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68000 Tablet Update


Written by Stephen
February 22, 2020

I have ordered some parts for the 68000 Tablet I have designed. The most expensive part was the PCB - it was $90 CAD for 5 tablet boards, as well as 5 programmer boards. About half of that cost was shipping. I also put together a BOM, of which most of the parts come from Ebay, along with some from Digikey. It's crazy how fast the cost of a few dozen logic gates add up! I placed the Ebay order, but not the one with Digikey. The latter will arrive within a day or two from shipment, so it's not as high of a priority.

I made a few part substitutions to save money. I swapped out the 1MB of RAM for 256K. This brings RAM cost from $14 to $8. I'm also using an EEPROM where the ROM goes. Again, I swapped out the 1MB for 256KB. One thing is that the A17 line connects directly up to the WE line, which is active low. This means that when the computer turns on, it will overwrite its ROM immediately. I will probably just not connect WE and leave it floating. I haven't had problems doing that in the past, but if it causes problems I'll run a wire to +5V. The last thing I did was swap a 74 series logic chip out, because it was discontinued. The new one is pin-compatible, but I had to update the schematic with the new part number.

I opted not to order the screen yet. At $40, it is the most expensive part. I can still debug the system fine without it, by using the bus LEDs and serial ports. I also didn't order the mechanical key switches. They're quite expensive and I can get by with shorting the pins out manually. Once the system is up and running, I'll get these ordered too.

I have already identified some changes I would like to make to rev1 of the board(currently on rev0). For one, the LEDs need to be able to be turned off via a hardware switch. Some simple calculations say that the lights will use up to 600mA when all are lit. This is on par with the consumption of the rest of the system. Being able to turn the lights off would increase battery life, and would make it less distracting to use.

I think it's also worth splitting the board up into two parts - one for the system and one for the keyboard. I'll probably end up making several revisions of the system, but I should only need one version of the keyboard. I'm also going to update the schematic to use an EEPROM instead of ROM. It's not the 90s any more!

The next step is to write some assembler code to make sure the computer works, once the parts arrive.

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