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.

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YAZ180 v1 at full configuration.

The new PCB for the YAZ180 v2 has been ordered.

These are some screenshots of the new PCB.

Update

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.

Update – RetroChallenge Day 1

I’ve decided to enter the RetroChallenge 2017/04 and my challenge is to read and write to an IDE drive using the newly configured IDE interface on the YAZ180v2. But before I can write the code for the IDE interface, there’s a bit of building and testing that needs to be done.

The new PCBs arrived a few days ago, and they look great. But Arduino Day and the first day of the RetroChallenge 2017/04, 1st April, seemed like a good day to lay them out.

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New PCBs. 2oz Copper, 2mm thick. Opulent.

I was hoping to lay build several boards at once, but somehow I forgot that there was only one RAM and one FT245 device in my component stocks. That means that I had to satisfy myself with just one board for now.

Note the suitably Retro PowerMac (circa 2001) driving the layout guide screen.

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Adhoc Workspace

This is the board just before cooking. Respect to anyone who notices the substantial noob layout mistake. Anyway, after a small smokey explosion, everything was rectified.

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Two YAZ180 versions, side by side.

This is the finished build of the YAZ180 v2. Looks very tasty. Retro goodness.

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Fully populated YAZ180 v2 PCB.

I’m still working on fixing an issue with my code, which I noticed when experimenting with the Am9511A APU, and inserting an Interrupt Jump Table. Basically, I’m getting jumps to odd or  random locations, which is detected buy filling unused locations with 0x76, the HALT, OpCode. The most common address where the HALT is executed at is 0x00C3.

Previously, I’d been filling unused locations with 0xFF, the RST 38H OpCode shared with the INT0 location 0x0038, which was causing the APU to be triggered inappropriately. This issue has me snookered. I can’t move on, in the software sense, until it is resolved .

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Update – RetroChallenge Day 8

Well this week was one of the most frustrating weeks ever, in terms of time spent vs. results obtained.

There are two major projects in hand. 1. Getting the YAZ180 V2 running, and 2. resolving the software issue plaguing my initialisation code.

Hardware issues

Bringing up a new piece of hardware is never easy. Initially, nothing can be trusted to work, and everything needs to be checked against the design, and then even the design checked for correctness. Bringing up the YAZ180 V1 was very time consuming, because I had to develop the PLD design during the process, as well as checking that all the hardware was sorking as it should. I thought that bringing up the YAZ180 V2 would be easy. Just solder it together and win. But it has not been so simple.

Essentially, after a week of working on this every evening, I don’t know why it is not working correctly. All the standard things, volts, clock, stuck address and data lines, etc are all working correctly. But it still doesn’t work. And, it may not be just one thing that is wrong, but if anything is not perfect it just won’t work.

After a few days of testing, I found that I’d programmed the PLD devices with an old version of the CUPL code. Nearly right, but not exactly right. Once I’d isolated that issue, by ensuring the new GAL devices worked perfectly in the V1 board, I thought it would be enough. But no. There’s still something wrong.

My current thought is that somehow, either electro-static damage or heat damage, the RAM is unreliable. But, I’m not sure enough of this to unsolder the RAM device and replace it. I’ll be spending this weekend on resolving this problem.

Software issues

Because of the effort I’ve been putting into resolving the hardware issues, I’ve not been able to solve the software issue apparent in the YAZ180 initialisation and serial code. I’ve documented the issue on Github.

My lesson learned is NOT to fill unused memory with 0xFF bytes. This causes RST 38H jumps to the INT0 location when the PC is incorrectly loaded, and can be very distracting. Best to fill unused memory with either 0x76 HALT bytes, to see where things became broken, or with 0xC9 RET bytes to just float over the underlying issue.

I’ll need to fix this properly, but it has consumed several weeks of effort, and I’m not much closer to resolution.

Update – RetroChallenge Day 10

The weekend was unkind, but today some new eyes (literally) have brought successes.

Hardware Issues

After doing quite a bit of further testing, I’m fairly sure that I’ve damaged the RAM and will need to replace it. So, I’ve ordered a hot-air solder gun. Should have had one for a long time. Finally, I’ve got a round-‘tuit. I’ll have to order some replacement components too, which will result in being able to make additional boards as well.

Software Issues

Finally, I’ve resolved my issue. What we had here was a classic “failure to understand”. Somewhat embarrassed to leave this here for Internet eternity.

  • Z80 vectors are supported by a JUMP table.
  • Z180 vectors are supported by an ADDRESS table.

Insert JP instructions into an address table and you will have a very very bad day.

Or in my case, quite a few of them.

This issue cost so much time. But at least on the up side, I’ve written robust Z80 and Z180 vector tables, improved my ASCI code, and cleaned up initialisation code, in trying to track this down.

Also finally, I now understand. Which is the entire point, anyway.

Update – RetroChallenge Day 17

Following up on the success of last weekend, I was hoping to have a lot of achievement to write about today. Unfortunately, it has been a grind this week too.

I have been distracted back into the original project that unearthed my previous software problem, and led me along the path to getting a much better understanding of the Z180 CPU, and then solving the issue. The original project was building an interrupt driven driver for the Am9511A-1 Arithmetic Processing Unit.

I’ve spent pretty much the past week on this code, and digging through it with a fine tooth comb. I’m now of a belief that my Am9511A driver code is correct, but my hardware is not correct and may never be correct.

The issue lies with the requirement for the Am9511A to have the Address lines and Chip Select signal remain valid for 30ns following raising of the Write signal. Unfortunately, the Z180 only maintains valid address lines for 5ns following Write. This means that writing to the AM9511A APU is very much a hit and miss affair, with miss being the most likely outcome. I’m still thinking about ways to bodge this to work. But, I think that it may just be too hard to get the old APU to work with a modern CPU. More on this later.

This week I’ll be working on the PaulMon IDE code, and migrating it from 8051 to Z80 nomenclature, and trying to get it to compile.

Update – RetroChallenge Day 21

Well the last couple of days have been exciting, as I found a way to make the Am9511A APU work. A hint from a fellow competitor (on working with the MC6809 CPU) inspired me to look further for information on options to fix the hardware interface.

The Z180 E CLOCK

The Z180 has an almost undocumented feature, called the E Clock. Yes, it is documented in datasheet that it exists, but there’s no real background that I can find as to why it exists, except that is for a Secondary Bus Interface. This pin and signal doesn’t exist on the Z80, for example. Anyway, since it has the same name as a signal on the MC6809, I thought it might be worth looking at it. It turns out that the E Clock provides a shortened version of the WR and RD cycles. Which is exactly what we need.

One caveat however, when running at doubled PHI rates (i.e. 1:1 PHI – CLK) the shortening of the E Clock signal is not consistent. The minimum of 30ns between release of WR and CS is not always held. This means that we’ll need to keep the PHI at half CLK whilst using the APU. In practice this means that using the APU requires we cut the CPU clock by 50%, to ensure the trailing 30ns is not lost.

Anyway. Good news. The Am9511A is working.

Am9511A FDIV

Am9511A APU Floating Divide in 115us

The E Clock is not an inverted signal, so to generate the active low APU_WR signal we have to first invert it, then OR it with the WR signal. For the purposes of testing, I’ve got a little breadboard with a GAL on the side, but later I’ll build a new PCB and add in a SN74LVC1G97 little logic device to provide the APU_WR signal.

Am9511A FDIV PUPI

Am9511A APU FDIV PUPI command interval 128us

Am9511A FDIV PHI6 Cycles

Am9511A APU FDIV in 179 Phi/6 Clock cycles

So now we see the Am9511A APU FDIV floating point divide takes about 101us to 115us when running at 1.536MHz, or from the datasheet 154 to 184 clock cycles. In 101us, the Z180 CPU at 36.864MHz produces 3,723 cycles. To produce a floating point divide using the Lawrence Livermore Library requires about 13,080 cycles, according to the AM9511A Floating Point Processor Manual by Steven Cheng. Therefore, we are still substantially faster than antique software on a modern Z180!

Anyway, back to the IDE interface….

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!

Testing

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.

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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.