tl;dr: TIL cheap "A4988" stepper drivers are probably HR4988s. I don't know where the HR4988 silicon falls between theft and compatible clean-sheet design. The datasheet is appallingly copy/pasted. Like really astonishingly appalling.

I've been using "A4988" stepper driver modules of the 0.6" x 0.8" 16-pin DIP sort pioneered by Pololu, but as re-designed by Joe Mosfet who openly published his version as (one quarter of a) "StepStick". High volume, low cost board houses churn these out for a price that's not very much different from free which makes them attractive for personal tinkering where vague provenance is acceptable.

Considering their origins, I wouldn't be surprised if the driver ICs didn't actually came from Allegro. Less euphemistically: I'd be surprised if they did.

TIL HR4988

I've just learned that HR4988 exists, and that essentially all of these cheap "A4988" units use HR4988 driver ICs. There are datasheets in Chinese from 深圳市永阜康科技有限公司 (Shenzhen Yongfukang Technology Co., Ltd.) and in English from Heroic Electronic Technology (Jiaxing) Co., Ltd.

The Chinese version (only):

• comments on A4988 compatibility
• describes additional microstepping options up to 1/128.

The English version remains silent on those points.

On one hand, I would like to acknowledge that giving their compatible part a distinct part number, a datasheet, and even some extra function smells more like commerce than counterfeiting.

...but...

On the other hand their datasheets are just the most appalling Junior High-level rip-off of Allegro's datasheet. Really. Apart from half a page of red-lettered "Cliffs Notes", the text and tables are either essentially all or really entirely ^A^C^V directly copied verbatim from Allegro's datasheet. With minor variations of appearance like different font, page layouts and pagination. Beyond that, all the diagrams and graphs are copied exactly. But not exactly exactly: where they could have at least given Allegro the respect of copying their vector graphics, instead they apparently just captured screenshots and copied Allegro's vector graphics as degraded raster images. The only authorial modification being to type over "A4988" with "HR4988" in the middle of the "typical application diagram", and add an incorrect minor note, in a non-matching font. Really. It's that bad. (I'm probably missing a detail here or there but not enough to blunt the point.)

Aside from the bald plagiarism, one thing the reader can know with confidence is that the graphs which show actual scope traces (right Allegro?) vs diagrammatic representations absolutely did not come from any HR4988 part.

how did that happen?

I don't know much about making chips. I don't know how much of the HR4988 silicon design is rip-off vs. original design of a work-alike part. Either way, I figure getting together whatever they had to send to a chip foundry to get working chips must take non-trivial knowledge, effort, and resource. How could they have been able to get that done and not have the wherewithal to come up with a datasheet by some means other than blatant verbatim copying? At least they could have said "it works like an A4988; configure extra microstep options like this otherwise refer to Allegro's A4988 datasheet because we did such a great job of not deviating from any of that."