Adding the CPU and ROM
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CPU and ROM
This is probably the most annoying part of the build, mainly because of the number of connections required.
Theory
The ULA and lower RAM are seperated from the CPU and ROM via a set of resistors and a pair of multiplexers. The idea of this is that the when the CPU is not accessing the Ram, it can go and do its own thing with no interruption, while the ULA accesses it instead.
When the ULA and the CPU both require access to the lower Ram, contention requires that the ULA get access, as the Video cannot wait for access. To do this the ULA stops the clock when it detects it is about to access it.
Under normal circumstances, the CPU can access ROM and any external peripherals without needing to be stopped, and the resistors stop the two busses interfering with each other.
Note that in the schematic below, the Decoupling capacitors across the 74ls157's are not shown, They should be put in. Also, the 680 ohm resistor going to the Rom chip select not required in my version. This is provided to allow external peripherals to disable the ROM from the edge connector. As i am not implementing an edge connector, it is not required. When i implement the 128K rom's later, I will simply remove the ROM and re-implement the memory map, thus the RomCS line from the ULA.
Schematic
This is the schematic of the hookups required>
The changes I made were as follows:
- I used 2n2369s instead of ztx313 for the transistors.
- Not so much a change as an oversight in the schematic: The 74LS157's need decoupling capacitors.
- I didnt bother with the 680 ohm resistor to RomCS, as the current rom is only really for testing and the need to disable it is not really required.
Finished product
Note, this rubbish picture is the finished board before i tied up all the wiring to make it neater. Also note, that this board originally used the old version of the colour section, and the third power wire is the +12V supply for the chroma.
Testing
At this stage, if everything is wired in, we have a 16k spectrum minus its keyboard. So simply plugging in the video section and the power should result in a the computer resetting and showing the standard "(C) Sinclair research 1982" prompt.
