Fullscreen HTML5Med Res (??MB)HTML4

By Tim Blythman Silicon Chip Nano Pong on your TV Atari’s Pong arcade game is nearly 50 years old and is remarkable for its time, inspiring many of the computer games that followed. Our Nano Pong game is modern and retro at the same time; it replaces the 70-odd discrete logic chips in the original with a single chip that costs about $1! But it still looks and plays much like the original game. I n Dr Hugo Holden’s in-depth article on recreating the original Pong arcade game (June 2021; siliconchip. com.au/Article/14884), he stated that none of the later versions of Pong were as good as the original. He specifically mentioned single-chip solutions as being inferior to the discrete version. This project is an attempt to change that! While this is a complete redesign of the circuitry to implement the Pong game, we have tried to be reasonably faithful to the original in terms of its graphical style, and how the game is played. Another inspiration for this design was our article about new 8-pin PIC microcontrollers (November 2020 issue; siliconchip.com.au/ Article/14648). One of the chips we looked at then was the PIC12F1572 microcontroller. It was about the cheapest 8-pin PIC we could find at the time, and despite that, it had superior features to the PIC12F675 that we have used for many years. As one of the smallest, cheapest microcontrollers around, we decided that it would be an interesting challenge to use it to recreate Pong. corrected a few bugs along the way (all described in the article linked above). But his faithful recreation depends on some parts which are becoming hard to get or expensive. Our version of this classic game is made using not much more than a small microcontroller and some passive components. It’s so tiny that we haven’t even specified a case for it; it can simply be wrapped up in a length of heatshrink tubing and left hanging behind the TV. A pair of controllers (‘paddles’) are built into small enclosures on flying leads, but if you’re interested in creating something more akin to the cabinets and consoles that would have existed at the time, you can do that too. Nano Pong is closely inspired by the original Pong; two players control on-screen bats that vie to keep the ball in play. The winner of the game is the first to win 11 rallies. We say ‘inspired’ as we haven’t attempted to make it identical. No doubt those who played the original game would notice some differences. But we have tried to emulate the style and gameplay of the older game. In doing so, we hope that those building this project can experience the joy of playing a 50-year-old computer game without the hassle of having to locate and solder a multitude of vintage logic chips. Like the original, the two player control paddles are potentiometers that translate the player’s paddle position to a corresponding on-screen paddle. We’ve also added a pushbutton (which isn’t in the original) to allow a player to ‘serve’ the ‘ball’. The PIC chip emulates the gameplay mechanics, and generates analog audio and video signals that can be fed to a PAL television’s AV inputs. Our Nano Pong project fits on a miniature 43 x 16.5mm PCB and relies on a single micro that only costs $1. Nano Pong Dr Hugo Holden’s recreation of Atari’s original Pong arcade game mainly kept with the original design. Still, he made the PCB smaller and 46  Silicon Chip Australia’s electronics magazine siliconchip.com.au Fig.1: this Nano version of Pong doesn’t need much in the way of hardware! A single 8-pin PIC microcontroller and surface-mounted passives are complemented by a handful of off-board parts for the player controls. Hardware Fig.1 shows the complete schematic – there is not much to it! Potentiometers VR1 & VR2 and pushbuttons S1 & S2 are not located on the PCB, but connected via flying leads. CON1 is the first modern flourish. A mini-USB socket provides 5V power to the circuit, significantly simpler than the original mains supply. Since many TVs now have a USB socket, the unit can be powered from the TV that it’s connected to. 5V power goes to pins 8 (GND) and 1 (Vdd) of IC1, a PIC12F1572 microcontroller, bypassed by a 100nF capacitor. IC1’s MCLR pin is pulled up to 5V by a 10kW resistor, so the PIC will run its internal program from flash memory as soon as power is applied. Pins 7 and 6 of IC1 are inputs to the ADC (analog-to-digital converter) peripheral and communicate the Player 1 and Player 2 control inputs to the PIC. Each player has a 1kW potentiometer padded on both track ends by a 470W resistor. The resistors are fitted to the PCB. The potentiometer flying leads connect to pins 1-3 of CON5 for Player 1, and CON4 for Player 2. With the 470W padding resistors in series with the 5V supply, the player paddle wipers vary between 1.25V and 3.75V depending on the potentiometer rotation. siliconchip.com.au We’ve specified standard 24mm potentiometers, but if you think your Nano Pong might be subjected to long periods of vigorous gameplay, you could upgrade them to more robust types. Although it would not be in line with the original, slide (rather than rotary) potentiometers could also be used to make a more intuitive interface, matching the straight-line motion of the bat on the screen. Pins 4 & 5 of CON4 and CON5 connect across normally-open momentary pushbutton switches. By pressing the button, the player pulls pin 2 of CON4/ CON5 (connected to the potentiometer wiper) to 0V. As the lowest voltage the pot can generate is around 1.25V, the microcontroller can distinguish this as a button press. The top padder resistor limits the worst-case current through the switch. Pin 5 of IC1 is the pulse-width modulated (PWM) sound output. It feeds a 1kW/470W divider, reducing the PWM amplitude from 5V peak-to-peak to around 1.6V peak-to-peak or 0.56V RMS. This is AC-coupled by a 1μF capacitor and biased to ground by a 100kW resistor before going to the output RCA plugs that connect to the TV. We’ve chosen these values to keep the sound signal well below 1V, as the audio (as per the original Pong) is a shrill-sounding square wave. The video signal is a standard CVBS (composite video baseband signal) in monochrome PAL format. Many of the differences between PAL and NTSC involve colour transmission, so many NTSC TVs should lock onto this signal. The main remaining difference is in the number of lines that are sent per frame. Modern TVs will usually detect and display the correct format. This signal is formed from digital levels at output pins 2 and 3 of IC1. Pin 2 is designated as luminance (LUM) and pin 3 as synchronisation (SYNC). The TV is assumed to have a 75W terminating impedance, so it Once finished, the PCB and cabling can be covered with heatshrink tubing. Australia’s electronics magazine August 2021  47 the colour picture information; since we are not transmitting such signals, the picture is decoded as monochrome. During the visible video area, the video intensity is determined by the signal voltage, between the black and white levels. A longer sync signal is used to trigger a vertical retrace. Often, the vertical sync signal is mixed with the horizontal sync signal to create a so-called ‘serrated’ sync signal that allows horizontal sync to be detected during the vertical retrace. This improves the TV’s ability to maintain horizontal hold. Thus, a single 1V peak-to-peak analog signal can encode raster intensity and both horizontal and vertical synchronisation to recreate a 2-D TV image. Software Scope 1: a scope grab of the video signal for a typical scan line, along with a portion of the display around that scan line, so you can see how they correspond. Each line is delimited by the dips in the trace to the low sync level (horizontal sync pulses), while the peaks correspond to a white raster on a black background. The red lines indicate that there is a substantial part of the signal outside of the visible area. Fig.2: the way that the screen is laid out makes it very easy to generate in a left-to-right fashion. Each horizontal scan line can display the Player 1 bat, Player 1 score, net, Player 2 score and Player 2 bat. The ball is produced separately by the PWM peripheral so that it can appear at any horizontal position. will see different voltages depending on the pin states. If both LUM and SYNC are low, then the output is 0V, which corresponds to the so-called ‘sync’ level. With SYNC high and LUM low, the TV sees around 300mV. This is known as black level, and corresponds to a black raster being displayed. Finally, with both pins high, a level near 1V is seen, which generates a white raster. Scope 1 shows the voltage generated over time for a typical horizontal scan line, along with several lines of video (including this scan line) above. Note the horizontal sync pulse troughs on either side of the displayed video. 48  Silicon Chip Analog video signal So how does a TV translate this signal to a two-dimensional picture? The TV continually scans its raster in left to right horizontal scan lines from top to bottom of the screen, with each scan line taking around 64μs. A 4-5μs low pulse indicates the start of a new horizontal line. The visible area takes up most, but not all, of the remaining scan line. The actual visible area takes 52μs to transmit, so it is bracketed by periods of black level called the ‘back porch’ and ‘front porch’. Colour transmissions contain signals in the back porch to help decode Australia’s electronics magazine Understanding the following is not necessary for getting Nano Pong to work. Still, it is interesting to compare it with how the original version operates, especially since the original version was purely hardware-based. Accurately emulating the logic chips in Pong would be a better job for an FPGA than a microcontroller, as the former allows everything to happen independently in parallel, a luxury we do not have. Our PIC needs to generate a tightly timed signal to maintain a steady picture. Most older PICs would require a crystal oscillator to provide an accurate enough clock to display a TV image, which would take up two of our eight pins. But here, we get accurate timing by running the PIC’s internal oscillator at 32MHz (requiring the use of the internal PLL), which gives an instruction clock of 8MHz. At 64μs per line, we can distinguish up to 512 horizontal positions per line. By scaling this to use 256 positions, we can use 8-bit bytes to hold pixel locations. In practice, the actual horizontal play area is around 200 positions. If you look closely at our images, there is a bit of horizontal jitter, which would not be present with a more precise crystal oscillator. But we don’t think it looks out of place in our recreation of 50-year-old technology. As PAL TV signals have 312 horizontal lines per field, we conveniently set the play area to be 256 lines, which neatly lines up with the visible area on most TVs. siliconchip.com.au With such tight timing needed, we have fallen back to using assembly language so that we know how long every part of our program will take to execute, ensuring the image quality does not suffer. The initial setup is written in the C language. It then calls our main assembly language subroutine. The main program is a loop of 312 subroutine calls, each corresponding to a horizontal display line variant. These, in turn, consist of numerous direct pin manipulation commands to set the necessary video output levels interspersed with calls to a delay routine to affect the timing. This starts with six vertical sync lines to start the field, followed by 28 blank lines. The blank lines are 5μs at sync level (SYNC and LUM low), followed by 59μs at black level (SYNC high and LUM low). The vertical sync line is serrated by delivering 5μs of black and 59μs of sync instead. After this are 256 active display lines. The counter LINECOUNT is used to keep track of which line is being displayed, and this is compared with the bat and ball positions, then flags are set to indicate whether the bat or ball should be displayed on the current line. These flags are set during the line’s horizontal sync period, so it does not affect the timing of the visible part of the display. The way these flags are set is a bit unusual. To ensure that each line runs for the same amount of time, as needed to maintain a steady picture, we avoid skipping over code we don’t want to run (as usually happens if a condition is false), which would change the program timing. Instead, we use the ‘skip on bit test’ assembler opcodes (BTFSC and BTFSS), which essentially treat the following opcode as a NOP (no operation) if a test is true. These sequences of commands all take the same time regardless of their outcome, retaining the necessary consistent timing. For lines where the ball is visible, we use the PWM peripheral to display it. The PWM peripheral on the PIC12F1572 is quite advanced, with phase and offset parameters. The ball’s horizontal position is determined by the PWM phase and its width by its duty cycle. This means we don’t have to keep track of when to turn the LUM output on and off. siliconchip.com.au Screen 1: a typical game of Nano Pong. The ball is in play after Player 1 has won the first point of the game. Screen 2: with a reasonable amount of program flash memory to spare, we added this splash screen when the unit is powered up. Screen 3: the start of a game, before Player 1 has served the ball. With that taken care of, the remainder of the visible lines can be neatly broken up into sections that can be handled sequentially. From left to right, these are the Player 1 bat, Player 1 score, the net, Player 2 score and Player 2 bat. For the bats and net, we briefly toggle the polarity of the PWM signal, thus getting the XOR effect as the ball passes over, so the ball does not ‘merge’ with them. Fig.2 shows how the horizontal lines are organised, and Screen 1 shows it without the lines. The scores are handled slightly Australia’s electronics magazine differently. These are effectively bitmaps hard-coded as brief assembly language sequences, so the PWM output is turned off while the scores are being displayed. Thus, the ball disappears behind the scores, which could provide an advantage for a canny player. After the 256 active lines, a further 21 blank lines are displayed, followed by a single customised blank line that handles all of the logic that updates the game’s state. The code for this line is two instructions shorter than the other blank lines, to account for the time taken to jump back to the start of the loop. August 2021  49 Most of the time that this final blank line is being generated, the game logic is processed. If it detects that the ball has struck a wall or bat, the ball vector is adjusted. This includes taking into account where it strikes the player’s bat, as this affects the ball’s vertical speed, like the original game. Also like the original game, each strike of the bat can also increase the ball speed. These events also trigger a sound to be played, generated by a different PWM channel playing a tone from pin 5 until it is reset on the next field. This gives a variety of differently toned beeps depending on what the ball has struck. The two ADC channels for the paddles are alternately sampled and allocated to their respective players. The relationship between the ADC value and on-screen position is adjusted to take into account the range set by the resistors. If the ADC value is outside this range, then the bat position is not updated, which also takes care of the case when the ADC pin is pulled low by the button press. Thus, trick serves are not possible. A point is registered whenever the ball reaches the screen edges (ie, missing the player’s bat), which increments the score counter. Flags are set to indicate that the player winning the point is to serve, and if the score has reached 11, that a win has occurred. In this case, a melody is played on the pin 5 PWM channel and the winning score is flashed. Timing for these events comes from different bits in the FIELDCOUNT parameter. Since the main program only uses about 2/3 of the available flash memory, we also added a splash screen, shown in Screen 2. This uses data from the score bitmap sequences in a hard-coded loop. An 8-bit timer counts down over 256 fields at 50Hz, so this screen shows for around five seconds. you can change the 470W resistors connected to the potentiometers at pins 1 and 3 of CON4 and CON5. Increasing their value will create a gap near the top and bottom of the screen that the bats can’t reach, as in the original Pong game. For example, replacing these four 470W resistors with 560W resistors will leave around a 3% gap at the top and bottom of the bat travel. If you have a 5V power source that doesn’t have a USB connector, it can be fed to pins 2 (positive) and 3 (negative) of CON2. We haven’t tested it, but the circuit should run from a 4.5V supply, such as three AA or AAA batteries in series. Construction In keeping with the theme of this being a modernised and miniaturised version of Pong, the PCB uses mainly SMD components. Since these are resistors and capacitors, with one IC in a relatively large 8-pin SOIC package plus the USB socket, assembly is not difficult. The double-sided PCB is coded 08105212 and it measures just 43 x 16.5mm. Refer to the PCB overlay and wiring diagram, Fig.3, during construction. Start by mounting those SMDs. We recommend that you have a temperature adjustable soldering iron, flux paste, tweezers, a magnifier and solder wicking braid, as well as some solder wire. The small PCB can be temporarily secured to your desk with some Blu-Tak or similar, so it doesn’t move around during assembly. Fume removal or ventilation is also recommended, as flux generates more smoke than typical solder wire. Start by fitting IC1 and CON1. Apply flux to the pads and rest CON1 in place, then add a small amount of flux to the top of the pads. Its small plastic pegs should align it to holes in the PCB. Clean the tip of the iron and add some fresh solder. Apply the iron’s tip to the two longer pads on the PCB; the flux should help the solder run up the leads. You only need to solder the two outer leads as this socket only supplies power. If you create a solder bridge, add some more flux and press the solder braid against the bridge until it draws up any excess solder. There should still be enough solder left to make a successful connection. Turn up the iron temperature slightly to solder the four larger pads to the PCB that mechanically secure the connector, then return the iron to its original setting. IC1 needs to be fitted in the correct orientation, with its pin 1 towards the USB socket. There should be corresponding marks on the PCB and the part itself. Apply flux paste to the PCB pads and rest the IC in place. Add a little solder to the iron top and touch it to one pin to tack the part in place. If the IC isn’t flat against the PCB or the pins are not aligned with their pads, carefully apply the iron again and adjust the position. Component notes With such a small PCB, there isn’t a lot that can be modified. If you find that the volume of the sounds doesn’t match your other TV sources, you can adjust the 1kW/470W divider connected to pin 5 of IC1. Reduce the 470W part value (or increase the 1kW part value) to reduce the volume. Alternatively, increase the 470W part value to increase the volume. If you want to make the game harder, 50  Silicon Chip Fig.3: with fewer than 20 onboard parts, the PCB is easy to assemble. Mount SMD parts IC1 & CON1 first, then the passives, then the connectors (if you are using connectors). Australia’s electronics magazine siliconchip.com.au Once it is correctly aligned, solder the remaining pins. Then, if you have bridged pins, use the braid to remove them as described above. The 100nF capacitor sits between IC1 and CON1. Using a similar technique to IC1, tack one lead, adjust and solder the other. Go back to the first lead and add a little flux paste or solder to freshen it up. Don’t be alarmed if your solder joints don’t have compact, concave fillets. The important thing is that the parts are connected firmly, and a large glossy solder joint that isn’t bridging to other parts is fine. Now fit the remaining SMD passives where shown in Fig.3. The resistors usually are marked (see the typical codes in the parts list), while the capacitors will only have their values printed on the packaging. Once all the surface mounted parts are fitted, you can clean the excess flux from the PCB using your preferred flux cleaning solution. Allow the board to dry out thoroughly before continuing. All of the through-hole headers are optional; CON2 is only required if you do not have a pre-programmed microcontroller, while CON3-CON5 can be regular headers or sockets as you need, or you can just solder wires (eg, sections of ribbon cable) directly to the PCB pads. Programming IC1 We expect most constructors will get a pre-programmed PIC from us. Otherwise, you can program the chip after soldering it to the board, but it’s a bit tricky. The problem is that the programming pins, pins 7 (ICSPDAT) and 6 (ICSPCLK) are also connected to the player paddle wipers. So it’s best to program the chip before connecting up those paddles. To do this, plug your programmer Parts List – Nano Pong 1 double-sided PCB coded 08105212, 43 x 16.5mm 1 PIC12F1572-I/SN (SOIC-8) programmed with 0810521B.HEX (IC1) 1 SMD mini Type-B USB socket (CON1) 1 10cm length of 20mm diameter clear heatshrink tubing 3 RCA plugs AND 1 3m length of shielded cable OR 1 triple RCA plug cable [eg, Jaycar WV7316] 1 5-way male pin header (CON2, optional for programming, see text) Capacitors (all 50V X7R SMD ceramic, M3216/1206-size) 1 1μF 1 100nF Resistors (all 1% metal film SMD, M3216/1206-size) 1 100kW (marked “104” or “1003”) 1 10kW (marked “103” or “1002”) 2 1kW (marked “102” or “1001”) 6 470W (marked “471”, “470R” or “4700”) Controller parts 2 small plastic enclosures (eg, UB5 Jiffy boxes) 2 1kW 24mm rotary potentiometers (VR1, VR2) [eg, Jaycar RP3504] 2 large knobs (up to 50mm) to suit potentiometers VR1 & VR2 2 momentary pushbuttons (S1,S2) [eg, Jaycar SP0716] 1 1m length (or longer) of 4-5 core wire for controllers [eg, Jaycar WB1590] 2 100mm cable ties into the ICSP header, CON2. You can solder a header strip to the pads, but we’ve had success by simply resting the header in place and applying gentle force to ensure contact. A PICkit 3 or PICkit 4 can be used, or even a Snap programmer, if you can supply power to the board (which the Snap cannot do by default). The easiest way to do this is using the miniUSB socket, CON1. Use software like Microchip’s MPLAB X IPE to upload the 0810521B. HEX file to the chip. There’s nothing obvious to indicate that the chip is working, apart from using the software option to verify that the file has been transferred correctly. Wiring it up We built the two player controls into plastic UB5 Jiffy boxes, but you could also mount all the parts in a single enclosure to imitate the hardware of the arcade version of Pong. Fig.4 shows the two possible ways that these can be wired. The only difference is that if the controls are wired remotely, one end of the switch can be wired directly to the pot wiper to save having to run an extra wire back to CON4 or CON5. Fig.5 shows the cutting diagram that suits the parts we have used (listed in the parts list). You might need to modify the hole sizes if you are using different parts. Our photos show how we have connected everything, but the design is quite flexible and can be adapted to different parts and enclosures. We’ll describe how we finished our version. This is what our player controls look like, with a separate UB5 box for each controller. Internally it’s very simple, comprising of a 1kW 1kW potentiometer and momentary pushbutton. siliconchip.com.au Australia’s electronics magazine August 2021  51 Fig.4: wiring up the two player paddles/ controllers externally only requires a fourcore cable. Each paddle is wired the same, with Player 1 connecting to CON5 and Player 2 connecting to CON4. If everything is being mounted in the same enclosure, you can run the pot and switch wires back to CON4 & CON5 separately. Drill holes in a pair of UB5 Jiffy boxes according to Fig.5, noting that this should include a hole in the end of the box for the wire. The bottom of the box becomes the top when held in the hand. Doing it this way means that we aren’t trying to juggle wires leading from the lid to the cable entry while mounting the lid. Cut down the potentiometer shafts to suits the knobs you are using, and use a file to tidy up any rough corners or edges. This is most easily done with a hacksaw while holding the end of the potentiometer shaft (rather than the body) in a vice. This avoids straining the potentiometer mechanism. Fit the pushbutton switches and potentiometers to the Jiffy boxes. You can also fit your knobs at this stage. We whipped up some 3D printed knobs to give a bit more grip; the files are available along with the firmware download from the Silicon Chip website. Solder and heatshrink the wires as shown in Fig.3 and our photos. Run the connecting cable out through the hole and secure a cable tie around each cable to prevent it from being pulled off the terminals inside the box. Next, solder the other ends of the wires to their respective pads on CON4 and CON5. If you want to test the paddle operation, apply power and check for 3.75V on the middle lead of each potentiometer in the fully clockwise position, and 1.25V anti-clockwise. This voltage should drop to 0V when the button is pressed. There are normally-closed variants of this switch, so if you find that the action is reversed, you might have this other variant. 52  Silicon Chip Fig.5: we built our paddles into UB5 Jiffy boxes, with holes drilled in their bases as shown. There also needs to be a hole in the side of the box for the cable to pass through. Check that the size of your potentiometers, switches and wires match the hole sizes before drilling. To make the RCA connections for the TV, we simply cut a three-way RCA cable in half. Strip back a good amount of insulation and collect all the braids together. Attach these to pin 1 of CON3 (marked on the back with “G”). We put a short length of heatshrink tubing over the braids for extra protection. Then bare the internal wires by a small amount. The video plug (which will usually be yellow) should be connected to pin 2 of CON3, marked “V”. Pins 3 should go to the left audio lead (white, “L”) lead, and pin 4 should go to the right audio lead (red, “R”). Plug the RCA leads into the AV connections of a television and apply power to CON1. You should see the splash screen followed by the main game screen. Check that everything operates correctly. If so, the main PCB can be sealed up by enclosing it in a length of 20mm diameter heatshrink tubing. Ensure that the CON1 end does not overhang the connector. A 10cm length should ensure that the cables are secure and have some strain relief. Let’s play! At the start of the game, the ball will be in front of one player’s bat. Pressing the button on that controller will cause the ball to be served. Rotate the potentiometer to move the bats on the screen to keep the ball in play. If a player misses the ball, the other player wins a point. Once one player reaches eleven points, the game is over. The winning score will flash and a melody will play. Serving the ball starts SC a new game. This is how we built our Nano Pong setup. It uses a separate controller for each player, and has a composite video connector. Australia’s electronics magazine siliconchip.com.au