ULA and lower ram section and testing

The ULA and Ram

The ULA is responsible for a variety of tasks in the speccy, the main one being video generation. To do this, it requires priority access to the lower 16k of ram (From $4000-$7FFF). This is controlled by monitoring various CPU address lines. The CPU is bus decoupled from the video bus via some resistors and Chips, but this isnt dealt with here.

Theory

To get a video signal, the ULA must be provided with a crystal. Other than that, it must be given some memory. In the original zx-spectrum, this was done with 8, 1bit chips of type 4116, which was a very popular ram chip in the time period.

Fortunately, things moved on, and later in the 80's to avoid having to have users spend hours inserting individual chips, They were grouped together on small PCBs with a standard interface called SIMM's.

These simplify the wiring immensely, as instead of having to hookup 16 pins, 8 times with the errors that inevitably creep in, There are a total of 30 wires, half of which are wired directly to the ULA, and the rest directly to 0v. Also, all the Decoupling is on board, so other than a small capacitor over two pins, you dont have to worry about that.

In addition, a small passive section needs to be provided for the analog section.

Schematic

This is the modified schematic with the SIMM in it

The changes I made were as follows:

• I used a 30 pin SIMM in place of the 8 4116's

Testing

Testing is simple, if it works, you should see a display. If it doesnt, well, you dont. You cant totally test if the SIMM is wired correctly without connecting a CPU and ROM, which is the next stop, however some basic checks can be done.

You can remove the Simm from its holder (You did use a holder didnt you?) and short the data wires, one by one to VCC. The video should change with lines going down the screen in the appropriate positions.

Parts list

• Active
  • 6C ULA
  • 30 pin Simm
• Passive
  • 14Mhz crystal x1
  • Capacitors
    • 47nf x3
    • 22uf
    • 100nf
    • 16pf 5%
  • Resistors
    • 15 ohm 1W resistor.