OMEN Kilo: The 6809-based minimal computer

Photo by Ugi K. / Unsplash

What else to start with but the physical wiring of the 6809…

Perhaps unsurprisingly, there is some similarity to the 6502 processor. This is logical because, as we've said several times, both the 6502 and 6809 are based ideologically on the same predecessor, the 6800.

Pin description

The processor has both main buses, the data bus (D0 - D7) and the address bus (A0 - A15). There are no surprises here. The signals are not multiplexed and are fully available.

The /RESET input is used to initialize the processor to its default state. The input is a Schmitt flip-flop circuit, so a simple circuit with capacitor and resistor is sufficient, similar to previous computers. After RESET, the address (2 bytes) is read from the FFFEh and FFFFh addresses, and the value obtained is used to set the PC program counter.Remember that the 6809 uses Big Endian style storage, i.e., the higher part of the address first, then the lower. So the higher part will be at address FFFEh, the lower at FFFFh.

OMEN Kilo at Hackster

See Kilo

The R/W output informs the environment whether the processor intends to read (=1) or write (=0). Again, as with the 6502, no distinction is made between peripherals and memory, everything is "memory" for the processor.

The XTAL and EXTAL pins are used to connect the crystal. If you use an external timing pulse generator, feed the signal to the EXTAL input and leave XTAL unconnected.

Inside, the processor will run at a frequency that is equal to a quarter of the crystal frequency (or the supplied frequency). So for the OMEN Kilo, I used a frequency of 7.3728 MHz, which when divided by four gives our favorite operating frequency of 1.8432 MHz.

There are three variants of this processor, which differ in their maximum operating frequency: the 6809, 68A09, and 68B09. The variant without a letter operates at 1 MHz, variant A uses 1.5 MHz, and variant B can operate at up to 2 MHz (that is, the external crystal can have a frequency of 4, 6, and 8 MHz, respectively).

The clock signals E and Q are used to control the external circuitry. E has the same function as the PHI2 output on the 6502. A falling edge on the E output tells the processor that an operating cycle is about to begin. A rising edge on the Q output then comes in to indicate that there is a valid address on the address bus. This is followed by a rising edge on signal E, a falling edge on signal Q (with no special meaning), and the entire cycle is terminated (and a new one started) by a falling edge on signal E, during which the processor reads data from the address bus.

For most simple cases, we can ignore the Q (Quadrature time) signal and just work with the E signal, as with the 6502.

Beware of the 6809E variant! This variant has no internal oscillator, requires an external two-phase clock generator, and the pinout is slightly different.

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Martin Maly

Martin Maly

Programmer, journalist, writer and electronic hobbyist. Vintage CPU lover. Creating new computers with the spirit of 80's.