PCB suggestions

[Pilkerton]

The BOM <-> schematic mismatch tripped me up when I assembled a v3.3a board from the Mouser BOM:

Current BOM       Schematic                                   Customer Part # on BOM should change to
-------------------------------------------------------------------------------------------------------
U2 Socket           U1                                              U1 Socket
IC2                     U3                                              U3
IC1                     U2                                              U2
U2                      U1                                              U1
RN52                  U4                                              U4
LED1-4               BT1, CAN1, CAN2, POWER            BT1, CAN1, CAN2, POWER
-------------------------------------------------------------------------------------------------------

Recommendations for future PCB revs:
• 'IC3' should ideally be 'U5', but this would require schematic and silkscreen changes
• Top side silkscreen should somehow indicate polarity for all LEDs
• Square pad for Atmega U1 pin 1
• Bottom side silkscreen alignment marks near RN52 pins 1 and 44 should be moved out slightly to help 'frame' the component during placement; currently, these marks are obscured when the part is in place.

Other thoughts:
• Is the Mini USB connector really necessary? It's exclusively for RN52 firmware updates, but as of RN52 firmware v1.16, it defaults to updates over the UART pins. Also I was unable to get the update utility to communicate with the RN52 in DFU mode over USB using Windows 7 64-bit.

• Why not switch to an SMT package for the Atmega? It seemed like I spent a disproportionate amount of time soldering that single DIP package.

[Karlis]

Pilkerton wrote

The BOM <-> schematic mismatch tripped me up when I assembled a v3.3a board from the Mouser BOM:

Current BOM       Schematic                                   Customer Part # on BOM should change to
-------------------------------------------------------------------------------------------------------
U2 Socket           U1                                              U1 Socket
IC2                     U3                                              U3
IC1                     U2                                              U2
U2                      U1                                              U1
RN52                  U4                                              U4
LED1-4               BT1, CAN1, CAN2, POWER            BT1, CAN1, CAN2, POWER
-------------------------------------------------------------------------------------------------------

Recommendations for future PCB revs:
• 'IC3' should ideally be 'U5', but this would require schematic and silkscreen changes
• Top side silkscreen should somehow indicate polarity for all LEDs
• Square pad for Atmega U1 pin 1
• Bottom side silkscreen alignment marks near RN52 pins 1 and 44 should be moved out slightly to help 'frame' the component during placement; currently, these marks are obscured when the part is in place.

I'll let Seth comment on this. He's our expert on hardware design...

Other thoughts:
• Is the Mini USB connector really necessary? It's exclusively for RN52 firmware updates, but as of RN52 firmware v1.16, it defaults to updates over the UART pins.

You are right. Starting FW version 1.16 USB connector is not mandatory as the updates can be done over UART. However, we might keep that USB connector and connect it to an onboard FTDI chip thus making code updates easier over mini USB cable.

 Also I was unable to get the update utility to communicate with the RN52 in DFU mode over USB using Windows 7 64-bit.

Did you flip the switch on the PCB from "Normal" to "DFU"?

• Why not switch to an SMT package for the Atmega? It seemed like I spent a disproportionate amount of time soldering that single DIP package.

We are currently exploring this approach. Initially we followed the "don't build anything you can't take apart" principle. As in just in case we need to replace the chip, or move it to an Arduino Uno board for re-programming or whatnot. Helps a lot during initial R&D phase. Now, I guess, we are at a point where ATMEGA328 handling is stable enough for us to switch to SMT type of ATMEGA chip to save some more space on PCB. This, of course, means more work for Seth. :)

[Seth]

I fixed the ref designators; I didn't notice they had changed, but none of the footprints are the same so nothing could get mixed up anyway.

The LED's already have silkscreen polarity indicators.

ATmega *could* have a square pin 1; The package in Eagle didn't have it and I'm not going to change it. The silkscreen indicates the direction.

Same thing with the RN52 footprint; that's how the library footprint came, I'm not going to change it.

The mini USB connector is only for RN52 firmware updates from 1.10 to 1.16. Karlis has purchased RN52 modules in the past with 1.10 (apparently old stock) which needed updating. You don't need to populate it if you don't want to (along with DFU switch, D3, R11, R12, C9).

Switching to SMD Atmega might be a pita. The chips won't come with a bootloader, so you will need a special socket just for that. I have one, but it's a $20 cable that someone will have to buy for a "one time use". I believe you can buy chips with bootloaders already programmed, but they are easily 2-3x more expensive. I'm not saying it's not possible, I'm saying it's easier for someone without a lot of experience/equipment to make a module with the through hole version instead. Plus there's just the ease of replacement if ever needed.