch – DigiKey # 360-2353-ND
• 2x28V @ 20 mA transformer (2x) – DigiKey # MT2129-ND
• 2x18V @ 65 mA tarnsformer – DigiKey # MT2122-ND
• 230V 3W signal lamp – RS Components # 3393254

The circuit is as described on that “Transformers – part 2” link listed above, except that I’ve added a 3W 230V incandescent light bulb in series with the primary windings (thx for the tip, Martyn!). They draw 13 mA when glowing, but when cold the filament resistance is only 2 kΩ – which is great, because this way over 85% of the AC mains will go to the transformer while it is unloaded, or very lightly loaded. Once the load increases, the light bulb acts as a PTC, lights up, and reduces the voltage left for the transformer. It’s an extra precaution – I’ll need to test it with a dummy load to see what happens to the voltage when drawing more current.

Also, I’ve decided not to ground a center tap on the secondary windings after all. The reason is that I want to be able to tie this to a regular oscilloscope probe (not just the new isolated differential probe). This automatically grounds one side (and would interfere with a grounded center tap), so the safe way to use this thing will be to stay in the lower voltage ranges. That means this setup is now simply an isolation transformer, like my other one.

Anyway, the point of this all is to have 10 different voltage steps to test with. Here’s how they came out:

• 17.8 Vrms
• 30.0 Vrms
• 35.5 Vrms
• 47.0 Vrms
• 58.3 Vrms
• 64.3 Vrms
• 75.6 Vrms
• 92.3 Vrms
• 119.6 Vrms
• 147.7 Vrms

This was measured with a 100 kΩ resistor over the output, to create a very light load. Unloaded, the maximum voltage will rise slightly further – to 155 Vrms.

Now maybe 155 Vrms doesn’t sound like that much, but that “rms” stands for root mean square. That’s sort of an average value. The peak voltage at the crest of this particular waveform is actually 240 V. On the scope, this is displayed as a wave with an amplitude of 480 Vpp peak-to-peak.

Speaking of waveforms – in my setup, it’s not really a sine wave that comes out:

The FFT analysis shows that there are all sorts of harmonics in there:

(a pure sine wave would only have that very first 50 Hz peak at the left, as far as I understand FFTs)

But hey, it’s a great setup to test low-power transformer-less power supplies. The lower voltage ranges are safe, currents are limited to tens of milliamps, it’s isolated, and the output voltage ought to collapse when loaded by more than 20..30 mA (this still needs to be verified).

It never occurred to me that RFI would be an issue, but it sure is. There’s lots of electromagnetic interference around my workbench here at JeeLabs. And it’s messing up my scope experiments – here’s a baseline:

That’s the signal, picked up by an unconnected probe lying on the table. The pane at the top is the raw signal, synchronized to the AC mains line frequency. The pane at the bottom is a Fast Fourier Transform – which has always fascinated me as a kid (yeah, I was born a geek) – the X axis is frequency instead of time. This analyzes a signal and decomposes it into a set of sine waves (every periodic signal can be treated as a sum of sine waves).

The peak at the left is “zero”, or rather, 50 Hz. There’s a weak peak at 128 KHz and a surprisingly strong peak at 675 KHz (update: probably a nearby 100 kW AM transmitter). That’s the noise the probe is picking up.

This was with my LED light strip powered from the (linear) 12V lab power supply. With a switched 12V supply, still powered on 100%, i.e. no PWM, I get a couple of extra spikes:

Looks like this SMPS is switching at about 55 KHz. So I think I’m going to drive these LEDs with a conventional linear power supply – unregulated, to keep losses low.

I’m averaging the FFT results to get a clean readout. This is possible because the signals I’m after are repetitive and constantly present. Now let’s change the scale a bit and look again at the probe’s pickup with no lights on:

Pretty clean – but watch what happens with the little fluorescent light of the magnifying glass I use for soldering:

It’s a pretty hefty 30 KHz transmitter! (the strong odd harmonics indicate a square wave, i.e. on/off switching)

The moral of this story: if you want to measure weak signals, clean up your desk – electromagnetically, that is!

After yesterday’s post, here are some more details about grounding do’s and dont’s …

First, let me reiterate that there is no such thing as a fixed “0 volt” level. Voltages are measured as “potential differences“. You can only have a voltage between points A and B – even if there is virtually no current flow.

Warning: from here on, I’m just going to invent an explanation for how ground works and how to deal with it. If I’m wrong, please correct me. The point is to try and get my intuition across – because IMO it’s not complicated.

The solution presented yesterday was to use a “star grounding” approach: all the big power consumers get their own power and ground wires to the power supply. As was explained there this prevents big currents from “raising the ground floor”, i.e. reducing the way ground level changes end up messing with low-power signals.

Here’s a delightfully simple and practical rule: tie all the big power consumers directly to the power supply, and do whatever you like with the rest. There’s still a risk of “ground loops” (which is a nightmare in audio circuits, because it can leads to audible hum), but it’s not nearly as important as getting the big currents under control.

So if ground levels can vary, how come this doesn’t lead to trouble when connecting multiple devices?

Well, the simple answer is: they could, but the trick is to avoid currents going through ground connections. Now, before all this gets too confusing, let me make the distinction more explicit with an image from Wikipedia:

The differences are quite subtle:

• signal ground denotes the return path for low-power, eh… signals
• chassis ground is the mechanical frame, if it is made of a conducting material
• earth ground is that rod in the ground I mentioned before

During my recent scope experiments, I’ve been quite puzzled by all this. How do you prevent damage when you tie sensitive circuits together, all at different potentials, different power supplies, and different grounding points?

Observation #1: voltages don’t cause damage, it’s the resulting current that does.

So as long as a circuit is high-impedance, you don’t really have to worry about damage. Poking around with a scope probe (which is usually 1..10 MΩ) won’t be a problem (unless you exceed the maximum voltage rating).

Observation #2: electricity only flows when there is a return path.

This is a biggie. Imagine a heavy power supply with lots of voltage and current capability to cause lots of damage:

A power supply is normally galvanically isolated from AC mains. This means it has no connection to it and no current can flow. Tying the “–” side of the supply to the chassis and enclosure will not change this. Likewise, tying this “chassis ground” to “earth ground” will not cause any current to flow. So far, so good!

Let’s look at two such installations, and let’s assume the second one is actually quite sensitive – such as a scope:

Again, tying its “GND” probe line to both chassis and earth ground will – in itself – not cause current to flow.

So far so good. Now let’s look at the bigger picture – since both appliances are tied into the same AC mains:

As shown in part 1, if there are large currents flowing through AC mains wiring, then there will be a voltage drop in the wires (red box), which means the different power inputs will start to “float” relative to each other.

Observation #3: the ground wire normally carries NO current!

This is the crucial bit which makes grounding sane. When there’s no current, there’s no voltage drop. In a 3-wire system, now common in most parts of the world, that ground wire is nice as reference and as fail-safe.

First the fail-safe: as you saw, it is common to tie all chassis and signal grounds to the “G” wire, which in turn is normally tied to earth ground somewhere in the basement (in one spot only). If there is ever a short between the other current-carrying (and dangerous) wires of AC mains and this “G” wire, then two things happen: 1) currents in the “L” (live) and “N” (neutral) wires no longer cancel, and 2) a current will start to flow through the “G” wire.

This is where the RCD or “RRCB” enters the picture. In modern house wiring setups, it kicks in the moment a current difference larger than 30 mA is detected between L and N. So what this does is shut down power as soon as 30 mA or so starts flowing through the “G” wire. An excellent safety device – it must have saved countless lives.

Observation #4: electricity takes the path of least resistance (or inductance in the case of HF).

So if the “G” wire normally carries no current and up to 30 mA during a fault, why is it as thick as “L” and “N”?

One reason that it’s an extra safety – if the RCD doesn’t trip, then the fuse will blow. But another reason is to keep the resistance of the “G” wire much lower than the resistance of say a probe’s “GND” tied to the “–” of the power supply. If there’s a fault current and it decides that it’s easier to go through the probe’s “GND” wire, then it could send a damaging spike through it (again, the scope is a very sensitive low-current device).

So the effect of that green-and-yellow “G” wire running through the house, is to create a splendid 0 V reference (when everything is operating properly) and to act as fail-safe if L or N are brought in contact with it.

G is not there to carry current, but to tie all isolated, i.e. floating, power supplies in all devices together!

If you review observation #2 in this light, then each appliance has its own isolated power supply and the current it produces always goes back “into it”, so to speak. To put it differently: each power supply takes care of its own electrons. The common ground connection merely “secures” one side of each supply to a safe reference point – and ties it firmly to all the conducting surfaces around us. The more surfaces we tie to “G”, the more we can be assured that faults will trip an RCD or a fuse – and leave us happily intact to tell stories like this and write weblog posts.

I’ve often wondered how “ground” works.

Let me explain: ground is of course the zero voltage reference level, which we implicitly think of when talking of a 3.3V or 5V signal, for example. You can only measure voltage between two points – it’s a relative concept.

But we all (well, I do, anyway) loosely talk about 0V and “ground” as if it’s an obvious thing. Not so, if you talk to electricians or engineers involved with power circuits.

First of all, there’s no such thing as a single ground potential. You can push a metal rod into the ground and think you’ve hit a solid reference but you might be surprised what two such metal rods pushed into the ground at some distance will do in a thunderstorm!

So why is grounding and ground level so tricky?

The problem is resistance. Every copper wire has some resistance. And where current flows in a resistance, you get a voltage drop (Ohm’s law, again – it’s everywhere!).

Let’s take a simple circuit, and let’s stay in the domain of JeeNodes and Physical Computing:

A 3.3V power supply feeds a JeeNode, which in turn controls a lamp. In normal low-power cases, things behave exactly as you’d expect. Ground is tied to the power supply, but both the JeeNode and the lamp are also connected on one side to ground, i.e. 0V.

I’ll redraw this to make the wiring resistance explicit:

Those resistance values will be very small, let’s say about 10 milliohm, i.e. one hundredth of one ohm.

Now imagine that you’ve got a super-powerful 33 Watt lamp instead, which draws 10 A. And assume the power supply is a big fat one which can supply those currents, and that the Relay Plug is also a beefy souped-up version.

With the lamp off, and the JeeNode drawing about 10 mA, the voltage drop over R1 and R2 will be around 0.00001 V (10 µV). Virtually undetectable.

Let’s turn the lamp on. Now a 10 A current will flow through R1, R2, R3, and R4. That means the voltage drop over each one will be 0.1V – so the JeeNode, for example, will be running at 3.1V now!

But that’s not all. Since the “ground” level of the JeeNode, i.e. the wire between R2 and R4 is now floating 0.1V above the power supply ground, everything the JeeNode does starts to float. The signal to the relay will switch between 0.1 and 3.2V – it’ll never be 0V (to ground) in fact.

For a relay plug, that doesn’t really matter – it’ll still switch on and off as intended. But for things like analog signals and ADC/DAC conversions, this really messes things up. If you don’t plan for it, all your analogRead() measurements could easily be 0.2V off, that’s a 6% error.

Now imagine driving the lamp with a big fat MOSFET instead and using PWM. It’s not hard to see that the JeeNode is almost literally going to be in a roller-coaster ride as the switching ends up affecting everything!

Remember seeing the lights dim briefly when a big heater is turned on? That’s due to cable resistance (and inductance) and floating grounds, in essence. So forget about an absolute ground – there’s no such thing.

Does that mean we can’t measure properly? Of course we can. Because there are some simple ways to deal with these resistance effects. The first obvious trick is to use fat wires for stuff that draws a lot of current. This lowers the resistance, and hence the voltage drop. Reduced voltage drop is why cables are sometimes much thicker than strictly needed w.r.t. current-carrying capacity.

But that doesn’t mean you need to waste copper all over the place. Have a look at this design:

Same circuit, but the JeeNode’s “ground level” is back in the microvolt range, even though it’s still switching 10 A. That’s because the current drops caused by the lamp currents no longer affect the JeeNode. R1 and R2 carry just 10 mA for the JeeNode again, even when the lamp is on: thin wires from power supply to JeeNode are ok.

Tomorrow, I’ll try to apply some common-sense to AC mains and multi-device grounding…

This is to announce that from now on JeeNodes will be fitted with a different type of RFM12B wireless module:

Previous module on the left, new module on the right.

The difference? It’s just a bit flatter, that’s all. As you can see, it’s the same board – with a low-profile crystal:

For most purposes, the change is irrelevant. The module is electrically identical: same pinout & commands, same RF12 library, etc. But in some scenarios, the lower profile might be useful, i.e. when the MPU is also SMD.

This is also why the flat model is going to used with the new JeeNode Micros.