First I got the video counters working, and generated a frame with fixed data:

The CDP1861 PIXIE timing is used (262 scan lines, 128 scan lines active, 14 bytes per line of which 8 are active). The data byte is fixed.

I was pleasantly surprised that the video timing and voltage levels worked on the first attempt.

The CDP1861 PIXIE uses DMA to read memory and directly shifts the bits out as video. In my design, because the CPU may be run at various speeds, from 1x to 256x the original Elf speed, the PIXIE logic will write bytes into a dual port RAM, and dedicated logic will scan them out as NTSC monochrome video. In this test, the CPU is not hooked up, and the frame buffer data is statically initialized to match the screen shot at the beginning of the July 1977 Popular Electronics article, page 41.

Due to an error in my design, the rightmost pixel column is being displayed on the left. Here’s the image after that error has been corrected:

I still need to hook up the CDP1861 DMA logic to the CPU, so that I can run the actual program, and others. PCBs are still expected to arrive on Tuesday.

In this photo, it is running a hardware test using a Xilinx MicroBlaze soft core, rather than my 1802 core. I’ve tested the LEDs, but have not yet tested the switches and composite video output. I’ll test those tomorrow, then switch to my 1802 core.

I can’t believe how well fingerprint smudges show up on the switch panel. Yuck! I’m not bothering to clean this one, because I’ll have the new rev of the switch panel when the new PCBs arrive.

I ordered parts from Digi-Key to build two units. Of course, shortly after placing the order I realized that I’d forgotten to order power supplies. It doesn’t require anything too special, just regulated 5V 2A with a 2.1/5.5mm coax plug.

I expect most of the Digi-Key order to arrive on Friday, and the PCBs will likely be here on Monday.

Other stuff I ordered to build the project arrived today:

I have two soldering stations at home, but I’m not there so I had to order a Hakko FX-888D to use here. I also expect to breadboard a few more things, so I ordered five more solderless breadboards, and (not shown) more precut breadboard wires.

Unlike the hexadecimal displays, the LTP-305HR contains only 36 bare LEDs (including one decimal point, which I won’t use), with no drivers. Since they are in a matrix, with anodes connected to five column lines, and cathodes connected to seven row lines, they require multiplexed drive. I want to have up to six digits (to display both address and data), so the most efficient organization will require 15 anode drivers and 14 cathode drivers. Electrically this is an arrangement of two rows of displays by three columns, although physically they will be mounted on the PCB with all six in a single row.

While the Digilent Cmod-A7 FPGA module has 44 digital GPIOs available, I don’t want to dedicate 29 of them to the display, so I prefer to use driver chips controlled by an SPI synchronous serial interface. In the past I’ve used Allegro drivers such as the A6275 octal LED sink driver and the UCN5891A or UCN5895A octal source driver, but unfortunately these have all been discontinued.

The A6275 sink driver was quite convenient in that it used internal current mirrors, so it controlled the current on all outputs based on a single external resistor, avoiding the need for current-limiting resistors for each LED cathode line. Allegro also made a 16-bit version, the A6276.

It is not easy to find suitable parts still made in DIP (through-hole) packages, since so much of the world has moved to SMD. As it happens, Microchip, which acquired Micrel a few years back, still offers the MIC5891 source driver in a DIP package, though the saturation voltage specs is slightly worse than the Allegro UCN5891A, and much worse than the Allegro UCN5895A.

I wasn’t quite so lucky with the A6275. Toshiba made a pin-compatible part, the TB62705, but they’ve discontinued it. On Semiconductor offers the CAT4008, which is pin-compatible, but not available in through-hole. Finally I found the TI TLC5916, which is not pin-compatible, but is still offered in DIP. The TLC5916 actually has increased functionality compared to the A6275 et al., including programmable intensity compensation and fault detection, but I have no plan to use those features.

The board will run on nominally 5V from USB, provided through the MicroUSB connector of the Cmod-A7 module. However, the Vbus voltage at a USB device may be as low as 4V, and the Cmod-A7 has a schottky diode in series from the USB Vbus to its header pin, dropping another few hundred millivolts. The LEDs have a forward voltage of around 2V, and the saturation voltage of the MIC5891 can be as high as 2.5V when sourcing 350mA. The TIL5916 sink driver controls the current by varying its output voltage. After adding this all up, it’s unclear whether there’s enough voltage to drive the LEDs. The MIC5891 saturation voltage is lower when the current is lower, so I need to determine what LED current is needed at 15:1 multiplexing.

I’ve put a Cmod-A7-15T, TLC5916, MIC5891, and LTP-305HR on a breadboard for testing, along with a TIL311 hexadecimal display for comparison:

I used a MicroBlaze soft-core processor in the FPGA to control everything for test purposes. I am using PWM to control the enable of the source driver, so I can test running at 1/15 duty cycle, even though I don’t yet have the actual display multiplexing running.

I’ve found that with approximately 1/15 duty cycle and with a 1.5K ohm resistor on the sink drivers, I get LTP-305HR display intensity that seems comparable to the TIL311. The TLC5916 sink driver will sink current at 18.75/Rext A, so with a 1.5K resistor that’s about 12.5mA per LED. For the full matrix, a maximum of 14 LEDs will be on simultaneously, so the maximum source driver current will be 175mA, so the MIC5891 source driver saturation voltage should be under 2.4V, though I haven’t actually measured it.

The hexadecimal font to look like the traditional displays will never have more than three LEDs on in any single column of the center three columns of a character, so if I electrically stagger the two logical rows of displays, so that the left and right edge columns of the top row displays are not coincident with the edge columns of the bottom row displays, then no more than 10 LEDs will be illuminated simultaneously, which will further reduce the peak current to 125mA. I may also put some higher value capacitors next to the decoupling capacitors for the drivers, to further average the overall display current.

The FPGA output pins have Voh below 3.3V, while the display drivers, which run on 5V, have Vih(min) of 3.5V. I’m cheating on my breadboard, counting on the drivers to actually have a threshold somewhat below 3.3V, which in practice at they typically do, at least at room temperature. On the real FPGA-Elf board, I will use an SN74AHCT245 to act as a level translator for various signals. One channel of the ’245 will be used with a voltage divider to shift the data output of the last driver chip down to 3.3V level for input back to the FPGA.

One issue with the LTP-305HR compared to the older HP and TI hexadecimal displays is that it uses 635nm “high efficiency red” LEDs, rather than 650nm LEDs which were much more common in the past. To me, 635nm looks orange-red rather than red, but then, I have partial red-green color blindness, so the frequency response of the green pigment in my eyes is shifted closer to red, and I see some colors differently than people with normal color vision. I need to show the TIL311 and LTP-305HR to someone with normal color vision, and see whether they perceive as much difference in the hue.

At $52, it’s a lot more expensive than other alternatives.  However, for my situation it was ideal.  I’m in Colorado, but I needed the aluminum to be in Oregon.  I was able to get it there overnight for $56.  If I’d purchased it locally and shipped it myself, it would  have cost more than that.

I don’t even know what software was trying to start; I don’t think it was related to the browser. I clicked on the Details expander, and it expanded to show this:

It would have been nice if the responsible program could have at least put its name in the dialog somewhere.

“Man, I have suffered through illness caused by your brutality and persecution. I have run from your bruising foot and taken refuge here, for dust and ashes are gentler than man’s heart, these ruins ruins less melancholy than the soul of man. Be gone, you intruder from the world of misrule and injustice.

I am a miserable creature who served the son of Adam with faith and loyalty. I was man’s faithful companion. I guarded him day and night. I grieved during his absence and welcomed him with joy upon his return. I was contented with the crumbs that fell from his board, and happy with the bones that his teeth had stripped. But when I grew old and ill, he drove me from his home and left me to merciless boys of the alleys.

“Oh son of Adam, I see the similarity between me and your fellow men when age disables them. There are soldiers who fought for their country when they were in the prime of life, and who later tilled its soil. But now that the winter of their life has come and they are useful no longer, they are cast aside.

“I also see a resemblance between my lot and that of a woman who, during the days of her lovely maidenhood enlivened the heart of a young man; and who then, as a mother, devoted her life to her children. But now, grows old, she is ignored and avoided. How oppressive you are, son of Adam, and how cruel!”

Thus spoke the speechless animal whom my heart had understood.
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