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Altera_Forum
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14 years ago

Cyclone III with 5v inputs

I'm trying to connect approximately (16) 5v TTL inputs into a Cyclone III eval board, the terasic de0 [1]. These are all input towards the FPGA only. The eval board has (2) 40-pin IDC headers for the free I/O pins.

I've read something about a first generation Cyclone that using the internal PCI clamps, and an external resistor to bring 5v into the board. I don't believe this will work for the III, but needed advice before I proceed to spend $$$ and time on an alternative solution.

I'm also concerned about AN 447: Interfacing Cyclone III and Cyclone IV Devices with 3.3/3.0/2.5-V LVTTL/LVCMOS I/O Systems. [2] I guess I should worry about the overshoot.

Of course I searched the forums before posting, and did get some valuable information from one recent post. [3] Searching the forum for very short keywords like "5v" and "ttl", or even "5v ttl" doesn't work well.

I was going to create a small custom PCB that sits between the eval board and the inputs. The output towards the FPGA board is a 40-pin IDC connector using a standard old school hard drive cable.

The input to the board will be via a single row of .1 on center male header pins. I'm going to use 6" cables from those pins to a chip clip that will connect to a DIP in another system that is primarily 5v only.

The PCB will have at the center (2) 74LCX245 sold by NKC electronics. [4]. [5].

I was also going to put a ~500ohm resistor in line with the output of the transceiver towards the FPGA with the hopes of both meeting the sample circuit in the 245 datasheet and also for the series termination/minimizing reflection purposes in the 447 application note.

The eval board has VCCIO locked to 3.3v and presents 3.3v on the 40-pin header, which I can use to power VCC on the LCX245.

The signals are at either 7.15mhz or twice that rate, 14.3mhz.

Does this sound reasonable? I'd appreciate any ideas on improving it. This is a one-off prototype design that will probably never leave my basement, but will likely be documented online. This will be my first custom PCB.

Thanks,

Keith

[1] http://www.terasic.com.tw/cgi-bin/page/archive.pl?language=english&no=364

[2] http://www.altera.com/literature/an/an447.pdf

[3] http://www.alteraforum.com/forum/showthread.php?t=30591

[4] http://www.nkcelectronics.com/5v-to-33v-logic-level-shifter-bo.html

[5] http://www.fairchildsemi.com/ds/74/74lcx245.pdf

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  • Altera_Forum's avatar
    Altera_Forum
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    14 years ago

    --- Quote Start ---

    The TI 74CBTD3384C

    http://www.ti.com/lit/ds/symlink/sn74cbtd3384c.pdf

    buffers used on the MAX II PCI development kit are bus-switches with the diode internal to the device. You can power them from 5V and they'll clamp bidirectional signals to 3.3V levels.

    I've used a bunch a bus switches on lots of designs, but never realized they had parts with the diodes internal to the devices.

    Cheers,

    Dave

    --- Quote End ---

    Thanks Dave for your help.

    I would certainly prefer something with less external parts required.

    I sampled these today from TI, although they only had the DBQ(SSOP/QSOP) parts available. I prefer SOIC with the larger 1.27mm pitch. I've soldered the SOIC before, and they weren't bad. The SSOP have half the pitch.

    Any experience hand soldering these?

    Thanks,

    Keith
  • Altera_Forum's avatar
    Altera_Forum
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    14 years ago

    --- Quote Start ---

    I sampled these today from TI, although they only had the DBQ(SSOP/QSOP) parts available. I prefer SOIC with the larger 1.27mm pitch. I've soldered the SOIC before, and they weren't bad. The SSOP have half the pitch.

    Any experience hand soldering these?

    --- Quote End ---

    Soldering SSOP is easy. You tack the diagonal corners to get the pins aligned, and then wipe a flux pen over the pins, then solder, creating shorts by accident, then you go back with solder wick and clear the shorts. Wet the solder wick with a flux pen first, and the shorts are easily removed.

    Cheers,

    Dave
  • Altera_Forum's avatar
    Altera_Forum
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    14 years ago

    Before completely hijacking my own thread,

    Solder type and size? Temperature setting? Tip size and type? Solder braid size?

    Thanks

    Keith
  • Altera_Forum's avatar
    Altera_Forum
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    14 years ago

    --- Quote Start ---

    Before completely hijacking my own thread,

    Solder type and size? Temperature setting? Tip size and type? Solder braid size?

    --- Quote End ---

    Nothing particularly special, just whatever I have lying around, lets see ...

    * Solder wick = PRO WICK 1801-10F 0.035" wide (i.e., narrow)

    * Small solder tip

    * Solder station set to 700F (Weller or Hakko)

    There's a bunch of soldering videos on YouTube, check them out to see what people do (I recall SparkFun Electronics might have some videos too, but that may be for a DIY reflow oven ...)

    Cheers,

    Dave
  • Altera_Forum's avatar
    Altera_Forum
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    14 years ago

    This video is a good example: http://www.youtube.com/watch?v=ll8gcqqxb2e. When you have more experience you can drag the tip along the pins and solder a full row in one go.

    The most important part is the flux, it makes everything so easy! You should also have a magnifying glass, and ensure that the solder tip is perfectly for a good heat conduction.
  • Altera_Forum's avatar
    Altera_Forum
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    14 years ago

    @jshamlet: Thanks for the info. I've definitely seen those IDT quickswitches before, but I've never used one.

    TI is mega fast on the samples. I ordered yesterday 1pm, this morning at 9am it was in my door. Maybe I'll dead bug one soon just to watch it work, and then design my converter PCB around it.

    Thanks for the link to the videos.

    can i attach the output of the ti chip directly to the connector going to the cyclone iii? ie without a series resistor?

    The application note (http://www.ti.com/lit/an/scda003b/scda003b.pdf) and datasheet (http://www.ti.com/lit/ds/symlink/sn74cbtd3384.pdf) doesn't seem to cover it explicitly.

    should i connect the fpga ground to the ground of the system i'm receiving the 5v ttl from?

    Thanks,

    Keith
  • Altera_Forum's avatar
    Altera_Forum
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    14 years ago

    --- Quote Start ---

    can i attach the output of the ti chip directly to the connector going to the cyclone iii? ie without a series resistor?

    --- Quote End ---

    Yes. If a source termination is required, it would be needed back at the source driving the input to the bus switch.

    --- Quote Start ---

    should i connect the fpga ground to the ground of the system i'm receiving the 5v ttl from?

    --- Quote End ---

    Yes. Without it, your interface will not work.

    If you need an isolated interface, then Analog Devices has some nice parts; look for iCoupler.

    Cheers,

    Dave
  • Altera_Forum's avatar
    Altera_Forum
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    14 years ago

    --- Quote Start ---

    if I use pull-downs on the buffer inputs like you mention here (http://www.alteraforum.com/forum/showpost.php?p=123853&postcount=8)

    --- Quote End ---

    There is a big difference between using a buffer and a bus-switch.

    A bus-switch can be used for bi-directional clamping of signals.

    The bi-directional nature of this solution means that you can use the pull-ups on the FPGA I/O pins to guarantee that the I/O pins are at a valid logic level at all times.

    No need for pull-ups on your bus switch board.

    Cheers,

    Dave