What size smd resistors should I use for my PIC16 project? - Page 1
Forum section: electronics / general

author: Alexandros81
author_rank: Regular Contributor
timestamp: June 07, 2020, 05:35:52 pm
content: Hi!First time I am using smd resistors for my pic project.What smd size should I use for the resistors R5, R6, R8? (see the attached image).I am using PIC16*F19155.Thanks

author: Siwastaja
author_rank: Super Contributor
timestamp: June 07, 2020, 05:43:46 pm
content: I prefer 0402 for general-purpose passives but depends on your soldering skills and patience. 0603 is easier to handle, but going to 0805 is typically not warranted.So 0402 for small size solution, still hand-solderable, or 0603 for not as good but still acceptable size solution, easily hand-solderable.

author: cgroen
author_rank: Unknown
timestamp: June 07, 2020, 05:47:41 pm
content: From the looks of IC1, I would guess DIP (or perhaps SOIC) ?So why on earth struggle with 0402 in that caseUse 0603 as a minimum or 0805, as this is your first "SMD resistor" project. No need to go insane on space on the first project

author: cv007
author_rank: Frequent Contributor
timestamp: June 07, 2020, 06:30:34 pm
content: I would start large with first use, then work your way down when you get a good feel for the sizes. An 805 is doable for most anyone, but also nothing wrong with using something like a 1206, either. It would be better if you are surprised how easy it is, rather than end up being frustrated working with grains of salt. Remember you also have to get them out of some packaging, pick them up with a tweezers, etc. Machines can do that without any problem, human hands not as well.When it comes time to order parts, just order some of each 1206, 805, 603, 402. so you can actually compare the sizes. As time passes, your skills increase and you can move down in size if/when needed/wanted.

author: mariush
author_rank: Super Contributor
timestamp: June 07, 2020, 06:56:12 pm
content: 0805 is big enough to be solderable between two 0.1" spaced pads / holes on a circuit board or a prototyping board / stripboard.0603 is just a bit smaller, and it can still be soldered between two holes spaced 0.1" apartIt's still big enough to handle and solder without magnifying glasses, the power rating should still be 0.1w as the 0805 and most likely they'll be cheaper than 0805.As a beginner, I don't think you should go lower than 0603 and for this particular project there's no real need .. it's just 3 components on a board that's gonna big enough anyway due to the large soic/dip microcontroller.components wise ... is the 50k resistor really necessary? If you can go with 47k that would be a more common valueOr, if it has to be 50k, consider placing footprint for 2 resistors, so you could parallel two 100k resistors to get your 50k resistor.This way, you can either go with a single 50k, or you can parallel 2x100kalso put a decoupling capacitor right by the voltage and ground pins of the micro, a 0.01uF or 0.1uF 16v or higher ceramic would work well.

author: MarkF
author_rank: Super Contributor
timestamp: June 07, 2020, 08:01:24 pm
content: I use 1206 size for everything because I'm old, have poor eyes and shaky hands.But, 805 would be good for a beginner.One other comment on the ICSP header for programming:If I do not need any of the programming pins for I/O, I connect the ICSP directly to the chip.Also, I disable the MCLR function in the fuses so I do not need a pull-up on the Vpp/MCLR pin.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 07, 2020, 09:02:40 pm
content: Quite frankly, I feel the suggestion for 1206 is a really bad one, I have heard it before and totally disagree. Yes, I have seen beginners learn and have taught them. And I do see that 0402 is a bit scary. Hence the suggestion for 0603 which seems to be generally well managed. I accept 0805 may work better for some. Try 0603 first, though, you lose nothing.For prototyping, you need to think about the removal of the components as well. (Destructive removal is OK, the passives cost some $0.001 a pop.)0603 is kind of sweet spot because it's large enough to see, handle, align and solder pad-by-pad - say using a 1.6mm to 2.0mm chisel tip - yet it's small enough to bedesolderedusing the very same tip, heating both pads at once with some tin on the tip, then just getting rid of the component stuck in the tip with the brass tip cleaner thing (or the wet sponge, if you like that). 0805 still works the same way, just a bit slower, but I feel 1206 is too tedious to desolder. You typically need hot air, or two soldering irons to do it. Or need to change to a massive tip.And yes, in prototyping and learning, you will be changing the passive components all the time. (Installing gazillion of potentiometers isn't trendy anymore. Good riddance.)I witness beginners being impatient supplying enough heat (i.e., enoughtimewith their less-than-ideal iron tip contact areas) to solder large components. Low thermal mass of smaller components and the associated small copper pads plays in your advantage, as long as you stay away from components that are too small to see and position.Hand shaking is more a process issue, using big components to compensate offers only limited help. The key is to learn how to rest your hands properly. If you try to "freehand" it, even 1206 is too small, and the solder joints will be cold joints. Instead, rest your hand's weight on the table, grab the SMD part in the tweezers,and rest the tweezer tip (with the part) against the board. No shaking no matter how small the part is; location stays where it is. The iron tip must not be too small and pointy, a 2.0mm chisel type is a great all-arounder compromise tip for SMD work.The fact is that if you want to use all the modern components available - which you should, because it makes your life so much easier, many new components are very EASY to work with, think about modern regulators not being picky about the output capacitor type anymore; or in general, single ICs doing more things with fewer pins - you need to be able to deal with small packages. Now in 2020 no one deliberately limits theirselves to DIP packages anymore, we don't have to have forum fights about the resistance of getting into SMD soldering anymore; good riddance. But the same is starting to happen with SOIC and large passive packages, I think.The FACT is, soldering a 0603, or a 0.5mm pitch IC, is not MUCH more difficult than sodlering a 1206 or a DIP part. Just suck it and try!

author: MarkF
author_rank: Super Contributor
timestamp: June 07, 2020, 09:15:59 pm
content: When you get to be in your 60shave to wear glasses with bifocalshad 6 months of chemo-therapy where your fingers are now numb and shake...See if you can solder those 603 and 402 parts!There is NO amount of bracing that can do those small sizes.Just holding the tweezers is a challenge some days.Oh, to be young again...

author: westfw
author_rank: Super Contributor
timestamp: June 08, 2020, 01:33:11 am
content: I can barely solder 0805 parrts, but it's not clear whether 0603 would be more difficult or not (I'm getting better. But my eyesight is getting worse at about the same rate.)1206 ends up using about as much PCB space as a vertically mounted TH resistor, which is pretty depressing (but... only on one side of the PCB, which CAN be an advantage...)QuoteWell, the sweet point, price-wise, involves buying quite a lot of parts (whole reel from China, partial reel from eBay, at least 100 pieces from digikey, etc), and it's depressing to wind up with parts that are too tiny to use, or don't save you as much space on your PCB as you were hoping.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 08, 2020, 06:16:56 am
content: Quote from: MarkF on June 07, 2020, 09:15:59 pmBut did the OP say they are in their 60s and had 6 months of chemotherapy?I bet if this was the case, it would have been mentioned.Mostbeginners asking for advice getting into soldering are young and have no problem. Good advice mentions special cases that may apply, but does not make them the default.Quote from: westfw on June 08, 2020, 01:33:11 amTHIS! And 0805 passives next to a 1.27mm SOIC suck, you can't construct pull-ups/downs/feedback networks/etc. without escaping the tracks first. And if you have say a 14-pin SOIC with 10 passives around it, you are going to escape them far away. And I do recognize that many people are unwilling to route a track below an SMD passive. 0402s, on the other hand, fit perfectly right next to the IC, easing the PCB routing work. 0603 tends to be acceptable with only limited amount of trace escaping requred.For the same reason, 0201 would be an optimal component size around 0.5mm pitch components, like microcontroller power bypass caps or signal resistors, but 0201 is so tricky to handle that 0402 must suffice with some amount of track escaping necessary. But try to use 1206 with a 0.5mm pitch microcontoller for power decoupling, series termination (or LED current limiting) and pullups/downs and you are screaming in pain while routing the board.

author: Kjelt
author_rank: Super Contributor
timestamp: June 08, 2020, 06:20:43 am
content: Another advantage of 0603 is that the component value is still printed on the package. However this can be changed nowadays due to costs , not sure.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 08, 2020, 06:30:10 am
content: Quote from: westfw on June 08, 2020, 01:33:11 amIt's equally depressing to have a huge collection of 1206 parts that can't be used to modify or repair anything because no one uses that size, or build any future projects with them because you yourself started using sizes between 0402 to 0805 like everyone else, for the same reason everyone else.

author: Alexandros81
author_rank: Regular Contributor
timestamp: June 08, 2020, 12:57:06 pm
content: Quote from: cv007 on June 07, 2020, 06:30:34 pmYes. Good idea. I'll order some of each 1206, 0805, 0603, 0402 and compare the sizes!

author: Alexandros81
author_rank: Regular Contributor
timestamp: June 08, 2020, 01:02:03 pm
content: Quote from: MarkF on June 07, 2020, 08:01:24 pmThanks for the diagram. I'll keep that in mind. Where did you find it?

author: Alexandros81
author_rank: Regular Contributor
timestamp: June 08, 2020, 01:06:07 pm
content: Quote from: Siwastaja on June 07, 2020, 09:02:40 pmLots of good advice. Thanks

author: Ian.M
author_rank: Super Contributor
timestamp: June 08, 2020, 01:58:35 pm
content: The O.P's /MCLR reset circuit is no good, and the one MarkF posted out of the PICkit 2 User Guide isn't much better. See discussions here:https://www.microchip.com/forums/m692754.aspxand here:https://www.microchip.com/forums/m933052.aspxOn resistor size, first calculate the max possible dissipation and derate for high ambient temperature conditions. That will determine the smallest possible package size. Then consider whether the ability to run one or more tracks crossing under a SMD passive will reduce the number of PCB layers your design requires, which can bias you towards larger footprint passives. If none of the previous constraints apply, it comes down to the smallest footprint you can reliably and efficiently handle and solder. For early prototyping ease of handling and clarity of marking should take precedence over board area savings.Also, as ceramic capacitors are typically unmarked, while laying out hand-soldered prototypes, if possible, you should avoid having two different values/ratings of capacitor with the same package, colour and general appearance on the same PCB. If you cant avoid it, and you aren't reflowing the whole board, during assembly its a good idea to 'tag' each capacitor with a colored marker pen after its soldered, using a different color pen for each value/type and only have one value/type at a time out on your bench, not getting the next value/type out until all the previous value have been placed, soldered and 'tagged'. If you are doing whole board reflow with manually placed components you need to be dammed careful. e.g. stick with only having one value/type out at a time, and photograph the board after all the caps of the same value/type have been placed, and log the photo number with the reel/strip label details before switching to the next value/type as a record of what's been placed where.Ideally, a decent DMM that can read capacitance (or a DMM and a separate LCR meter), with SMD tweezer probes should be kept on the bench for confirming the value of passives and polarity of two pin diodes. Check the first part of any value/type before you place it to catch supply chain mistakes and mixups in your prototyping component storage.

author: Alexandros81
author_rank: Regular Contributor
timestamp: June 08, 2020, 01:59:46 pm
content: Well, thanks for your advice and quick response.Regarding the decoupling capacitor of the PIC I am using a 0.1uF. What smd size should I use?Also in my project I am using an eeprom memory 24AA64F-I/SN: Industrial Temperature, 1.7V, SOIC package. I am using a decoupling cap of 0.1uF.What smd size should I use?Also in my project I am using mcp9808 MSOP package. I am using a decoupling cap of 0.1uF.What smd size should I use?Because the eeprom memory and mcp9808 are interfaced via I2C I have a 10K pull-up resistrors for the SCL, SDA lines.What value of pull-up for the ALERT pin of the MCP9808? What smd size for the pull-ups?I have also an I2C LCD that requires 4.7k pull-ups.So what value should I use for the SCL/SDA lines for all 3 ICs ?(eeprom memory. mcp9808 and LCD)Another question I have is regarding soldering the passive components.I was thinking to let JLCPCB to solder the passive components along with MCP9808.I don't know how much it would cost though to do that. Thinking of ordering 5-10 pcbs.If not going for PCBA with JLCPCB what other methods could I use to solder the passives along with the ICs?

author: Siwastaja
author_rank: Super Contributor
timestamp: June 08, 2020, 02:59:06 pm
content: The power decoupling caps: note that the primary function of these parts is to providelow-inductance powerto the IC. The parameter to be optimized is inductance, smaller better; capacitance is somewhat irrelevant. Smallest case size gets you smallest inductance, both in the capacitor itself, and by the fact that you can place it closer. This is good for power integrity (reliability), and EMC. Of course, if you put a 0201 capacitor and place it 100 micrometers from the PIC power pins, it's overkill engineering. A 0805 capacitor 5mm away works equally well for such simple IC. But it's good to understand what the idea is to be able to expand it later when you have more demanding circuits.Ceramic capacitor capacitance ratings are like "scams", when they sound too good to be true, they aren't; the available true capacitance drops under DC voltage bias. The smaller the case, the bigger the effect (obviously, a larger capacitor can store more energy; the smallest are "too good to be true"). But the unimportance of the exact capacitance plays the role here: if you decouple a 5V DC power source using 100nF capacitors of sizes 0402, 0603, or 0805, respectively, the effective capacitances might be something like 50nF, 60nF and 70nF, respectively (just ballparking something up as an example). All work for the purpose; but the 0402 provides lowest inductance itself, and can be placed closest to the power pin.The requirements for the decoupling caps can change if you have special things like higher voltages, or large capacitive loads the device needs to drive. An MOSFET gate driver would be a good example; it works with, say, Vcc=12V, much more than your typical 1..5V logic levels. In addition, it quickly charges a MOSFET gate which might be 100nF equivalent. This charge needs to come from somewhere, quickly. So you would, as per example schematic, put there a 1.0uF capacitor instead of your bog-standard 100nF. Here you can expect there is a good reason to have that much capacitance, so you wouldn't pick the smallest and cheapest 0402 part because it could actually lose quite a bit of rated capacitance under such high voltage. So that could be a 0603 or even a 0805 part. Of course, instead of such handwaving, it is a better idea to look at the actual specs to verify these assumptions.For all the resistors, refer to Ian's reply; actually do calculate the power dissipation!I recommend you learn to hand-solder the components. It's simple: apply a bit of solder to one of the pads. Grab the component with tweezers, melt the solder again, while sticking the component in place with tweezers. Later solder the remaining pad. Home-brew reflow is the next step later, it will save time and enable use of some additional component cases (those with no apparent legs, but pads under the case), in more complex designs.Larger parts may be easier to handle, but smaller parts make PCB layout work easier; you can actually afford more placeholder/dead space between subcircuits, for example; you don't need to escape traces far away from ICs, routing traces is much simpler as you don't have to avoid bunches of components. For example, if a 1.27mm pitch SOIC has power and ground pins next to each other, a 0402 capacitor fitsrightnext to the pins, and the tracks leaving other pins around it do not need to take any detours. This also makes the circuiteasier to follow by eye, with smaller components, the layout becomes closer to the schematic, components can be placed in the logical order because they fit better.JLCPCB has this super-cheap assembly service now if you use parts in their parts library. Still, you can't avoid soldering, so it pays back to get a bit better on that. Despite, the process oforderingassembly is a learning process itself. I wouldn't invest time and money to that initially, only after you start feeling like you are bored soldering too much and want to pay someone to take part of the burden off.

author: Alexandros81
author_rank: Regular Contributor
timestamp: June 08, 2020, 03:16:32 pm
content: Quote from: Siwastaja on June 08, 2020, 02:59:06 pmHi. Regarding 'low-inductance power to the IC', where do I learn more?

author: Siwastaja
author_rank: Super Contributor
timestamp: June 08, 2020, 03:24:50 pm
content: Google for "power decoupling capacitor" or similar. A lot of appnotes and discussion available. Possibly too much.

author: Alexandros81
author_rank: Regular Contributor
timestamp: June 09, 2020, 01:32:01 pm
content: Hi.Should my ICSP circuit look like the one attached? my VDD is 3.3VAlso for the resistors R1, R2, R2 how do I calculate the max possible dissipation and derate for high ambient temperature condition?

author: Ian.M
author_rank: Super Contributor
timestamp: June 09, 2020, 03:01:00 pm
content: ICSP & /MCLR looks Ok.P=V2/RWith a 3.3V supply and no circuits that can boost the voltage, max V2is 10.9, so any R>250 ohms will dissipate less than 31.25mW. To derate components with temperature, reduce their max permissible dissipation (power) linearly from their nominal rating at usually 25 deg C, to zero at their max operating temperature (see datasheet). However you can bet that any resistor is going to have a max temperature >125 deg C, so if you don't dissipate more than 31.25mW in a 62.5mW rated resistor its certainly good up to 75 deg C.If you are determined to use 0201 'flysh!t' components, or are designing a furnace or engine control board so expect elevated ambient temperatures, you would need to do these calculations properly, otherwise with a 3.3V supply you can bet that any generic 0603 or larger >250R resistor doesn't need checking, so R2 and R3 are certainly OK.R1 only has to handle the transient dissipation of discharging C1, as R2 and R3 limit the current to it from both Vdd and Vpp during programming so it cant dissipate more than about 20uW if you hold the reset button down (assuming max Vpp is 3.3V. If you are using a legacy PIC that needs 13V Vpp it might go as high as 2.7mW). Even if you only assume a peak surge rating of 5x, a generic 0603 will be fine here.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: June 09, 2020, 03:29:43 pm
content: With no other constraints I'd say 0805 is the optimum passive size. 0603, while being perfectly hand-solderable, is significantly more fiddly, more so if vision is an issue.Also some 0603 manufacturers are no longer marking resistors. AFAIK no 0402 R's are ever marked.

author: NorthGuy
author_rank: Super Contributor
timestamp: June 09, 2020, 07:45:36 pm
content: I would go with 0805 if you only want to solder manually, 0603 if you want to do some sort of production, or smaller if you want to save space.The MCLR circuit is bad - it may not work at all. You don't really need any circuit. If you're worrying about noise, you can use a pull-up - 10k is typical - cannot make it much stronger than that.

author: Ian.M
author_rank: Super Contributor
timestamp: June 09, 2020, 08:03:37 pm
content: @NorthGuy,Why do you think the /MCLR reset circuit is bad? Its got a 10K/100nF CR delay with a switch to pull it low (to ~1% of Vdd, well within the logic '0' threshold) and a 10K isolation resistor to allow the ICSP Vpp signal to override it, without the 100nF slowing down the edges. Arguably, the isolation and pullup resistors resistor could be reduced to 5K1 if one needed a stiffer pullup for a high EMI environment.

author: TK
author_rank: Super Contributor
timestamp: June 09, 2020, 08:34:29 pm
content: I use 0603 and Kicad 0603 hand soldering footprint for both resistors and capacitors. The 0603 hand soldering footprint gives you some extra pad space to work with the soldering iron.

author: NorthGuy
author_rank: Super Contributor
timestamp: June 09, 2020, 09:13:26 pm
content: Quote from: Ian.M on June 09, 2020, 08:03:37 pmThe isolating resistor is in the wrong place - when you use the capacitor, the resistor is needed to isolate the capacitor from the programmer (as in MarkF diagram). The programmer should be connected to MCLR with little or no series resistor. This could be achieved if you disconnect the programmer from where it is now and connect it to the MCLR pin directly.Even if you do the change, the 470R resistor may be too small, and it will increase rise times for Vpp which you do not want. That's probably why OP has a switch there. If you drop the switch, you need to increase the resistor value. The bigger the resistor the less the stabilizing benefit of the capacitor.The purpose of the capacitor is not clear. If you use it to delay PIC start on power-up (as might be recommended for dsPIC33), this is not necessary for this PIC16, and if you want it, just do it in software. If you want to prevent resets, it's easier to simply disable resets by fuses (aka config bits). If you're afraid that the voltage on MCLR goes up and triggers HVP programming mode, the capacitor circuit is not much better than a simple pull-up to Vdd.A 10k pull up (or nothing at all because the chip has an internal pull-up) is the best in the vast majority of the situations.

author: Ian.M
author_rank: Super Contributor
timestamp: June 09, 2020, 09:23:05 pm
content: Errr . . . didn't you spot his revised circuit in reply #20?Quote from: Alexandros81 on June 09, 2020, 01:32:01 pmAs I said earlier, it looks fine to me and is appropriate if you need a manual reset button and want better reset timing control than can be got by simply configuring PWRT and BOR.OTOH I agree his original circuit, based on the one in the Microchip PICkit 2 User Guide is a reeking pile of sh!t.

author: NorthGuy
author_rank: Super Contributor
timestamp: June 10, 2020, 01:48:33 am
content: Quote from: Ian.M on June 09, 2020, 09:23:05 pmSorry. Totally my fault. This circuit is good.If the OP insists on his original idea, I would suggest making R3 smaller - about 2.2k.I personally would remove R1 (short), R3 (short), and C1 (open).

author: Alexandros81
author_rank: Regular Contributor
timestamp: June 10, 2020, 11:55:14 am
content: Quote from: Ian.M on June 09, 2020, 03:01:00 pmBy the way I was trying to analyze your circuit in post #9 inhttps://www.microchip.com/forums/m692754.aspx.I think is important to me to understand it better. What is the internal digram of /MCLR pin?How do I calculate the Voltage at points A, B? Is point A a voltage divider?How do I calculate I1, I2, I3?By doing the above calculations it would be easy then to perform the power dissipation calculations.

author: Ian.M
author_rank: Super Contributor
timestamp: June 10, 2020, 12:59:13 pm
content: For all FLASH PICs, when /MCLR is configured as an input without internal pullup, its Hi-Z and draws less than +/-5uA (or +/-1uA for reasonably modern PICs) while its voltage is between Vdd and Vss (ground). The pullup is always on when external /MCLR is enabled. The pullup current isn't well defined and you'll need to root around in individual datasheets for the max /MCLR pullup current if given or the max. permissible value for the /MCLR isolation resister to still get a valid low level for /MCLR reset.FLASH PICs that use HV programming may draw a bit more /MCLR input current when its at Vpp, typically up to 1 mA, but as Vpp on the ICSP connector should connect direct to the /MCLR pin, this has little effect on the preferred external /MCLR circuit. /MCLR doesn't supply power to the internal programming circuit as there's an internal charge pump supplied from Vdd, that generates the programming voltage for the FLASH memory array, enabled by /MCLR being at Vpp. Old CMOS PICs (PIC16C... PIC18C... OTP and windowed CERDIP) don't have the internal charge pump so /MCLR must supply all the EPROM programming current so your programmer must be capable of supplying >50mA to /MCLR while its at Vpp for successful programming.That should give you enough info to analyse the suggested reset circuit.N.B. the only critical resistor value in the /MCLR circuit is usually the /MCLR isolation resistor, as the voltage drop across it due to the /MCLR internal pullup current must be small enough when the switch is closed for a valid logic low level on the /MCLR pin to put the PIC in reset, but it is inadvisable to use an isolation resistor of less than 5K as some Microchip development programmers aren't rated to drive loads with less than 5K resistance on Vpp. Production programmers *may* have more Vpp drive capability so may well permit a lower isolation resistor (which is where the 470R suggestion from Microchip probably originally comes from) but unless you know the specs of the programmer on your production line, and are never going to out-source assembly and programming, it would be dumb to rely on that.

author: Alexandros81
author_rank: Regular Contributor
timestamp: June 10, 2020, 03:08:30 pm
content: Quote from: Ian.M on June 10, 2020, 12:59:13 pmSo in my case I am enabling the external /MCLR.Is the max /MCLR weak pull-up current the one I found in the device's datasheet? See attached files.

author: Ian.M
author_rank: Super Contributor
timestamp: June 10, 2020, 03:58:50 pm
content: There's nothing in that datasheet to suggest otherwise, and the /MCLR low threshold is 0.2 x Vdd, so that 200uA max pullup current sets a hard upper limit for the isolation resistor of 3.3K, assuming an external reset circuit that pulls right down to ground. You need some margin, and have 100R in series with the switch, so try 1.5K (for a worst case 0.1 x Vdd /MCLR low level), which should be OK to drive for any programmer that's modern enough to support the device. It breaches the >5K Vpp load requirement for legacy FLASH PIC development programmers, but as they don't support your PIC, that's irrelevant. You can leave the pullup resistor to the reset timing capacitor at 10K , though if you decrease it, don't go under 4K7 to keep the worst case current dumped from Vpp into the Vdd rail during HV programming under 1mA.As a matter of interest, why not take your actual PIC, ground /MCLR through a 300uA range on your DMM, and check if the actual current is anywhere near what you'd expect from the fig. 40-41 graph?

author: NorthGuy
author_rank: Super Contributor
timestamp: June 10, 2020, 04:06:51 pm
content: Quote from: Alexandros81 on June 10, 2020, 03:08:30 pmYes. 200 uA @ 3V suggests roughly 15k impedance for the weak pull-up - that's the strongest you can expect. When you press the reset button, you'll be pulling down with your R3 = 10k against this. Thus the highest voltage level you'll get on MCLR will be roughly 5V*10k/(10k+15k) = 2V. The D305 parameter, just above the portion of the table you have posted, suggests than you need to pull below 1V (0.2 Vdd) to reset the PIC. So, with your selection of the R3 resistor, the PIC is not guaranteed to reset when you press your reset button (although most of the time it will because weak internal pull-ups will be not as strong as the maximum specs and the real input voltage levels are likely to be more relaxed). To meet the specs, you need to make R3 stronger.

author: Ian.M
author_rank: Super Contributor
timestamp: June 10, 2020, 04:13:58 pm
content: Quote from: NorthGuy on June 10, 2020, 04:06:51 pmErrr..... it appears to be a 3.3V (nom.) PIC so 0.2 x Vdd is 0.66V not 1V. Otherwise we seem to be in agreement.

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 10, 2020, 04:29:54 pm
content: You're not saying what kind of other packages you use on your board, including the PIC chip.But if you're only using 1.27mm pitch packages, for instance, I don't really see a point going below 0805 for passives. Just pick whatever is consistent with your board design and density. IMHO.Of course the other factor is power dissipation.

author: NorthGuy
author_rank: Super Contributor
timestamp: June 10, 2020, 05:34:32 pm
content: Quote from: Ian.M on June 10, 2020, 04:13:58 pmThe specs refer to the real Vdd - the one which is applied to the PIC. Thus the levels change depending on the Vdd you use.

author: Ian.M
author_rank: Super Contributor
timestamp: June 10, 2020, 05:56:28 pm
content: My mistake - somehow I'd got the idea the O.P. was operating at 3.3V with a 3.3V (3.6V max) PIC. If he's actually running at 5V Vdd the 1V threshold in your post is correct, and the max pullup current from fig. 40-41 is 300uA @5V Vdd, which (fortunately) also leads to an isolation resistor upper limit of 3.3K, so fortunately the rest of my reply #32 remains valid.

author: TomS_
author_rank: Frequent Contributor
timestamp: June 11, 2020, 07:04:26 am
content: If youre still unsure, jump onto ebay or amazon or aliexpress or whoever and search for SMD soldering practice kits of various sizes and give different size components a go.This is what I did, and I decided to start my SMD soldering adventures with 0805 components, and I still use them today, some years later.Why?Because Im not building millions of units where I need to cost optimise as much as possible (components and board size).Because while there is no need to use unneccessarily large components, there is also no need to use unneccessarily small components.Because people sometimes dont like change.Might I go to 0603 or smaller some time in the future? Sure - 0805 is a piece of cake, and I suspect that with the skill and experience I have gained over the years that 0603 will now also be fine.

author: Alexandros81
author_rank: Regular Contributor
timestamp: June 15, 2020, 04:20:28 pm
content: Hi.I have three switches in my circuit.Can you check that my pull-up resistors should be 10k? See attached circuit.My Vdd is 3V. The board is running at 3V.Also I am going to be using leds. Four to be exact. Using the 1206 package.I am ordering from ebay and it says the have nomimal current 20mA. So for an active hi configuration I should be using 150Ω resistors?See attached circuit.Thanks

author: CatalinaWOW
author_rank: Super Contributor
timestamp: June 15, 2020, 05:58:14 pm
content: I hesitate to give any advice on component size. There are many factors.Routing difficulty: There are advantages to both small and large sizes, depending on your application and the capabilities of your board fab of choice. Particularly if you are making your own boards larger sizes are an advantage.Physical skills: Young people in general can deal with smaller sizes better, though not everyone has good vision and motor skills in their youth. And we all hopefully will get older, and those of us who have achieved that can testify that things get worse with age. Slowly for some, rapidly for others but the sign of the slope is the same for everyone. Will you want to lose access to your earlier work as you age?Design goals: Some projects are space sensitive, most are not. As you grow through the hobby you may find it useful to use challengingly small sizes to achieve a particular goal, while still preferring larger sizes for other projects. Some projects need to dissipate power and need larger sizes.Budget: If you can afford high quality stereo microscopes, soldering stations and the like the limits on small size go down. If you must deal on a shoestring using salvaged components it may go differently.

author: phil from seattle
author_rank: Super Contributor
timestamp: June 16, 2020, 05:54:14 am
content: Quote from: Alexandros81 on June 15, 2020, 04:20:28 pmYou will need to know the Vf of the LEDs - different colors have different values. You also need to read up on how to calculate LED current limiting resistors (uses Vf). Try this -https://www.allaboutcircuits.com/tools/led-resistor-calculator/