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Project by Tim Blythman PICkit Basic Power Breakout Board The PICkit BASIC programmer/debugger is compact, robust and works with most modern PIC chips. But it could really use the ability to provide power to the chip being programmed. This small PCB fixes that! Compact inline unit Makes 5V & 3.3V available from a USB-C cable Passes USB 2.0 data through, allowing a PICkit Basic to be connected A s we noted in our review of the PICkit Basic, starting on page 33 of this issue, it appears to be an updated version of the Snap programmer. Two of the most obvious improvements are the plastic case and a USB-C socket. Like the Snap, it does not offer high-voltage programming (HVP). HVP involves applying 9-13V to one of the microcontroller pins to enter its programming mode. This was the only way to program older chips like the PIC16F84. Later chips still support HVP, but many parts now support LVP (low-voltage programming). So you can use the PICkit Basic to program most PICs we have used in our projects over the last few years (plus some other non-PIC chips). The feature that’s most notably absent is the ability to provide ‘target power’, to run the chip being programmed (the target). Neither the Snap nor PICkit Basic can do this. Both these programmers still have a power pin. On the PICkit Basic, it is labelled as Vdd or Vtg. The programmer uses this to check for the presence of a suitable supply LED indicators for 5V & 3.3V presence Jumper wire for voltage selection voltage for the selected microcontroller before communicating with it. Programming parts out-of-circuit, like we do for the programmed chips we sell, can be done using a device like our TQFP Programming Adaptors (siliconchip.au/Article/15977). These adaptors have separate terminals that can be used to provide power, so it’s not necessary for the programmer to offer target power, although it is still convenient that you don’t need a separate power supply. For in-circuit programming, such as during a project’s development or for a part that cannot be easily programmed out-of-circuit, it might be possible to use the onboard power supply. However, that might not be feasible for projects powered by high voltages, such as from the mains. At the other extreme, some circuits use batteries or coin cells for power. For those cases, it makes sense to use an external power supply to avoid discharging the battery during development. Thus, it often makes sense to supply power via the programming header. So we need a way of injecting an appropriate voltage into the Vdd/Vtg pin. Most modern micros will happily work at 3.3V, including just about all the parts we use these days. So we have chosen that as one of the available voltages. The USB cable also means that 5V is available. The Breakout Board replicates a handy feature we added to our Snap programmer. The yellow wire extracts 5V or 3.3V from suitable points on the Snap and feeds it to the Vdd/Vtg pin of the Snap’s programming headers, supplying power to the connected circuit. 38 Silicon Chip Australia's electronics magazine In our PIC Programming Helper project, we noted that the Snap has pads that expose 5V and 3.3V rails. We modified our Snap to add a threeway header socket so that these rails can be easily accessed (June 2021; siliconchip.au/Article/14889). The third position of the header is not connected to anything and provides a location for a wire in cases where a voltage is not required. The photo at lower left shows our modified Snap, with one end of a jumper wire soldered to the Snap’s Vdd/Vtg pin. Of course, the PICkit Basic’s plastic case makes direct access to its PCB more difficult. It does have marked internal pads for 5V and 3.3V, but they are not as conveniently arranged as on the Snap. So the intent of the Power Adaptor is to provide these power breakouts without needing to modify the PICkit Basic. USB 2.0 The PICkit Basic uses only USB full-speed (USB 2.0) communications, although it is fitted with a USB-C connector. It makes sense to fit the Power Breakout with a USB-C plug and socket so that it can be connected inline, without needing an extra USB lead. USB-C brings along with it the delightful possibility of up to 48V being present if a USB-PD (power delivery) device is connected. To avoid that, we have designed the circuitry so that the USB-PD control lines are not carried through. The Power Breakout makes it appear that the upstream source is a USB 2.0 legacy 5V host by taking over the USB-PD control lines. This means that the Power Breakout siliconchip.com.au ◀ Fig.1: the circuit requests and offers a 5V legacy power source, turning a fully featured USB-C connection into a basic 5V USB-2.0 connection. The regulator and capacitors derive 3.3V from the USB 5V supply, while the LEDs and resistors act as power indicators. Fig.2: after soldering the USB socket and plug, the remaining parts are easy. has a second use: if you have a non-­ compliant device with a USB-C port that does not get 5V power when connected via a USB-C to USB-C cable, inserting the Power Breakout inline will fix this. Blocking USB-PD might seem a bit overly cautious, but we suspect that constructors might find other uses in situations where devices don’t play well with the newer features of USB-C and power delivery. Circuit details Fig.1 shows the circuit; CON1 is a USB-C receptacle. This variant sports 12 pins and breaks out power, USB 2.0 data, the CC (configuration channel) and SBU (sideband use) pins. CON2 is the corresponding plug, allowing the Power Adaptor to be fitted inline. It is a nine-pin part, providing access to power, USB 2.0 data, one SS (‘Superspeed’) pair and one CC pin. Only one CC pin is needed, since this is the point at which the cable orientation is detected in a USB-C cable arrangement. CON1’s CC pins are connected to separate 5.1kW resistors to ground, in the well-known legacy arrangement that marks this as a power sink. This means only 5V is requested from the power source. CON2’s CC pin implements the corresponding source arrangement, with a 56kW resistor connecting it to Vusb. Technically, arrangements like this are not strictly allowed by the USB-C specification. But since we are not interested in higher voltages, currents or USB data speeds, it is very unlikely to cause any problems. The remainder of the circuit is straightforward. REG1 and its two siliconchip.com.au bypass capacitors derive 3.3V from the nominal 5V Vusb rail. LED1 & LED2 are connected (with dropping resistors) to indicate that the two rails are present. CON3 is a three-way header that provides the same connections as our modified Snap. One position has 5V, one 3.3V, and the last is not connected. CON4 is a similar three-way header, but all pin positions are connected to ground, in case grounds are needed. We used a stackable header to intercept the connections for connecting the Power Breakout to the PICkit Basic. A jumper wire soldered to the appropriate pin allows power to be injected when the other end of the jumper wire is plugged into CON3. You can see this in our photos overleaf. If you just need 5V or 3.3V from a USB-C cable, you could assemble the Power Adaptor without CON2 and break out the requisite voltages from the pins of CON3 and CON4. Construction The main assembly is a PCB fitted with small surface-mounting parts and some fine-pitch USB connectors, as shown in the overlay diagram, Fig.2. The layout is fairly simple, so you might get by using the PCB silkscreen markings. The pin pitch is around 0.5mm on CON1, so you’ll need surface-mount soldering tools and gear, including a good magnifier. Flux paste and solder-­wicking braid are highly recommended, too. Add a thin layer of flux to the component pads on the PCB as you go. Start by soldering CON1; it shouldn’t be too hard to align correctly, since it has locating pins. Tack one of these in place and confirm that the leads are centred on the pads and that the part is flat against the PCB. Add flux to the pads and solder the pins. We’ve extended the pads slightly, so you should be able to touch the iron to the pads and see the solder flow onto the leads. Solder the remaining locating pins and check for solder bridges between the pins. CON2 should be similarly easy to locate. It has a slightly wider 0.65mm Parts List – PICkit Basic Power Breakout 1 double-sided 42 × 14mm PCB coded 18106251 1 MCP1700T-3302E/TT 3.3V LDO regulator, SOT-23 (REG1) 2 red SMD LEDs, M2012/0805 size (LED1, LED2) 2 1μF 50V X7R M2012/0805-size SMD MLCC capacitors 1 USB 2.0 type-C receptacle (CON1) [GCT USB4105-GF-A] 1 edge-mounting USB 2.0 type-C plug (CON2) 1-2 3-way 2.54mm/0.1in pitch socket headers (CON3, CON4; optional) 1 8-way stackable header strip 1 jumper wire or similar pluggable arrangement to suit CON3 1 4cm length of 20-25mm diameter clear heatshrink tubing a small amount of neutral-cure silicone sealant or thick glue Resistors (all SMD M2012/0805 size, ±1% ⅛W) 1 56kW 2 5.1kW 1 1kW 1 470W SC7512 Kit ($20 + P&P): includes all parts except the jumper wire and glue We built some cables like this, with five-way plug and socket headers, to provide a flexible connection between our Snap and boards that we have been developing. Soldering an extra wire allows the connection to be made to the Power Breakout. ◀ pin pitch and fewer pins, so you can folThis header simply low much the same has half a jumper wire soldered to the second process. pin of a stackable header. The Breakout Fit REG1 next, notBoard sits upstream of the programmer, ing the correct orienwhile the stackable header connects tation. Tack one lead, downstream, to the ICSP header. confirm the part is flat and square and then dry. Inspect it under a magnifier and fix solder the remaining any bridges or dry joints that you see. leads. The two LEDs Testing and completion should be aligned with their cathodes towards The PCB can now be tested by plugthe ‘K’ marks on the ging CON1 into a USB power source. PCB. The cathode is You should see LED1 and LED2 illuoften marked with a minate. If you have a multimeter, you small green dot, although should measure close to 5V or 3.3V at we have seen some parts the marked pins on CON3. You can use where the anode is any pin of CON4 or the USB connecmarked instead. tor shells for ground. The remaining parts You can also double-check that a are not polarised. The device can be connected downstream, capacitors will not be marked, but forto CON2. Any USB 2.0 device should tunately, only one value is used in this work just as well as if it had been conproject, on either side of REG1. Fit the nected directly with a USB-C cable. resistors next, matching the values to Any problems here point to a soldering the silkscreen. problem with CON1 or CON2. That completes the SMD parts, so Next, solder CON3 in place. We clean any excess flux using an approdon’t plan to use CON4, so we left that priate solvent and allow the PCB to off our prototype. Cut the heatshrink 40 Silicon Chip Australia's electronics magazine into a piece 1cm long and another piece 3cm long and shrink in place on either side of CON3. Make sure to cover CON3’s pins on the underside. That completes the unit. You can see from our photos that we used a stackable header to feed power into the downstream target. Half a jumper wire is used to provide a pluggable connection to CON3. Solder the jumper wire to the second position on the stackable header. This needs to align with the red Vdd/ Vtg markings on the PICkit BASIC, so a red jumper wire is preferred. Follow by adding some glue to the pins where they join the housing of the header. Run the glue up on the insulation of the jumper wire as well and allow the glue to cure. We recommend a fairly thick geltype glue or silicone sealant, since a thinner glue may flow into the header housing and gum it up. This happened to one of our prototypes. The glue has two purposes. Since the pins are only a press-fit into the housing, this will stop them from coming loose. The glue on the jumper wire will also offer some strain relief and prevent the wire breaking at the solder joint. You might also like to mark the heatshrink with the 5V and 3.3V markings if they aren’t otherwise visible. Using it Plug the header and PCB into the PICkit Basic as shown in the photos, aligning the wire with the Vdd/Vtg markings. Plug the jumper wire into the 5V or 3.3V position, depending on your needs. Most newer PIC micros can be programmed with a 3.3V supply, so that is a fairly safe option. Connect the header socket to the ICSP header of the target board and plug a USB-C cable into the PICkit Basic. The photos show another arrangement we tried. For a while now, we have used a short, flexible five-way lead to provide a degree of strain relief between our Snap programmer and target. It’s a bit less precarious than plugging the programmer directly into the PCB and having it balance vertically. By adding an extra orange power wire to the assembly, we can avoid the need to rig up the header socket. It’s frustrating when a loose or intermittent connection causes problems, so this eliminates a potential point SC of failure. siliconchip.com.au