Deciding on a deisgn for SBC

While trying to decide what processor and logic to use for our homebrew computer design, we kicked around a few ideas and tried to think of the pros and cons of each.

First, we were thinking of two designs:

1. ARM microcontroller, Cool Runner 2 CPLD memory interface and framebuffer:

This system comes with the advantage that the pin assignments on the CPLD are pretty much a free for all, but the disadvantage that two chips, likely QFP 144 (MCU) and 208 (CPLD) are needed. The reason for 208 on the CPLD is that the QFP144 versions might just barely not have enough I/O pins for everything we want to do, since memory bus interfaces to both the MCU and the RAM are needed.

2. FPGA with soft CPU
For the same price as an ARM microcontroller + CPLD, we could get a Spartan 3E in 208 QFP package. This should be large enough to hold a capable soft CPU such as the OpenRISC 1k, with room left for framebuffer and other logic. This has the advantage that pin assignments are very flexible, but the disadvantage that the Spartan 3 requires three different supply voltages, making the power distribution system a bit more complicated to design. Especially for a two-layer PCB, which is basically what we are limited to based on cost.

Finally we settled on a third option:
NXP LPC2478 ARM7 microcontroller. This contains a display controller on board, so only one main chip is needed, which saves us some space. The pin assignments look a little inconvenient, but should be possible on two layers, somehow or other.

The specs we are planning are:
Serial port to FTDI chip TTL-232R cable for loading bootloader to flash, no need for MAX 232 level converter
SD card slot to store linux kernel and data
Interface to our analog RGB LCD, as well as VGA and composite video for TV out if possible
Sound.. MP3 Codec chip? (TBD)
Ethernet PHY? (TBD)
PS/2 keyboard/mouse interface? (TBD)

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