Nathans Z80 Project Mark 2

Posted on Feb 6, 2014

The basic requirement was for some sort of output display and some sort of alpha-numeric input. So the simplest solution I could think of was a UART, attached to something with a screen and keyboard running a serial terminal. I looked at using the PIC already in the system as a UART as well, but decided that it would be better used as a fast DMA c

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ontroller with all its pins allocated to that. I had a 6402 UART chip of a similar vintage to the Z80 CPU lying around so that became the core of the UART peripheral. There are three peripheral chips in the UART system, a 74HC4060 clock/divider chip, a MAX232 RS232 level shifter and a 74LS541, an 8 bit tri-state buffer. The clock/divider chip provides an independent clock source for the UART from the CPU clock. The UART clock has to match the baud-rate so needs to be a fairly weird frequency. The 6402 needs 16 times the baud-rate as an input clock, so I`ve used a 4. 9152MHz crystal. Taking the Q6 (divide by 64) output this gives 4800 baud, the Q5 (divide by 32) output gives 9600 baud and so on. I`ve used a jumper block to allow a baud rate from the range 4800, 9600, 19200 or 38400. The MAX232 chip provides the big voltages needed for "true" RS232 signalling. If you`ve made anything with a serial interface before you`ll probably be familiar with this or a similar chip (e. g. the MAX202). There`s a bunch of electrolytic caps attached to this to make the suitable voltage step-ups. If you`re using a TTL level USB interface e. g. an FT232 based device, you need to skip the level translator, but I`m planning on plugging this into a real serial port on some old hardware I have. The final peripheral chip is the tri-state buffer. This buffers the status flags from the UART and allows them to drive the data bus when a "status...

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