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Desining a new kit, thoughts from the blue desk

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blueroomelectronics

Well-Known Member
In the following series of notes I'll outline the state of a new kit called Ladybug. You can see first hand reports on why a designer makes particular choices and changes as a project progresses from concept to completion.


Of course feel free to make notes of your own.


Part 1 (TQFP or not TQFP)

Although the Ladybug an Ethernet based PIC project has already been through dozens of different design phases I’ll begin this thread with the design in its current state.

For this kit I wanted an easy to assemble (through hole) design and the first prototypes used the ENC28J60 and a PIC18F4620. This popular combination is the basis of several Ethernet controllers but I’m leaning towards something a little better…

Here’s a brief description of Ladybug

MPU
PIC18F67J60, 128K RAM, 3.8K SRAM, onboard Ethernet 39 I/O pins

Peripherals
Two 8 bit I/O connectors
Ethernet
RS485
RS232
SD Card socket

So

I looked at the PIC18F67J60, a powerful single IC that contains both the microcontroller and an Ethernet controller, more memory, more I/O…

BUT

The 18F67J20 is only available in a TQFP package (from 64 to 100pins). A TQFP unlike a PDIP does not have self-aligning features. You must get it positioned exactly right (masking tape helps) before soldering. You also need flux and a fine tipped soldering iron OR you can blob down the solder and wick it up with solder wick.

No matter how you do it’s still open to errors but lots of folks said they have no issues with TQFP soldering…

Solution(s)
1. partially assembled PCB with the PIC pre soldered
2. have Schmartboard make the PCB (the TQFP fits into grooves on the PCB)
3. let the user solder down the TQFP

I’m going with 1 & 3, this will reduce the PCB price and the kit price.

Part 2 coming soon, oh were oh where does the I/O go…


* Actually my desk is well worn walnut veneer
 

crutschow

Well-Known Member
Most Helpful Member
The availability of through-hole packages for new semiconductors will become increasingly rare, so it's probably time to start introducing surface mount packages into the world of electronic kits.

I don't know what the price premium is, but option 2 would seem to be a good way to locate the part for ease of soldering.

But perhaps I'm underestimating the ability of hobbyists to align the part with masking tape and soldering it. In either case (2 or 3) much emphasis must be made in the assembly instructions on using a low-power (or temperature controlled) fine tip iron, small gauge solder, and separate flux application, for proper soldering of surface-mount devices, especially those with many fine pitched leads. Trying to solder such a device with a normal soldering iron and standard, large gauge solder will likely result in many solder bridges, a possible overheated chip, and a frustrated hobbyist.

Including a practice board and dummy (perhaps some rejected) chips with the kit to learn how to solder such a device may also be helpful.
 

blueroomelectronics

Well-Known Member
Well the Schmartboard PCBs would be about double the cost. This would add about $10 to the retail cost of the kits. The Schmartboard system does work well though.
 

Hero999

Banned
I love some SMT packages, especially the SOIC-8/16 and 805 packages.

The think I like most is not having to drill loads of holes.

Unfortunately I do have loads of through hole components and virtually no SMT components in stock.
 

Mr RB

Well-Known Member
If you go with 1. and get the PIC SMD soldered for you it requires panelisation etc to suit the SMD placement machines. If you go to that level of hassle it's worth just making the entire PCB as SMD and selling it as a finished PCB with all the parts pre-soldered (or maybe make the user just do the last couple of through-hole connectors etc).

If you make the user solder a TQFP I can see it's gonna bite you in terms of support and people wrecking the PIC (the most expensive part of the product) and failures due to soldering shorts etc.

I know people here will say "sure I can solder TQFP easily" but TQFP soldering requires a lot more expertise and experience than a SOIC which most beginners can handle with decent instructions.
 

blueroomelectronics

Well-Known Member
I feel the same way, my kits all currently use through hole parts and so far nobody has made a dogs breakfast out of them.

The Schmartboard PCBs are about a foolproof a it gets for TQFP. You do need a fine tip soldering iron, flux and some scotch tape. You just slide the solder onto the TQFP (the PCB is already pre-tinned with enough solder)

It'll be one of the last decisions before the kits go to market.

My current quandary is to swap out the SDCard socket for a large SPI EEPROM (128k x 8) like the 25AA1024 or similar. This would also make the PCB slightly narrower.

New feature, adding a socket for an XBee or XBee Pro module.
 

Mr RB

Well-Known Member
...
My current quandary is to swap out the SDCard socket for a large SPI EEPROM (128k x 8) like the 25AA1024 or similar. This would also make the PCB slightly narrower.
...

I think that would be a massive reduction in the possible applications and versatility, and a big reduction in the value of the product. Being able to record onto cards (and/or receive data on a card) is a HUGE value in something that will be used for ethernet/comms etc.

Since a card connector is cheaper than a 24xx1024 eeprom anyway I just think it would be crazy.
 

BeeBop

Active Member
Why not mount the PIC on a carrier board which slides onto double row headers on your main board? That way the TQFP could be soldered in place and the rest of the kit could be assembled by the user.
I agree with MrRB; keep the SD socket. :)
 

blueroomelectronics

Well-Known Member
It's the cost of custom headers and larger dual PCB layout that makes me want to avoid that solution. Wish the part was available in PLCC form. The Schmartboard is looking better and better, also the kits is not for first timers so I would hope the more experienced PIC users / builders would be most interested.

Ok, can't beat a solid argument I'll keep the SD card (and space permitting a EEPROM socket option)

5V switch mode wall wart with a 3.3V linear regulator (5V is for relay power on the I/O connectors which will be pin compatible with the EasyPIC series devices)
Or
9 - 15VDC unregulated (also for relay power on optional octal relay board, and/or other future add ons such a octal opto input board)

PCB layout will be the following connectors along the same side

2.5mm PWR Jack, RS485 (3 screw terminals), RS232 DCE (DE9M), Ethernet RJ45, SD card

Internal connectors / misc will be

Two 8 bit I/O expansion connectors (10pin), XBee, ICD connector, Reset button

As of today the current I/O and pincount (we have 39 on the 18F67J60)
I/O required plus description

16 Two 8 bit I/O conectors (2x5 jack or 10 pin inline undecided)
2 ICD & SD RW & Insert (jumper selectable) PGC & PGD are 5V tolerent so will work with Junebug / Inchworm / PICkit2 / ICD2 etc...
2 Ethernet LEDs
4 SDCard (SPI)
4 RS232 EUSART with CTS / RTS
2 RS485 Software UART
4 XBee Software UART with CTS / RTS
4 25MHz & 32,768Hz crystals

Total = 38

One pin left... what to do? LED, iButton, analog input, interrupt input, CCPx I/O, 433MHz RF TX or RX?

A typical PCI18F67J60 design looks like this one from Olimex (pdf attachment)
 

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DSGarcia

New Member
If there is room on your board, could you place SMD pads on your board along with a couple of rows of holes for headers? You could then give the user two options:

1) Solder the SMD onto the board
2) Solder headers onto the board and purchase an optional daughter board that has the processor pre-soldered (and the desired lines running to the mating connector) which will plug into the headers.

Dale
 

blueroomelectronics

Well-Known Member
Hmm, not bad, I can make the PCB any size of course smaller is cheaper. But might be worth it for the faint of heart as an option. Something like this?
8531-PCB-LQFP64.jpg
 

fernando_g

New Member
"5V switch mode wall wart with a 3.3V linear regulator (5V is for relay power on the I/O connectors which will be pin compatible with the EasyPIC series devices)
Or
9 - 15VDC unregulated (also for relay power on optional octal relay board, and/or other future add ons such a octal opto input board)"

If you plan to sell worldwide, a switch mode would be best, since they are (for the most part) universal voltage input. You would still have to provide for the different plug configurations, though. But I'm sure far-east manufacturers have products that could support the requirements.
 

DSGarcia

New Member
Hmm, not bad, I can make the PCB any size of course smaller is cheaper. But might be worth it for the faint of heart as an option. Something like this?
That's exactly what I was talking about. You would also have the same layout on your motherboard for those willing to solder the SMD directly on the motherboard and then they would ignore the headers.
Dale
 

BeeBop

Active Member
That's exactly what I was talking about. You would also have the same layout on your motherboard for those willing to solder the SMD directly on the motherboard and then they would ignore the headers.
Dale
That sounds like a great idea! :)
 

blueroomelectronics

Well-Known Member
I'll be putting the adapter on the PCB, I'm really on the fence about the SD card. Pretty soon standard SD will be disappearing in favor of SDHC & SDXC (both use a 4bit mode)
 

blueroomelectronics

Well-Known Member
Well the SD card will be sacrificed in favor of an 8pin DIP/SOIC SPI EEPROM / Flash. These inexspensive ICs should be plenty big for an embedded web server upto 8M bytes can be had for less than $5.
An addition will be 4 type C relays NO/C/NC and two opto coupled inputs on RB0,1
 

blueroomelectronics

Well-Known Member
On the fence.

One of Ladybug's functions (at least in my opinion) will be a HVAC / furnace / AC controller with Ethernet

5V Switchmode DC supply LM2575 (Relays / 3.3V regulator)
3.3V linear supply

Standard peripherals (100% sure)
One Ethernet jack with magnetics
Four Panasonic JS1-5V Relays
Two opto isolated inputs HCPL-2630
SPI EEPROM / Flash upto 8Mb
At least one 8 pin I/O port EasyPIC compatable

Likely to be there (80%)
DE9-Male DCE RS232 with HW/SW UART (jumper selectable)
RS485 SW UART

On the fence (50% either way)
XBee / XBee Pro socket with HW/SW UART (jumper selectable)

Do we need them?
433MHz RF receiver (handy for digital thermometers)?
NO/C/NC or NO/C relay outputs?
32,768Hz watch crystal?


Gone
SD card socket.
iButton connector (just use an I/O port if you need it)
Temperature sensors (also can be added via DIY I/O expansion)

As always your opinions on this would be appreciated.
 
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