usb - Repurposing FT232 UART pins as parallel out

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...

Answer

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.