If one MCU supports 32 I/Os and you desire to goto 64 the most straight forward thing to do is to use two of the current MCU setups in parallel. There are multiple ways to get the host PC to be able to control two separate MCU subsystems...here are some ideas:

1) Setup a multi-drop bus such as RS485 to be able to connect to both I/O MCUs on the same communications link. In this case your PC may require an RS485 adapter off its Comm port.

2) Let each MCU have its own serial port and connect those two ports back to the PC via two COMM ports.

3) Use idea of #2 above but use two USB to RS232 adapters to get the two COMM ports out of the PC.

4) Instead of the RS232 type USB adapter cables look at using the TTL232R-5V cable (discussed in another thread here a day or so ago) to a 1x6 header on your MCU board. For two cables use two 1x6 headers. These would connect as TTL level signals direct to the UART pins on the MCUs.

5) Simplify the connection back to the PC to a single USB cable. Then inside the product circuit board add a small USB hub chip (SMSC is a company to look at here and they sell through Mouser/Diikey). Have two downwind ports of the hub connect to two FTDI FT232R chips on your board to convert USB to UART serial which you then in turn connect to the UART pins of the MCUs.

6) You can get rid of the USB to Serial chips in the design by directly using an MCU that has a USB port on it. Use of an embedded hub chip still lets you be able to talk to multiple USB enabled MCUs and yet still have just one USB cable back to the PC.

7) Finally you could look for another MCU that has lots more I/O pins. SiLabs has several that offer eight ports for a total of 64 I/Os.

Michael Karas