Yet Another Z180 (YAZ180 v2)

Testing on the YAZ180 v1 , shown below, is now complete. I don’t want to use it for further driver and platform development, because the PLCC socket for the 256kB Flash is becoming worn-out.

It will continue to operate as an augmented Nascom Basic machine, with an integrated Intel HEX loader (HexLoadr) supporting direct loaded assembler or C applications.


YAZ180 v1 at full configuration.

The new PCB for the YAZ180 v2 has been ordered.

These are some screenshots of the new PCB.


Pi Day, March 14 2017.

After dwelling on the fact that the V2 PCB was really just a clean up the V1 PCB, with no additional features, I decided not to build the beautiful new PCBs that arrived today.

But rather, to create a new PCB with additional features.

New Features

When I originally designed the YAZ180 the breakout for the 82C55 was simply an interim design, to enable me to test the board. I was thinking of making an Arduino style pin-out, or something along those lines. But this is something much better.

Recently, after reading Paul’s page on interfacing an IDE drive to an 8051 microprocessor with the 82C55, I decided that adding IDE to the YAZ180 was a must-have feature.

So there is a new connector on the YAZ180 to break out the 82C55 pins, in IDE 44-pin 2mm format. I have not followed the design provided by Paul exactly. I’d note that his design and the earlier design by Peter Fraasse were specialist designs, which don’t support the generalised usage of the 82C55 chip, beyond the IDE functionality.

By the above statement I mean that in Mode 1 and Mode 2 for Port A and Port B, the PC2, PC4, and PC6 pins of the 82C55 device are designated registered strobe input pins /STB in input mode, or peripheral acknowledge /ACK in output mode. If an inverting output buffer is connected on these lines, then the registered input and output mode capability is lost. This would restrict the functionality of the 82C55 to simply Mode 0, being the mode that is used to create the IDE functionality.

As I’ve connected the three IDE address selection pins to PC2, PC4, and PC6, and these pins are not passed through an inverting buffer in the design, it is possible to use the 82C55 in any of its modes, and therefore to use the IDE 2.5″ 44-pin form factor to connect the YAZ180 82C55 ports to extension PCBs of any type or design.

As a connected IDE drive or other extension board may need to interrupt the CPU, I have connected the IDE INTRQ pin to the remaining inverting buffer to provide an input to the CPU on /INT0. As the /INT0 (or actually the INTRQ) input terminates on the IDE header, either a IDE drive through INTRQ, or either of the two 82C55 INTR pins, PC3 or PC0, can originate the interrupt.

I have reconfigured the Am9511A-1 to use the /NMI interrupt, as previously the /INT0 was configured.

The new YAZ180 V2 PCB has been ordered. YAZ180_V2_Schematic.

Happy Pi Day.

Characterising Am9511A-1 APU

I’ve built a Z180 based board, supporting the AM9511A APU. Partly for historical enjoyment. Partly because it is actually still faster than “modern” Z80 devices.

For example, from the Am9511/Am9512 Floating Point Processor Manual by Steven Cheng, we have comparison tables. On average the Am9511A APU (at 1.966MHz) produces a hardware floating point divide in 165.9 cycles (of a 2MHz 8080 processor). Converted to my Am9511A implementation (at 2.304MHz), we have the equivalent in 141.5 cycles of the 2MHz 8080. Converted to best case modern Z180 terms (overclocked to 36.864MHz) this is 2,609 CPU cycles to return a hardware floating point divide.

To produce an equivalent software floating point divide, using the LLL floating point library, requires 13,080 cycles.

This means that floating point on the 40 year old AM9511A APU is still 5.0 times faster than an overclocked Z180. Sweet!


I’m integrating an Am9511A-1 APU device into my YAZ180 build. The basic device is capable of operating at 3MHz. But, I’ve found that driving one sample at 3.072MHz doesn’t work. But, it works fine at 1.536MHz.

This one example has 83.33ns delay between RD or WR and PAUSE signal being operated. This means that it should comfortably operate with the minimum of one wait state when the Z8S180 is running with a 18.432MHz bus.

And, I’ve got to say that these devices run hot… OHS issue hot. There is a reason they are provided in a ceramic package. They sink 70mA at 12V plus another 70mA at 5V, and all that energy has to go somewhere.

The timing for the Am9511A-1 is generated by dividing the Z8S180 18.432MHz system clock by 6.  The divide FCPU by 6 for FAPU CLK 3.072MHz is done with a SN74LS92N device. And for test purposes, the Z8S180 is also half rate clocked at 9.216 MHz, producing 1.576MHz for the APU.


YAZ180 Test Rig – Am9511A-1

The test is initially pretty simple. Will they properly push and pop data at 3.072MHz?
If not, then I’ll need to redesign the YAZ180 to operate the Am9511A-1 at a lower APU clock. But, if my initial samples are just not the up to specification, then they can be secondary devices.

Ok, now what kind of devices have I got to hand, and lets see what the results are…

Front Serial ID Country & Rear ID 3.072MHz 1.576MHz
239KH2Z Malaysia 9237FP Fail Pass
301MBZP Malaysia 9252CP Fail Pass
3368YF8 Malaysia 9335CP Fail Pass
348W76S Malaysia 9347DP Fail Pass
921BDIV Philippines 8917WM Fail Pass

Kind of boring really. Pretty clear that AMD oversold the capability of its Am9511A-1 to run at 3MHz. Or, I’m not feeding them with the correct timing.

I’ll need to redesign the YAZ180 to provide a divided by 8 clock at 2.304MHz, and cross my fingers that they work at that clock rate.

There will be more here as I test different versions. Perhaps, one day I’ll find a magical device that will run at 3.072MHz…?

Freewrite – First Thoughts

Having taken delivery of a beautifully sculpted object, the Freewrite, I believe it was worth the wait, and the money, in every way. I’ve built both hardware and software, and I know that it is unbelievably difficult to get a piece of hardware (including aluminium, plastic, PCB, components, assembly, testing, and the list of items goes on) constructed and shipped globally. Rightly, getting this construction process completed has been the focus of the Astrohaus team over the past year.

freewrite crop

During the many months of waiting for delivery of my Freewrite, I have purchased Cherry mechanical keyboards from both WASD and Das Keyboard (costing US$150 and more), and I use them during my day to day work and pleasure. There is nothing even remotely close to the pleasure of having Cherry MX switches under your fingertips (I prefer Clear and Brown varieties).

These mechanical keyboards can work with a Chromebook (costing US$300 or more) to produce a functionality similar to the Freewrite. But, then there are two pieces to be transported, and the whole package is not a single writing tool. Spending the time to juggle the pieces of the drafting package leads to further distractions, and the result is not substantially cheaper than the Freewrite.

The Astrohaus software game has been mediocre, as this article notes. Hopefully, getting the hardware out the door will allow them to focus on the many minor software shortcomings. As it is just software, it should be easy for them to provide OTA upgrades of the Freewrite to remedy the situation.