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Background

I am trying to build a USB Bridge circuit as part of a larger project. What I mean by that is, I wish to control a parallel out circuit based on information transmitted by the host computer.

Application Details

The idea behind the project is to have a USB powered Nixie Clock. A Nixie Display unit has a high operating voltage of about 150V. The power supply part of the project will take care of stepping up 5V DC (up to 500mA due to USB limit) to 150 V (Vdc), 5mA (0.75W).

We want our initial design to have no onboard clock circuitry, instead, we want the time and date information to come over a server lookup from the USB host computer.

Parallel Pin requirement:

Due to a power limitation, our design will rapidly switch between the 6 Nixie Tubes (HH MM SS) such that only one is lit up at a time. This is where the parallel out requirement stems from.

We need 6 pins to select the tube whose anode is pulled up to Vdc and 10 pins to choose the digit that currently glows. These final outputs can come from a decoder, but the system will still have 16 states.

So atleast 4 parallel pins are a requirement. However it would be better to have 3pins for tube control + 4 pins for digit control Since these will be interfaced to decoder ICs I believe, there is no significant current requirement for them.

Possible Solutions

A good solution could be to use a general purpose USB microcontroller such as the TUSB - 3210

However to keep things simpler, I have considered using a USB - UART interface chip such as the FTDI series of devices - FT232R (Single UART) and FT2232 (Dual UART, for more functional pins)

So what I want to be able to do is to program the pins available as per my will, without regard for their UART protocol functionality. I wish to avoid using the FT232R on-board EEPROM due to a lack of experience with using such programmers. This prevents me from using the control bus (CBUS) pins.

My questions about doing this are

  1. Is doing this a good idea, is it a common application of FT232R?
  2. I am on a tight power budget since I want to make a Nixie clock front end work using just USB power. Would the 3210 prove better for this application, considering the fact that USB controller will be a single package solution whereas a UART based solution would need some extra (Although low power) Decoder ICs?

Some useful information about the FT232R datasheet

  • Page 7 - 10 pin functionality.

I am very new to big controller based projects, please provide feedback on how I could make the question better, provide more details...

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  • \$\begingroup\$ While this is possible it is not going to make things simpler - if anything the reverse, because you're going to be in a host side driver support mess. Simple would be doing everything in an MCU and borrowing whatever you decide is the most portable USB functionality (virtual serial? HID? abused mass storage?) to do nothing more than set the time. Fake mass storage tends to result in annoying meaningless error messages, but just grabbing the file write time could be fun. Two cheap buttons to set the time makes it work on a USB charger and makes host drivers entirely optional... \$\endgroup\$ – Chris Stratton Feb 19 at 16:31
  • \$\begingroup\$ @ChrisStratton thanks for pointing out that a USB host is not really required since the aim is to simply display time. I understand this but part of our project is to make a server lookup and implement communication between the host and the slave over USB. Thanks also for your additional inputs towards the design choice. \$\endgroup\$ – ijuneja Feb 19 at 16:42
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So, you want to light one digit at a time, which means you need to select 1 of 6 anodes to drive (3 bits) and 1 of 10 cathodes (4 bits), for a total of 7 bits of output.

The FT232R has modes that directly support this. Read the paragraphs "Asynchronous Bit Bang Mode with RD# and WR# Strobes." and "Synchronous Bit Bang Mode." on page 14 of the datasheet. Synchronous is probably what you want. There's an application note that goes into additional detail.

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  • \$\begingroup\$ Thanks for a pointer in the right direction. Also, the question Title - 'Repurposing seems incorrect' after reading your answer \$\endgroup\$ – ijuneja Feb 19 at 16:58
  • \$\begingroup\$ I have asked a related question at the below link, would really appreciate if you could take a look. electronics.stackexchange.com/questions/425875/… \$\endgroup\$ – ijuneja Mar 6 at 14:10

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