Driving a stepper with bitwise manipulation of the PORT

In a previous article I described driving a steppermotor with an arduino. I like to use that as an example for writing directly to the ATMega328’s registers to drive the stepper.
Normally you would first set the chosen Digital pins on the Arduino to Output with pinMode and then you would set the Value of the pins with DigitalWrite. If you are using four pins to drive your stepper, this would require 4 DigitalWrite cycli per step.

“digitalWrite” is not the most efficient or fastest way to write to a port as it takes many cycli. Also, the pins are written to sequentially instead of at the same time.
That is not really a big problem for steppers as those are not time critical. It is possible however to write to all 4 pins at the same time.

If you use digital pins 8-11 to drive the motor, you are using the port ‘B’ (pins 8-13). That port can be manipulated by 3 registers and the arduino website describes those.  We need to set two registers, the DDRB and the PORTB register.

The DDRB register determines which pins will become output and if we use pins 8-11, the statement:
would set pins 8-11 to output.

There is a problem however because we also write to the higher pins –12 and 13-  and actualy to some more places that might already have a certain value that we are now altyering. It is therefore better to state:
DDRB= DDRB | B00001111;
This ‘logical OR’ function, does not change the upper four bits that might have been set to 1, it just sets the four lower bits to HIGH.
This basically takes care of the 4 pinMode statements.

The four ‘digitalWrite’ statements are taken care of by the PORTB register, but then we run into a bit of a problem: if for our first step we need only the first and the fourth bit to be HIGH and the 2nd and 3rd bit to be LOW, we cannot just write:
Yes, it will set the required pins high and low, but like with the DDRB register, we may have previously set the upper 4 bits of the register and those are now set to ‘0’ again.

We can also not use the: 
PORTB=PORTB | B00001001; because  this will leave any pins in the lower 4 bits (D8-D11)  that were previously set to HIGH untouched and they remain HIGH.

So we need a function that leaves the upper 4 bits untouched, but sets the lower 4 pins to the exact value we want.
That is done with a mask:

This mask has the pins that we want to change set to HIGH and the pins that need to remain as they are set to LOW. That allows us to write to the exact pins we want in the register with the following statement:
PORTB = (PORTB &~mask) | (B00001001); 
(actually:   PORTB = (PORTB &~mask) | (B00001001 & mask);//gives the same result )
so, the entire program will look like this:

// uses two coils and thus has extra torque
//pins D8-11 are used for the motor control

int delayTime = 100;
int mask=B00001111;// sets only the last four bits
void setup() {
 DDRB = DDRB | B00001111; //set D8-D11 to output                        
void loop() {
 PORTB = (PORTB &~mask) | (B00001001); 
 PORTB = (PORTB &~mask) |  (B00000101);
 PORTB = (PORTB &~mask) | (B00000110);
 PORTB =  (PORTB &~mask) | (B00001010);

Identifying and using a stepper motor on Arduino

If you buy a new steppermotor for a project there is usually some documentation telling you how to connect it.
However, we do not always buy steppermotors with documentation: sometimes you have an old one, sometimes you pull it out of an old unused device or get it from a surplus store.

I found a steppermotor that I had probably bought 30-35 years ago and never used (over ambitious). It did not offer much info other than: 55SI-25D AWC 7.5 DEG 36 Ohm and that it was from Oct 83 and came from Japan. It had six wires, but other than some handwritten labels mentioning colors and numbers (that were even corrected), it offered no clue on what connection was what. Initially, also the internet offered no help other than some questions about that type of motor (Although I recently found some info [now gone]).

So, how to identify what was what? Well first the number of  connections –6-   (brown, yellow, blue, red, white and white) suggested it was a unipolar steppermotor and I suspected the two white ones to be the common connection. To establish how the rest of the wires where connected it was time for the multimeter and I set up a small table in which I could enter the resistance values I measured:

white 1 white 2 red blue yellow brown
white 1 x 36Ω 36Ω
white 2 x 36Ω 36Ω
red 36Ω x 72Ω
blue 36Ω x 72Ω
yellow 36Ω 72Ω x
brown 36Ω 72Ω x

For those having trouble reading the symbols: ∞ means that there was an infinite resistance (i.e. not connected) and x means that there is no need to measure it as it is the same wire.

If you don’t have a multimeter, it becomes a bit harder but then you can also find which coils belong together: if you short a coil the resistance in manually turning the axis increases

unipolarcoilsObviously this was a motor with 4 coils, that each have a white common connector and the table tells us that red and brown are the coils connected to ‘white 1’ and that yellow and blue are the pair with ‘white 2’ as a common connector.

However, with a unipolar stepper the coils need to be driven in a certain sequence, otherwise the motor won’t turn at all, or will have only little power and though from the table we now know what coils belong to a pair, we still do not know what the sequence of the coils is.

In other words take for instance the red and the brown pair. Is red coil 1 and brown coil 2 or is it the other way around?  Well, that is a bit tricky because even the numbering of the coils is not always standard.

There is a way of measuring which coil is which, by applying voltage to one coil and then adding voltage to another coil, while feeling if the shaft turns at all and what way. But that is actually just as much work as connecting it to an arduino and switching a few pairs of wires to test. Once you have established –as we did in the table above-  what pairs are connected. It is really not that much work anymore.

coilsSo for now let’s forget about coil numbers and look a bit more at the functions and how to drive the coils.
So, for the time being we will just call them coil 1a,1b, 2a and 2b. we don’t know which one is a and b but we do know that it is the red-brown pair vs the blue yellow pair.

As none of the coils belonging to the same pair should be energized at the same time, the sequence must be either:
red yellow brown blue
red blue brown yellow.
As it turns out, it is red-yellow-brown blue to turn the motor clockwise and yellow-red-blue-brown to turn the motor counter clockwise

There are in fact 4 ways of  driving a steppermotor:

  • Wave driving (sometimes also called (“one-phase on, full step” ) is the simplest as only one coil is on at any one time. This uses least current but also produces the least amount of torque. The coil activation sequence would be: 1a-2a-2b-1b-1a etc
  • Half stepping is done by alternating between activating a single coil and two coils at the same time. Due to the smaller step angle, this mode provides twice the resolution and smoother operation. Half stepping produces roughly 15% less torque than dual phase full stepping. Modified half stepping eliminates this torque decrease by increasing the current applied to the motor when a single phase is energized. The torque fluctuates between utilising a single coil and both coils. the coil activation sequence would be:
  • Full stepping (sometimes also called  “two-phase on, full step”) is when two coils are always energised at the same time. This gives the motor maximum torque but will require more current more current. This looks like: 1a2a-2a1b-1b2b-2b1a.
  • Microstepping is a technique that increases motor resolution by controlling both the direction and amplitude of current flow in each winding. Current is proportioned in the windings according to sine and cosine functions. Microstepping is at the expense of drastic loss in torque. On the Arduino microstepping can be achieved  with PWM. However, unless you really need it, I wouldnt bother

To drive the motor continuously, we just apply power to the two windings in sequence. Assuming positive logic, where a 1 means turning on the current through a coil, the following two control sequences will each spin the motor clockwise 24 steps:

  Winding 1a 1000100010001000100010001
  Winding 1b 0010001000100010001000100
  Winding 2a 0100010001000100010001000
  Winding 2b 0001000100010001000100010
              time --->

  Winding 1a 1100110011001100110011001
  Winding 1b 0011001100110011001100110
  Winding 2a 0110011001100110011001100
  Winding 2b 1001100110011001100110011
              time --->

Note that the two halves of each winding are never energized at the same time. Both sequences shown above will rotate a permanent magnet one step at a time. The top sequence only powers one winding at a time, as illustrated in the figure above; thus, it uses less power. The bottom sequence involves powering two windings at a time and generally produces a torque about 1.4 times greater than the top sequence while using twice as much power.

Anyway, a simple program to test the motor would be:

int motorPin1 = 8;
int motorPin2 = 11;
int motorPin3 = 10;
int motorPin4 = 9;
int delayTime = 500;

void setup() {
  pinMode(motorPin1, OUTPUT);
  pinMode(motorPin2, OUTPUT);
  pinMode(motorPin3, OUTPUT);
  pinMode(motorPin4, OUTPUT);

void loop() {
  digitalWrite(motorPin4, LOW);
  digitalWrite(motorPin2, LOW);
  digitalWrite(motorPin3, LOW);
  digitalWrite(motorPin1, HIGH);
  digitalWrite(motorPin1, LOW);
  digitalWrite(motorPin2, LOW);
  digitalWrite(motorPin3, LOW);
  digitalWrite(motorPin3, HIGH);
  digitalWrite(motorPin1, LOW);
  digitalWrite(motorPin4, LOW);
  digitalWrite(motorPin3, LOW);
  digitalWrite(motorPin2, HIGH);
  digitalWrite(motorPin1, LOW);
  digitalWrite(motorPin2, LOW);
  digitalWrite(motorPin3, LOW);
  digitalWrite(motorPin4, HIGH);

Now you cannot of course run the motor directly from the arduino ports. You can do this with special Motordrive IC’s like the L298D or with discrete components such as power transistors or FET’s, but a fairly cheap and simple IC that can be used is the ULN2003 darlington driver (or the MC1413).

A note though: with 6 wire unipolar steppermotors you can decide to forget about the  middle connectors (the ‘whites’ in this case) and drive them like a bipolar. That is something you cannot do  with a ULN2003, you need an H- bridge for that

If you do not know the voltage of a motor, then just start with 5 Volts and go up. Many steppers are actually 12 V. The ULN2003 can take 50 Volts and deliver 500mA per gate. As the motor I use has a coil resistance of 36 Ohm and the Voltage is 12Volt, that will be 333mA so that should be OK. Nevertheless, if you plan on using the motor at high speed, some cooling of the ULN2003 might be a good idea. There are DIL heat sinks available but you could also epoxy a piece of copper or aluminium U-tube on it. The ULN2003 is not the most efficient chip and the Darlington has a high voltage drop. The chip therefore dissipates a lot of energy.

Wire it up as follows:


The connections of the header are chosen as above since that was the sequence of the header on the motor (but remember red-brown are a pair and yellow red are a pair).

On using the test program above, the following connections would make the motor step properly:

D8-> red

This seems as one would expect coz it is the 1a,2a,1b,2b or at least the 1x2x1y2y sequence. Similarly, one would expect that

D8 –>red

(basically switching ‘1’ and ‘2’)  would make the motor step backwards and it does.

When the motor is used with the ‘Stepper.h’ library from the Arduino (e.g. in the Tom Igoe “Stepper_oneStepAtATime”), we see the same results.

For those interested to try the stronger torque program and try some direct bit manipulation at the same time have a look here:

As the steppermotor takes up 4 of the 7 outputs, you may want to consider using the other 3 to drive relais or a couple of small dc motors. The outputs of the ULN2003 may be parallelled for higher output current. The 3 darlington’s left therefore could drive a 1.5 Amp load.

If you need to drive two steppermotors consider the 2803 as that contains 8 darlington drivers rather than 7.

However, there is something else you can do with the spare gates, Remember the stepping sequence t o drive the motor?

Steps Coil 1 Coil 2 Coil 3 Coil 4
1 1 0 1 0
2 1 0 0 1
3 0 1 0 1
4 0 1 1 0
1 1 0 1 0

There is a pattern in this table: Coil 1 is always the opposite of Coil 2 and the same goes for Coil 3 and 4. It is therefore possible to cut down on the number of pins from the Arduino (or other microcontroller) to just two, by using additional NOT gates.

The ULN2003 can provide these:


Programming then is very easy by toggling the two pins high and low.

If doing work with the Arduino and a stepper motor, consider AccelStepper as a library. It is more flexible.
Good site here (Spanish) and translated

Driving an Attiny? or here.
Steppermotor wiring.

Protection for Arduino ports

If you are using long wires to transport a signal to e.g. the analog entrances of your arduino, then voltages sometimes can be generated in those wires that are a bit too much for your Arduino and you may blow it up.

protection_circuitA small protection circuit may prevent a lot of problems and aggravation. Of course the Zener and resistor should be placed close to the chip.

Should you be considering building the simple Arduino board, described earlier, then take advantage of the space that is still present on the strip board, right in front of the Analog gates. Cutting the strip, bridging that cut with a resistor and connecting the zener between anolog and ground would be easy to do.

Serial communication with Arduino using a 7404

7404-rs232Serial communication with a DIY Arduino remains a topic of interest as the USB connection with an FTDI chip is not always easy to DIY.
A MAX232 as shown before can do the job and so can a few discrete components. But if you have 2 gates left in e.g. a 7404 (a HEX inverter) you can use that one too:

If you have 3 gates left, you could add one to the above circuit to transfer the DTR signal of pin 4 (on a DB9) to use that as a Reset for the DIY Arduino. Just make sure you feed that signal to the input of a spare gate via a voltage divider (2x10K).

Oh well, let me do that for you. It would look like this:

Simple Arduino with 3.3 Volt

The Simple Arduino published earlier can easily be outfitted with a 3.3 Volt outlet, as there is space enough on the stripboard
Adding 3.3 Volts is easily done with a 3.3 V regulator for which I have chosen the lowdrop TS2950C33. This regulator can deliver 150 mA which should be enough for most applications.

The circuit is quite straight forward, just use the input of the 5 Volt regulated power supply of the board as a feed for the TS2950. If you pick another 3.3 Volt regulator, make sure that 5V input is enough, otherwise you may need to feed it from the unregulated power line coming from Diode D1. The capacitor in the 3.3 V line does not need to be that big: 1uF even 220nF is probably enough already, but I just happened to have a 10uF.

It is very easy to find space in the setup as originally proposed by Iñigo Zuluaga, as shown here:

It requires only 2 extra wires, a TS2950C33, a capacitor and of course a one pin larger connector and it nicely fills the ‘empty space in the upper right corner.

The Stripboard looks like this: It requires cutting a strip and running 2 wires. (indicated in red). If you wish, you could extend the header with even 1 more hole to not only include the 3.3 V pin, but run an extra Ground right next to it. The ground is already running on that strip

Mind you, it is the stripboard as seen from the copperside, thus looking at the under side of the physically not visible components (an X-Ray view so to speak).

Simple SD card interface for Arduino

In an earlier circuit I showed a simple way of  adding an SD card to an Arduino. That works well. Sadly because the Arduino is 5 Volts and the SD card needs 3,3 volt levels, a level adapter was used, consisting of resistors.
As said, it works well, but it is not really an elegant solution. It is better to use another chip (The 74HC125)  to do the level shifting, as shown in the figure below:
SD card2

To keep the circuit a bit simple, I have just drawn the buffer gates of this chip, but there are a couple of more pins that need to be connected: Pin 1,4, 7 and 10 Need to be connected to ground. Pin 14 (Vcc) needs to be connected to 3.3 V.

The reason why so many of the pins need to go to ground is because Pin 7 is the regular ground of the chip, but pin 1,4 and 10 are the negative control gates of the buffers that are used. The 74HC125 is a Quad bus buffer so you will have one buffer to spare. It might be wise to connect the input of that buffer (pin 12)  and it’s negative gate (pin 13) to 3.3 V as well to avoid unpredictable behaviour.