Pullup resistors on TTL lines

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NXTreme

Senior Member
I'm trying to connect a Nyko Kama (knockoff of Wii Nunchuck) to a Picaxe 20x2. I tried some code I found here on the forums but wasn't able to get anything to work. I powered the 20x2 at 5V because I'm using a USB FTDI cable, which, AFAIK only work at 5V. I used the SparkFun Logic Level Converter to lower the Data and Clock lines to 3.3V for the Nunchuck and a 3.3V LDO v-reg to power it.

I then figured that if I ran the Picaxe at 3.3V along with the Nunchuck, it would eliminate most interfacing problems except for one, connecting to the programming cable (necessary for debug).

To connect the Picaxe running at 3.3V to the 5V FTDI cable, I decided to reuse the SF LLC. This is where I'm starting to get a bit confused. I think I have pretty much figured out the circuit, except that the LLC has 10K pullups installed. I'm not sure how they'd affect the communication with the computer, would it be best to remove them? Would the BSS138 FET installed on the LLC still function without the pullups?

I found this thread (post #12) which states that it might be safe to use pullups but that's for comms between 2 Picaxe chips, not the computer and Picaxe.

BTW, what is the correct way to refer to serial comms between Picaxe chips (or any other, for that matter) at 5-3.3V. TTL? Serial? UART? I'm confused...
 
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Andrew Cowan

Senior Member
With the download circuit, you'll be fine using the standard programming resistors. They'll limit the current sufficiently that the 5v logic from the USB cable is limitted at 3v3 by the PIC's internal diodes.

Between PICs, you'll often be fine just by putting high value (say 22K) resistors in the serial lines between the chips.

Andrew
 
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NXTreme

Senior Member
Thanks for the quick reply! Unfortunately, the tests I ran a while back showed that the FTDI USB cable won't register 3.3V as a "high" level. Maybe I should try again. Thanks!
 
hippy

hippy

Ex-Staff (retired)
NXTreme said:
Thanks for the quick reply! Unfortunately, the tests I ran a while back showed that the FTDI USB cable won't register 3.3V as a "high" level. Maybe I should try again. Thanks!
Click to expand...
Perhaps time to consider buying an AXE027 ?

I've had no problems with an AXE027 to a PC with PICAXE at 3.3V and even lower; working with a 2V powered 18M2 at present.
 
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NXTreme

Senior Member
Well, I tested it at 3.3V and guess what? It works! :p*

hippy said:
Perhaps time to consider buying an AXE027 ?

I've had no problems with an AXE027 to a PC with PICAXE at 3.3V and even lower; working with a 2V powered 18M2 at present.
Click to expand...
I would get a AXE027 except that I already have purchased a SparkFun USB Picaxe programmer, which is similar to the AXE027. I normally use an old laptop with a serial port to do all my programming but a couple months back while I was traveling, somebody wanted me to prototype a product for him, and to keep costs down I bought the SF programmer instead.

Since the old laptop died (anyone know of a place to get cheap PATA drives?) I've been using my more up to date laptop for programming. As such, I'm still using the SF programmer. It does work... even if it is slightly less protected than the AXE027 :eek:!

Thanks again for all the help, and for ignoring my stupidity...

*-EDIT- That is, the Picaxe will program at 3.3V but I still haven't been able to get the Nunchuck (Kama) working. Moral of the story: don't buy third party, buy first party!
 
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