P2 Custom PCB
hillgoff_space
Posts: 6
in Propeller 2
I’m finally breaking away from the Arduino platform for a Flight Controller for TVC model rockets. This is my first custom PCB and I just want to make sure I’m doing everything right. Do I just need to use a FT232RQ chip between the USB and P2?
Comments
Yes, a FT232R would be fine, FT231X is also used by Parallax on their P2 products and the newer prop plugs.
In the past there has been a parasitic issue whereby the P1 or P2's transmit pin can power the FTDI chip just enough that it starts up and issues a reset pulse, causing a cyclic reset every few seconds. It can only happen when the FTDI is connected but the USB cable isn't (in ormal connection to a PC, everything is fine). To fix this pay attention to the resistors around the TX line in the Parallax schematics.
Welcome to the P2 forums, Sounds like a really fun project
You do need a means to serially load P2, some P2 designs include that on the PCB for ease of use, and some space-paranoid designs use an external module to serially load.
Note the FT232R is quite an old and expensive part these days,
There are many offerings out there, it depends on what package you prefer, and how important serial link speed is.
As above in #2, the FT231X is newer, lower cost sibling to FT232R from FTDI. FTDI also have larger-packaged HS-USB parts, which need a crystal.
Others out there
Silabs: CP2102N in QFN20 (3x3), (a newer version of CP2102)
Prolific PL2303 series, with the PL2303Gx being newer and faster
WuChip CH34x, with the CH343 being newer and faster
Exar (now Maxlinear) XR21B1420IL28-F this part has the best baud granularity and highest sustained FS-USB speeds, With handshakes and 12MBd it can average above 10MBd.
and these days, you likely want to consider availability too
Thank you for the responses and the warm welcome! I will definitely check out some of those other chips. Just for clarification: would I connect the DTR pin to the P2 RESET pin? The last thing I want is to spend a couple hundred $ on hardware just to mess up on something simple like that.
Almost, you need ideally a capacitor, transistor, and couple of resistors - one from transistor base to ground, one from P2 reset to 3v3 (since the P2 doesn't have this internally like the P1 did).
That circuit gives you a simple edge-triggered reset, so the P2 will only reset once when you exit a PC side application, rather than staying in reset due to a low or absent DTR signal
See above, the lowest parts count method is to get a Pre biased Dual transistor, and use one half as a free pullup. Just add a single C to that
Digikey call these Transistors - Bipolar (BJT) - Arrays, Pre-Biased
@hillgoff_space We've assumed you're creating a board but you mention a few hundred $ of hardware, does this mean you already have a parallax board?
@Tubular I do not currently have a parallax board. See below for what I’m trying to make.
Well glad I asked, because that looks fantastic.
Who wouldn't want a board with 4 Pyro connections... @erco
@hillgoff_space I got to know: what is that exactly? Uber cool looking, but pyros? Thats serious next-level mischief! Half the board is probably thinking the same thing.
@JRoark its a flight control computer for hobby rockets. The pyro channels can ignite engines, deploy parachutes, and separate booster stages.
The board will also use a dual servo gimbal to control the direction of the engine output and a reaction wheel for spin stabilization. All while providing telemetry downlink for mission control. My favorite part is that it does all of this in a 2”x3” package!
Nyce board!
Looks good. Power supply capacitors are on the back side, I guess?
Yeah, caps, resistors, SD socket, etc. are all on the back side. Still working on the layout, but I can share a pic later if anyone is interested.
Nice board it is.
It would be interesting to know how do you intend to provide any external clock reference, to be connected to P2 XI, supposing you don't want your options to be restricted to the internally available ones.
Also, by considering the possibility of any adverse combined effects of thru vias and internal gold wires inductances, perhaps you could find a way to preview some combination of bulk, and also hi-frequency bypass capacitors, at the same side the Propeller is to be soldered, closer to V2831 pin (47) and the two nearest VDD pins (44 and 52).
Henrique
@Yanomani I've attached a screenshot of the backside. Please pardon the mess, as it is still in progress. I’ve got an external clock, which can easily be swapped out if desired.
I will absolutely take this into consideration. Thank you for the suggestion! I am not an electrical engineer (yet), so any advice will be greatly appreciated.
You are welcome, @hillgoff_space.
Bottom side image is easier to my eyes (less blurry, due to zoomed scale), and your coments are elucidative too, since I was suposing the BGA-alike pattern at the top layer, near XI/XO, was some kind of external oscillator.
To be clear, I'm not an engineer too, and will never be (at least on paper) but 53 years of experience doing electronics could help a lot, sometimes.
As for signal integrity, better letting some good habits flow inside while you're young, than needing to ablate/reform them, as age and also too many hours expended in front of a scope, trying to find how/why that damm woobling signal is interfering so much, in the outcome of any design.
So, if possible, consider swaping the layers, leting the clock reference near and at the same side of P2, with short connections as possible, without any interveining vias.
By the way, rockets are fantastic; I'm a big fan of them, but firecrackers are a lot cheapier to blast in the air. So, if you love your projects, as it seems to be, make sure everything will work as intended, pray God, and cross all your fingers.
Henrique