My4TH is a minimalist computer designed by Dennis Kuschel. This computer is extremely simplified in its design, employing a serial ALU and utilizing 16 74HC series CMOS logic devices along with microcodes from ROM for CPU. It runs the Forth system. Here are some experiments related to this computer.

Increasing the running speed

To ensure the state of the bus during reset, the original version of My4TH employs 10k resistors to pull down the data bus to GND. This introduces compatibility issues between TTL and HCMOS voltage levels: the outputs of RAM 62256 and ROM 27C256 conform to TTL levels, with a minimum high-level output voltage (\(V_\mathrm{OH}\)) of 2.4V; whereas the minimum high-level input voltage (\(V_\mathrm{IH}\)) for 74HC devices is \(0.7 V_\mathrm{cc}\), which is 3.5 V when powered at 5 V.

Basic principles

The circuit is based on the N1BYT’s WBR receiver, as shown in the schematic below. Q1 is the core of a Colpitts oscillator circuit, and RV2 and RV5 adjust whether the circuit oscillates (i.e., regeneration strength). L2, C7, C8, and six diodes D1 to D6 form the oscillation tank circuit. The input signal is attenuated by RV1, and then fed into the middle tap of L2 with L1. The DC path of the variable capacitance diode anode is provided by L1.

Mirror site of www.fcz-lab.com (March 28 2017)