The Acorn Archimedes, well-known L’ for its speed and good graphics facilities, has a video interface that allows programmers to design a variety of screen modes with the aid of a programmable controller. As so often, this versatility has a drawback: since the controller uses a fixed clock at 24 MHz, the frame frequency decreases the more pixels are used in the screen image. As a result, high-resolution screen modes have a tendency to cause flicker. Fortunately, this drawback may be eliminated or nearly so by increasing the clock to 36 MHz: this has already been incorporated in the new A540 computers.
All that is necessary to increase the clock, s an integrated crystal oscillator and an Ic 11-pe 7400. To ensure that the circuit in the diagram is at all times compatible with the existing software, it may be arranged to be switched on (by software) only when the screen mode requires a higher clock. The design allows the circuit to be fitted simply to the existing connectors in the computer. The 111 -oscillator and two small connectors are fitted at one side of the board (not available ready-made), while the other side houses a surface-mount version of the 74HCTOO.
The board itself is fitted with the four pins of PL,3 in A300 computers or PL4 in A400 computers.
The connection with the i/o line that arranges the switching between the two clock frequencies is made with a short length of circuit wire soldered to pin 3 of PL10 on the motherboard.
The supply for the board is obtained by soldering two short lengths of circuit wire to the supply lines on the backplane (the card with the extension connectors).