Re: New Elf called Mar1e - dumb question, before I start modding #Homebrew #Electronics

Lee Hart

John wrote:
I really want to display the current address when I'm loading a program...
If I want to latch the lowest 8 bits of the address bus and show them on
LEDs, can I use a 74HCT373 octal latch with its strobe input connected
directly to TPB? ... I expect it's not that simple...
In the case of the 1802, it really *is* simple. :-)

To display the low 8 bytes of the address, all you need are 8 high-efficiency LEDs and a resistor network with 8 (or more) resistors. Individual resistors can also be used. The 1802 can drive them directly as long as the LED current isn't more than about 1 ma. I suggest resistor values in the range of 2.2k to 4.7k.

Connect the common pin of the resistor network to GND.
Connect each resistor pin to each LED cathode.
Connect each LED anode to the 1802 MA0-MA7 pins.
The LEDs will light for any MA0-7 bit that is "high".

In LOAD mode, the address bus continuously outputs the low byte of the address. TPA pulses and the high address byte are not present, except during the actual DMA-IN cycle. So, you don't need a latch. :-)

If you also want to see the high byte, you need a 74HC373 latch as you desscribed (connected to TPA), and 8 more LEDs and resistors connected to the outputs of this latch as described above.

Lots of modern LEDs are efficient enough to be bright at 1ma. If your DMM has a "diode test" range, this usually applies about 1ma as its test current. Connect your LED to it to read the LED's forward voltage drop at 1ma.

The red LEDs I supply with the 1802 Membership Card are highly visible at 1ma, and they are just garden-variety parts. The "best" LEDs are likely to be green or blue, which can be highly visible even at 0.25ma.

Fools ignore complexity. Pragmatists suffer it. The wise avoid it.
Geniuses remove it. -- Alan Perlis, "Epigrams on Programming"
Lee Hart, 814 8th Ave N, Sartell MN 56377,

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