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A project log for RPi WiFi

Fast RPi WiFi without USB

ajlittajlitt 02/09/2016 at 19:3611 Comments

This is a partial render of my next board spin. The big changes here are 4-bit SDIO (more about this later), removing the audio filters, and adding a 5V boost converter onboard.

I am working to add a boost converter to the Zero that should allow operation from 2.3V or so up to 5V. The converter should be good for about 1A above 3V, which is plenty for the Zero + ESP8266 + a typical USB device.

Power supply is one area where all the Pis are weak, not just the Zero. I think it would benefit a lot of projects to run the Pi from 2AA batteries, or the existing power supply of a quadcopter or robot, or a standard LiPo pack. Having a simple boost converter makes all these options possible. I know the Zero (and A+) can potentially work at less than 5V, but there's some uncertainty as to how reliable it is or which USB peripherals will work reliably.

The 10 pin header on the bottom edge is gone, replaced by a 5 pin header on the right. This header has the same pinout for pins 1-5 as the old version, so an FTDI 6-pin cable can be used if the last pin is left hanging. This connector can be used to feed the boost converter's input and to control the boost converter enable signal (after cutting a jumper that keeps it always-on). The enable pin can be used with a switch or another micro to turn the Pi off and on remotely.

Inset from the edge is a 2-pin JST PH header, same as used by many battery packs and charge controllers. The distance from the edge is to give clearance to route wires out of the way of the USB and HDMI jacks. This header is not populated by default since it adds significant height to the board, but for batteries with high discharge capability it's safer to use a locking keyed connector like this.

As a compromise, the audio is gone. The power supply noise it picks up from the Pi's 3.3V is terrible, and the complexity to add a stable supply and buffering is too much work for something I feel will see much less use than the boost converter. I know some people are interested in adding back lost functionality of the Zero, but in my opinion a dedicated DAC HAT or one of the newer Pis are a better way to get audio out.

Meanwhile, I'm planning a small run of boards. These would be hand-assembled by me, so I'm thinking of only 20-30. I have some of the core parts on order, but I'll hold off on PCBs until I finish and test a few of this next-gen. And yes, these would be for sale.

Discussions

txdo.msk wrote 02/17/2016 at 19:00 point

I could use a couple of these boards...

What cost are you thinking?

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ajlitt wrote 02/17/2016 at 21:22 point

I don't want to price it just yet since I haven't finalized the design. I'm waiting until I have parts for the pilot run in hand and have made a few.
I plan to make it more than a Pi Zero but less than buying a USB WiFi dongle and boost converter from one of the usual electronics shops.

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txdo.msk wrote 02/17/2016 at 21:59 point

well, I just got a TP link usb wifi for $10 on sale.  Use your local costs as a basis.

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txdo.msk wrote 02/19/2016 at 23:51 point

Another thought:  How about a board and a BOM?

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ajlitt wrote 02/20/2016 at 01:13 point

KiCad project is in github to the left :)

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txdo.msk wrote 02/20/2016 at 01:25 point

d'oh!

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deʃhipu wrote 02/09/2016 at 22:14 point

Looks nice. If you skipped the battery connector and replaced the male angled pins with a female pin header soldered flat, you could have everything surface-mounted, and thus have the other side of the board flat against the pi... just add some holes where pi has stuff sticking out.

By the way, I see you only have 6 pins circled. What happened to CH_PD? It's not connected to a GPIO anymore?

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ajlitt wrote 02/09/2016 at 22:33 point

Although I'm still pushing the "pants" idea I still want this to be compatible with other Pi models.  I plan to ship these without the HAT header populated so the user can choose between mounting under a Zero with a long pin male header or using a stack-through female header to mount on top.

The battery connector is optional for exactly that reason.  You can mount it as shown in the render, or turn it 180 degrees and put it on the opposite side.  The header is the same way: solder it on whichever side you like.  The male pins keep the plug flush with the board edge when plugged in to make cable routing easier.  I've also had bad luck with SMT single row box headers lifting pads.  PTH is a much cheaper and more reliable solution.

I didn't forget CH_PD.  It's on ID_SD and no longer has a cut-and-jumper option for other GPIOs.  The ID_SD trick seems very reliable.  I did neglect to circle the UART pins.  I might mark the UART pins and ID_SD differently so that anyone using a stack-through header knows which pins they might want to cut to keep from extending to other boards.  The SDIO pins definitely shouldn't be shared, but ID_SD and the UART should be OK.

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David Lowe wrote 02/10/2016 at 00:24 point

No big loss on the audio. You can add mono audio top-side on the Zero without sacrificing a GPIO on the 40-pin header - GPIO45 alt0 = PWM1 is available near the unpopulated  U13. Good enough for beeps and squeaks.

A boost converter is a much better use of the rest of the PCB!

Re pads lifting, the micro-USBs on all Pis seem to be surface mount connectors, but anchored with PTHs where there might otherwise be SMT pads - is that the solution?

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ajlitt wrote 02/10/2016 at 04:40 point

Interesting, I didn't know about that GPIO.

Agreed the microUSB with the through-hole stakes on the shield are much sturdier than the fully SMT style.  Unfortunately the small pin count box headers don't have anything but the pads to secure them.  In this case I'd use a couple of vias in each pad to help anchor it to the board, but that only helps so much.  Larger holes in the pad would help in that the solder would make a good anchor, but the holes would wick away too much solder paste during reflow and the joint wouldn't be any good.

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David Lowe wrote 02/10/2016 at 12:20 point

There are two unused GPIOs popping out at each end of the row of 4 pads above U13, near the raspberry logo. The one on the left is 44, far right 45:

GPIO, DEFAULT PULL, ALT0, ALT1, ALT2, ALT3, ALT4, ALT5
44, NONE, GPCLK1, SDA0, SDA1, TE0, SPI2_CE1_N, -
45, NONE, PWM1, SCL0, SCL1, TE1, SPI2_CE2_N, -

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