Fullscreen HTML5Med Res (??MB)HTML4

Make it with Micromite Phil Boyce – hands on with the mighty PIC-powered, BASIC microcontroller Part 24: Counting pulses, rotary encoders and a digital safe T his month, we are going to explore three useful MMBASIC commands that relate to counting digital pulses. Using these built-in commands greatly simplifies tasks such as measuring the frequency of a signal, measuring the time period of a pulse cycle, or simply counting the number of pulses present on an input pin. Linked to these topics is something that follows on nicely from last month, where we used a rotary potentiometer to generate a voltage between 0V and 3.3V, which in turn was used to control the position of a servo actuator arm. Several readers contacted us and asked if there was an alternative to a potentiometer; something that can instead be continually turned in either direction (unlike a potentiometer which is mechanically limited – typically to around 270°‚ not even a complete 360° revolution). This is where rotaryencoders come into play, and if you have never used one before, they can be an extremely useful input device. To make this a fun, we will show you how to use a rotary encoder to simulate entering a digital safe combination number. What exactly are digital pulses? Digital pulses comprise transitions between low and high logic levels on a signal line. The result is a digital signal. These can vary considerably, yet are often simply drawn as the examples shown in Fig.1. The pulses in a digital signal can be repetitive and symmetrical; an example is a square-wave – see Fig.1a. Here they are shown with a fixed frequency (ie, the length of a pulse cycle is constant), and with a duty cycle of 50% (meaning that for 50% of the pulse cycle, the logic level Questions? Please email Phil at: contactus<at>micromite.org 50 is high, and for the remaining P ulse cycle time it is at a low logic level). We have seen, and used, square-waves earlier in the 1b series when we used a PWM signal to drive a piezo sounder to generate musical notes. 1c Pulses can also be repetitive but non-symmetrical ie, where the frequency is constant, but Fig.1. Examples of various digital signals. 1a represents the duty cycle is not 50% – a square wave, meaning a duty cycle of 50%. 1b see Fig.1b. A great example represents a digital signal with the same frequency (ie, of this is a servomotor signal fixed time-cycle) as 1a, but with a lower duty-cycle. 1c (ie, PWM signal) where, as is a random signal with a varying frequency (and cycle we saw last month, the duty time period) and with varying duty-cycles. cycle controls the position of a servo motor actuator arm. Digital pulses that the boundaries of each pulse cycle are may also be random (ie, non-repetitive indicated by the dotted lines in Fig.1a and and non-symmetrical) – see Fig.1c where Fig.1b. In terms of logic-level transitions, neither the frequency nor the duty cycle a pulse cycle in a digital signal is shown is constant. in Fig.1 as starting with a low-to-high Consider each of these digital signals transition followed by a high-to-low, and being drawn as a graph where the x-axis finishes on the next low-to-high transition represents time, and the y-axis represents (which is effectively the start of the next the digital logic level; ie, either a logic pulse cycle, and so on…). Understanding high (3.3V) or logic low (0V). The point this concept of a pulse cycle allows us here is that at any moment in time, the to measure two important parameters – state of the digital signal can be considered time period and frequency – by simply as either a low or a high logic state. detecting logic-level transitions. If we were to use a Micromite input pin to read the logic level of a digital Pulse time-period signal, then by detecting the transitions So, having just explained what a pulse between the low/high states within the cycle is in terms of logic-level transitions, signal, we can start to measure certain it now makes it easy to explain the pulse parameters about the digital signal. We’ll time period (sometimes referred to as ‘cycle discuss the theory first and then look at time’). The time period is simply the time the MMBASIC commands that simplify taken to complete one pulse cycle. In other the whole process. words, relating to the square-wave pulse cycle highlighted in bold in Fig.1, it is the time taken between two consecutive lowPulse cycle to-high logic-level transitions. More on A pulse cycle within a digital signal can this later when we discuss the MMBASIC be considered as a ‘complete wave’ that command to measure this timing. simply comprises a high-level logic pulse and also a low-level logic pulse (it doesn’t matter which comes first). Fig.1a has one Signal frequency square-wave pulse cycle highlighted in Frequency is often regarded as the number bold to make this a little clearer. Note too of pulses per second, and it is measured Practical Electronics | January | 2021 The SETPIN command As we have seen earlier in this series, the SETPIN command is used to configure a Micromite pin to behave in a certain way. The simplest syntax of this command is: SETPIN pin-number, configuration, option where: pin-number is the value of the physical pin number on the Micromite chip. It must be a valid value otherwise an error is generated (see the Micromite User Manual for specified values). configuration is a parameter that determines the function of the specified pin. When we first introduced the SETPIN command early on in the series we used it to define a pin as a digital input with SETPIN x,DIN (where x is a valid pin number) and used SETPIN x,DOUT to define a digital output. Last month, we also used SETPIN x,AIN to define an analogue input pin to which we connected a potentiometer, which supplied an analogue voltage between 0V and 3.3V. Fig.2. This rotary encoder comes with a built-in RGB LED and a push-button (activated by pressing on the shaft). A breakout board is available, making it a breadboard-friendly device. in hertz (abbreviated to ‘Hz’); but more technically correct, it is the number of complete pulse cycles per second. To determine the frequency of a digital signal, we could start a ‘one-second timer’ on a low-to-high transition, and then count the number of subsequent low-to-high transitions while the timer is ‘active’. When the one-second interval is up, the count reached would represent the frequency (in hertz) of the digital signal. An alternative is to use the wellknown equation: Freq = 1/(time period) If we measured a time of 0.1 seconds for a single pulse cycle, then the measured frequency would be 1/0.1 = 10Hz. Note that by using the equation method, we do not have to wait for the ‘one-second’ timer duration. The theory described above shows that for MMBASIC to measure a time period or the frequency of a digital signal, all it needs to do is detect logic-level transitions and accurately measure the time between such transitions. We could write a BASIC program to do this, but what if we want to measure a high frequency signal – there would come a point where our program code would simply be too slow to keep up. To overcome such a limitation, MMBASIC has three commands built into the firmware that automatically detect these logic-level transitions at an extremely fast rate. Practical Electronics | January | 2021 option is an optional parameter that may define further details; for example, previously we have used PULLUP or PULLDOWN in conjunction with DIN when using a push-button as an input. This avoided having to use a physical resistor to tie the input signal to a default logic level (with a push of the button then setting the opposite logic level). So, why have we explained all this? The answer is that the pulse-counting commands that we are exploring this month are also implemented with the SETPIN command, but with different configuration parameters. The FIN, PIN, and CIN parameters The SETPIN command pin-number parameter must be a valid value, and referring to the Micromite User Manual, it explains that four pins (pins 15-18) are valid for use as ‘COUNTing’ inputs on the 28-pin Micromite. Note that ‘COUNTing’ pins means that FIN, PIN and CIN can be used on any of these pins (and not just the counting (CIN) functionality). Now let’s demonstrate each in turn, starting with how the MKC can measure the frequency of a digital signal. Note that the voltage levels of any digital signal that we wish to measure must not exceed 5V, and ideally be at 3.3V for the MKC. If you wish to measure a digital signal that has a higher voltage, then simply add a potential divider between the high-voltage signal and 0V. For our demonstration purposes, we will simply use a Micromite output pin as the source of the digital signal, and so it will not exceed 3.3V for a high logic level. Using FIN To demonstrate how to measure the frequency of a digital signal, we will use pin 16 as the input pin. We could also use pin 15, 17 or 18, but pin 15 may already be in use if you have a touch-screen connected and enabled. For a digital test signal, we will use the PWM command to output a square wave on pin 4. There is no diagram for this circuit, since all we need to do is connect a jumper wire between pin 4 and pin 16. Once you have done this, connect your MKC to your computer, start the Terminal app, and enter the following: Freq=2000 SETPIN 16, FIN PWM 1, Freq, 50 DO PRINT PIN(16) LOOP Before you RUN the code, let’s explain what we’re doing. The first line sets a variable named Freq with a value of 2000. This will be the frequency (in hertz) of the generated square wave from the PWM pin (ie, the frequency of the digital test signal). The second line configures the appropriate input pin (pin 16) to which we are connecting the data signal that is having its frequency measured. The third line starts to output a square wave on pin 4 (PWM channel 1A). It is a ‘perfect’ square wave with a 50% duty-cycle; and the frequency is set to the value stored in the Freq variable. Next, a DO…LOOP, in which we continually PRINT the value of PIN(16) on the terminal screen. Pin 16 is configured with the FIN parameter, so when you PRINT PIN(16) you don’t get the logic value (as you would if we configured pin 16 with DIN), but instead the firmware measures the frequency by counting the number of low-to-high transitions that occur over one second. RUN the program and you should see is a stream of ‘0’s (zeros) initially displayed on the screen, followed possibly by a stream of values close to 2000, and finally after one second it should settle on a stream of 2000. If you do not see this, check your code matches that shown above, and also check you have pin 4 and pin 16 connected to each other. Note that the 2000 shown is a value in hertz; ie is equal to 2kHz. Gate time The reason for the initial zeros (and potentially a few non-2000 values) is that there is a one-second period of time during which the firmware is busy counting the low-to-high transitions in the background. This period of ‘counting time’ is referred to as the ‘gate time’; it must elapse before 51 A C lockw ise B Low- to- high transition A better to measure the pulse time period and then use the formula Freq = 1/(Time Period), This is what we will examine next by using the PIN configuration parameter instead. Using PIN To measure the cycle time period, the Micromite B firmware just needs to accurately measure the time between two Fig.3. The rotary encoder outputs two digital signals: ‘A’ successive low-to-high and ‘B’. By detecting a low-to-high transition on ‘A’, we transitions. By using can read the logic level of ‘B’ to determine the direction SETPIN 16, PIN in the of rotation. above program, we can see a frequency count can be provided. Gatethis time period measurement in action. time, therefore, determines how often the Change the second line (with no output result is updated (important if the option parameter), and also set the frequency of the data signal varies). value of Freq to 50 (ie, 50Hz). On running Now refer back to the syntax of the the program, you will see the value 20 SETPIN command and you’ll see there displayed, which means the period of is an option parameter. An important the 50Hz signal is 20ms. The result is point here is that when you use the always in ms – a value of 20 means 20ms FIN configuration parameter, the (0.02s). As a sense check, note that Freq option parameter defines the gate time = 1/(Time-Period) = 1/0.02 = 50Hz. in milliseconds (ms). So why would we When the configuration parameter want to adjust the gate time? Well, if the PIN is used with the SETPIN command, frequency of the data signal is very high, the option parameter can be used to then you could count many pulses in determine over how many pulse cycles a shorter space of time and achieve an the measurement is taken to average the accurate result quicker. In this scenario, period measurement. Valid values are 1 the output would be updated more often (default) to 10000. You would tend to set (shorter gate times mean the count result this to a higher value when measuring the is displayed sooner). Similarly, if the data time period of higher frequency signals. signal has a very low frequency, then a To see this in action, set the Freq value much longer gate time is required in order to 5000 (ie, 5kHz), and run the program. to count a number of pulse cycles in order The resultant time period displayed is not to measure the frequency. You couldn’t really accurate, so now add the option use a gate time of one second to measure a parameter with a value of 1000 so that signal that only changed every 10 seconds! 1000 pulse cycles are timed and averaged. To see the above in practice, stop the On running the program, you will now see program (Ctrl-C), and then EDIT the the value 0.2 displayed. This represents program to set the Freq variable to 200000 a time period of 0.2ms = 200µS, which (ie, 200kHz) – then RUN the program is the period of a 5kHz signal. again. We are still using a one second In summary, when measuring highgate time, so there is still an observable frequencies, it is better to use FIN, and ‘delay’ before the result is shown. when measuring low-frequencies, it is Next, shorten the gate time to 10ms by better to use PIN. changing the second line to SETPIN 16, FIN, 10 and on running the program Using CIN you should see the resultant frequency The configuration parameter CIN is displayed much quicker (every 10ms). used if you simply just want to count In summary, when using the F I N pulses (or to be precise, count low-toconfiguration parameter, ensure you high transitions). On configuring a pin use an appropriate gate time to allow with CIN, an internal counter is reset to MMBASIC to count a decent quantity a value of zero. Then, on every low-toof pulse cycles (which is ultimately high transition, the counter increments determined by the frequency of the input by one. Then, whenever you use PRINT data signal). The gate time parameter PIN(x) (where x is one of the four valid value must be between 10 and 100,000; COUNT pins), the value of the counter will and the outputted value is always in be displayed. To reset the counter back to hertz, regardless of the gate-time value. zero, simply use SETPIN x,CIN again. When measuring signals with a Note that the Micromite can detect input frequency less than 10Hz, it is often pulses as brief as 10ns, and hence if CIN A nticlockw ise 52 is used, for example, to try and count button presses on a push-button, then any mechanical bounce as the button is pushed will also be counted (which is why it’s important to debounce mechanical switches). Therefore, CIN is much more suited to counting pulses in a true digital signal as we will now explore. Note, there is no option parameter when using CIN. Using our existing code, set the Freq value to 1000, ie 1000 pulses will be generated every second. Then change the second line to SETPIN 16, CIN and run the program. You will see the pulses being counted, with effectively the number of ‘thousands’ being the number of seconds the program has been running for. Have a play by changing the value of Freq, and see how this affects the displayed value after each second. The above shows how to use the Micromite to perform various counting tasks on a digital signal. We will now look at a slight variation to this in the form of detecting pulses from a rotary encoder. Rotary encoders A rotary encoder is a rotary position sensor, but it can also be used as a ‘digital potentiometer’ which has no limitation on its rotation – it can be continually rotated in either direction. Instead of containing a variable resistance, a rotary encoder simply outputs a sequence of digital pulses, often as two separate digital outputs. By knowing how to interpret these pulses, we are able to determine the direction (and speed) of rotation – more on this shortly. Rotary encoders come in many different styles; however, we really like the one shown in Fig.2. It has a built-in RGB LED which can be made to illuminate through the clear plastic shaft; and by pushing down on the shaft, you can activate the built in push-button. In addition, while rotating the shaft, it has a soft ‘click action’ which provides nice tactile feedback. These features are not all normally found on a rotary encoder; and when you consider that it is readily available online at low cost, you can see why it is our favourite rotary encoder. To make it breadboard friendly, an optional breakout-board is also available (see Fig.2). Decoding the pulses The pulses generated by this rotary encoder are output on two digital signal lines – labelled ‘A’ and ‘B’ on the breakout board. These pulses are actually generating a 2-bit Grey-code signal. We won’t explain the details of Grey code here, but Fig.3 shows the kind of signals it creates. The point to observe here is that there are two possible rotations – you can turn it clockwise, or anti-clockwise. So how do we decode these pulses? The answer is actually quite simple once you apply the Practical Electronics | January | 2021 PULLUP option is used to tie the default logiclevel high (with the A C B Eq uiva lent circuit rotary encoder simply 3.3V pulling it low whenever it needs to output a low R ed Green B lue pulse). The second line NC R 1 R 2 configures pin 18 as an 470Ω 470Ω input so that the logic level on signal ‘B’ can 22 24 21 be read when required. + B SW G R The third line sets a R 1 470Ω NC variable that we have 22 3.3V named Rot_Value with a value of 50 – it also 24 R 2 prints this value on the 21 470Ω terminal screen. A simple DO…LOOP is Fig.4. The rotary encoder connects to the MKC with three included, and acts as the connections to read the output pulses. The Digital Safe project main program. It does adds connections for the RGB LED and the push-button. not do anything here, but is required so that the program can continue to run while waiting for a lowfollowing technique. Referring to Fig.3, to-high transition on pin 17 (ie, from the consider the moment in time when there rotary encoder’s signal A output. is a low-to-high transition in signal A. If The interrupt subroutine is the part that signal B is at a low logic level (highlighted then determines the direction of travel by the blue circle), then the shaft is rotating by reading the state of pin 18. Depending clockwise. However, if signal B is at a on the state of pin 18, Rot_Value is high logic level (highlighted by the red either incremented by 1 (clockwise), circle), then the shaft is rotating in an or decremented by 1 (anticlockwise). anti-clockwise direction. It’s that simple! And before the subroutine is exited, the Note that you could detect the transition new value of Rot_Value is displayed of signal B instead, and then read the on the screen. Note that the PAUSE logic level of signal A to determine the command adds a small delay to remove direction. And you can also use a highany mechanical contact bounce generated to-low transition – whatever option you by the rotary encoder. select, just stick with it. RUN the program to ensure it works Let’s put this into practice with a few correctly by spinning the rotary encoder. lines of code. First, refer to Fig.4 for the Upon each ‘click’, you should see the circuit diagram. Note at this stage we only value displayed on the screen increase need to connect the three contacts labelled or decrease according to the direction ‘A’, ‘B’, and ‘C’ to the MKC (respectively of rotation. to pins 17,18 and 19). A and B are the two If you see the above work, but ‘in signal lines, and C is effectively a common reverse’ (ie, increase with an antithat is connected to 0V. Connect this part clockwise rotation), then you can either of the circuit, and then enter the following make a hardware change (by swapping short program: the connections to pins 17 and 18), or you can change the code inside the interrupt SETPIN 17, INTH, MyInt, PULLUP subroutine – I will leave it to you to work SETPIN 18, DIN, PULLUP out what to change! Rot_Value=50 : PRINT Rot_Value 17 0V 18 DO : PAUSE 1 : LOOP SUB MyInt PAUSE 0.1 IF PIN(18)=0 THEN Rot_Value = Rot_Value + 1 ELSE Rot_Value = Rot_Value - 1 END IF PRINT Rot_VALUE END SUB The first line of code simply configures pin 17 to trigger an interrupt subroutine (called MyInt) whenever a low-to-high transition is detected on signal ‘A’. The Practical Electronics | January | 2021 Digital safe Having seen how to use a rotary encoder to increase and/or decrease the value of a variable depending on which direction it is ‘spun’, it is now time to have some fun by demonstrating how a rotary encoder can be put to good use in a practical project – we will simulate entering a safe’s combination-lock number in a short demo program. This will require the use of the red and green LEDs built into the rotary encoder, as well as the built-in push-button. Now connect the rest of the circuit shown in Fig.4 (ie, connect the MKC to the push-button SW contact, and LEDs R and G via 470Ω resistors). Next, download and RUN the DigitalSafe.txt program (available from the January 2021 page of the PE website). On running the program, you will see the current ‘combination dial’ value displayed on the terminal screen. Simply turn the rotary encoder in the appropriate direction to enter the three required values of the combination: 3, 46, 22. When you reach the first required value, push the encoder shaft to activate the push-button, which in turn ‘submits’ the number displayed on the terminal screen. Continue with the second and third required values above in a similar manner. If you entered the three-number combination correctly, then the green LED will illuminate, otherwise the red LED will illuminate, meaning you failed to open the safe! The code is commented throughout so we won’t go into the details here, but do take the time to look through the code to see how easy it is to create this ‘digital safe’. Challenges Once again, there are many things you can change in the program code to affect the way it works. In this example, why not add hardware too – here are some challenges: 1. Increase the range of each value from between 1-50 to between 1-99 (or greater) 2. Make the safe more secure by increasing the length of the combination number from three values to five values (or more) 3. Add a time-out feature; ie, the combination has to be entered within a certain amount of time 4. Add a piezo sounder to give audible feedback to signify a correct combination entry, and a different sound if incorrect 5. Add a relay and a solenoid to complete the lock! Next month We have covered a wide range of topics throughout the series, and along the way encouraged you to experiment and build your own Micromite-based projects. Several of you have written in to ask if there are more powerful versions of the Micromite. For example, devices that can run faster, or have more memory, or have additional features and commands for controlling different hardware, such as SD cards for storing data, or for driving larger displays with higher resolutions. The answer to all of these is very much yes, there are indeed different Micromites available, as well as different versions of MMBASIC. Next month, we will provide a summary overview of what else is available for those of you wanting to advance beyond the power of the Micromite Keyring Computer. Until then, have fun coding! 53