Question regarding to Model A Pi

[lilzz]

http://ecx.images-amazon.com/images/I/7 ... L1500_.jpg
1)Missing SMSC chip, I though it's a USB controller? Model B has it.
2)Voltage regulators, RG1, RG2, RG3. Why RG2 only has 2 legs and the other two has 3 legs?
3)What's that thing underneath C1 silkscreen? that has gold circle and brown surrounding and the green rectangle?
4)what's D1, and D18 , 3 legged diodes? I thought diode has 2 legs?
5)For volatge regulator RG2, the tab is solder to a large pad, the pad is just for cooling purpose not connecting to anything right? Therefore no need for a heatsink?
6)what's that weird circle thing above TP1, test point1?

[Jessie]

4. These are most likely switching diode pairs. I know that the schematic shows a bav99 but I don't recall if that is what you are referring to.

[FTrevorGowen]

W.r.t. 1) - Not a USB controller (that's in the SoC) but a 3-port** USB hub + LAN controller.
Trev.
** Two of which become a model B's ports (the model A's port connects directly to the USB on the SoC, IIRC).

[Ravenous]

http://ecx.images-amazon.com/images/I/7 ... L1500_.jpg
2)Voltage regulators, RG1, RG2, RG3. Why RG2 only has 2 legs and the other two has 3 legs?

I think it's one of these:
http://www.onsemi.com/pub_link/Collateral/NCP1117-D.PDF
in which case the tab is also a connection. I think...

[DirkS]

6)what's that weird circle thing above TP1, test point1?

There's 2 of these (2nd one at the other side of the DSI connector, near RG1).
Used for positioning the SD card slot (the slot has 2 little studs / pins; at least the one I soldered in has...)

Gr.
Dirk.

[plugwash]

http://ecx.images-amazon.com/images/I/7 ... L1500_.jpg
1)Missing SMSC chip, I though it's a USB controller? Model B has it.

The SMSC chip on the model B is a LAN9512 USB hub with ethernet. It's upstream port connects to the broadcom SOC and it's downstream USB ports and ethernet port are made available on external connectors.

On the model A that chip is not fitted and two zero ohm links are fitted (on the bottom iirc) to connect the single USB port directly to the SOC.

2)Voltage regulators, RG1, RG2, RG3. Why RG2 only has 2 legs and the other two has 3 legs?

On larger surface mount regulators the back of the regulator is usually soldered to the board, this can be used for both heat transfer and electrical connection and is sometimes used instead of one of the legs.

3)What's that thing underneath C1 silkscreen? that has gold circle and brown surrounding and the green rectangle?

This is a fiducal mark used for alignment by the assembly machines that place the components on the board.

4)what's D1, and D18 , 3 legged diodes? I thought diode has 2 legs?

Sometimes you also see single diodes supplied in three terminal packages with one terminal unused, not really sure why. You also often see diode pairs (useful for signal protection) in three terminal packages. I'm not sure what these ones are you'd have to look at the schematic.

5)For volatge regulator RG2, the tab is solder to a large pad, the pad is just for cooling purpose not connecting to anything right? Therefore no need for a heatsink?

The large pad is used for both electrical connection and heat transfer. For regulators with small drop that is sufficient to get rid of the heat.

6)what's that weird circle thing above TP1, test point1?

I intitally thought this was also a fiducal mark but looking more closely I think DirkS is right and it's a support peg for the SD card socket.

[Burngate]

4)what's D1, and D18 , 3 legged diodes? I thought diode has 2 legs?

Sometimes you also see single diodes supplied in three terminal packages with one terminal unused, not really sure why ...

So that the pick 'n mix machine can't put it in the wrong way round?