Issue 150 January 2003
Construct an ATA Hard Drive Controller

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Start • The Firmware • Getting Fat • Sources and PDF

Do you remember your first computer with a hard drive? What about when 5 MB was a lot of hard drive space? Have you ever wanted to put something of your own on a hard drive without having to rely on somebody else’s expensive and proprietary hardware and driver code? Ever want to read, write, and control a hard drive with a microcontroller?

If you answered "yes" to any one of these questions, then this project is for you. I’ll show you how to build an ATA hard drive controller with a microcontroller and a few common parts. In addition, you’ll learn how to write simple code that will form the basis for deploying a stand-alone, networkable, microcontroller-based data storage system.

Even if you aren’t interested in communicating with hard drives, there’s something here for those of you who don’t need a microcontroller-driven hard drive controller. The bonus track is in the hardware. Do you need an in-system programming-capable test stand for an Atmel ATmega128 with RS-232, Ethernet, and 64-KB of external 16-bit memory? Well, this project is for you too. Hard drives or no hard drives, let’s get started.

THE HARDWARE

Hanging a standard PC or laptop hard drive from the 40-pin connector shown in Photo 1 is the reason why we’re gathered here today. I wanted the ATA hard drive controller’s electronics to be flexible. So, in addition to the standard RS-232 port, which is driven by a Sipex SP233ECT, I added 10-Mbps Ethernet capabilities with the RTL8019AS/ LF1S022 combination.

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Photo 1—The board is clean and simple. All of the supporting capacitors and resistors are SMT parts mounted on the opposite side of the board.

The ATmega128 has plenty of internal SRAM (4 KB), but I thought adding 64 KB of 16-bit external SRAM would be nice. Adding the SRAM is sort of like buying rope: you can always make the rope shorter, but it’s a pain to add rope later if you need it.

The ATmega128 has enough I/O structure to service the big SRAM with some help from a couple of 74HCT573 latches. As you can see in Figure 1, the external SRAM is attached to the ATmega128 in the standard manner. This allows those of you who aren’t interested in placing bits on a spinning piece of magnetically coated aluminum to do your thing with the big chunk of SRAM and the raw power of the ATmega128. With the SRAM in this configuration, the results of the hard drive I/O operations can be buffered by the external SRAM or operated on by the AVR directly.

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Figure 1—No surprises here, either. The craftiness of this design lies in the way the firmware utilizes the hardware resources.

The Ethernet interface, shown in Figure 2, is actually a wire-by-wire copy of the Packet Whacker microcontroller NIC. The original Packet Whacker firmware that was written for the ATmega series of AVRs is used in the ATA hard drive controller code, as well. The Packet Whacker code already has hooks to allow for the easy transmission and reception of the hard drive data.

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Figure 2—If this looks familiar, it’s because it’s actually a Packet Whacker that’s been melded into the ATA controller design. The Packet Whacker code was reused, as well; it can be found wound into the ATA controller source.

The RS-232 port has a dual purpose. Running at 57.6 kbps, it’s fast enough to spit out a sector’s worth of ASCII data to a terminal emulator for debugging. In addition, it can be used efficiently in an application to transfer data and commands between the AVR-based hard drive controller and a peripheral device. The 10-pin header makes assembling a serial cable easy, if you use 9- or 25-pin IDC shell connector parts and ribbon cable.

I’ve standardized with 10-pin male headers for all of the external ports with the obvious exceptions of the Ethernet and hard drive I/O ports. As long as you put the right connector in the correct header socket, using the 10-pin headers with the keyed shrouds eliminates the possibility of inserting the ISP and serial connectors incorrectly.

The 10-pin arrangement is also standard for most of the ISP dongles that support the AVR series of ISP-capable microcontrollers. I used a Kanda AVR ISP dongle and a version of the company’s ISP software to program the hard drive controller’s ATmega128. Because the dongle interface is dedicated to certain pins on the AVR and power isn’t transferred within the programming cable, I was able to keep the dongle attached to the hard drive controller throughout the programming and debugging process.

The ATmega128 is clocked at 14.746 MHz to keep the data rate error percentage at a minimum for the 57.6-kbps serial port. For this project, the ATmega128 I used was a 5-V part that can run at 16 MHz. The 14.746 MHz is the closest standard crystal to the maximum clock speed that will clock the big AVR with the least amount of serial data bit error rate.

The first spin of the ATA hard drive controller used a 2-mm header that mated directly to the 44 I/O pins found on 2.5² laptop drives. My experiences with the 2-mm parts were not good ones. The pins and connectors are fragile, and I really don’t like working with the 1-mm ribbon cable.

In the process of attempting to work at 2 mm, I purchased a gaggle of new surplus 2.5" Hitachi 540-MB drives. That purchase, as it turns out, was a good thing. After junking the 2-mm idea, I purchased some surplus 850-MB, 3.5" drives that turned out to be mostly junk. I never really had any inclination to put real data on them, so it’s not a total loss. At less than $10 per drive, what did I expect?

When I bought the laptop drives, I also purchased some 2-mm to 0.1² (or 2.5² to 3.5²) converter boards. The idea was to be able to attach the laptop drives to a PC for the debugging and verification of the ATA drive controller’s firmware and hardware.

The moral of the 2-mm hard drive story is that, thanks to my foresight, you’ll see how I brought the ATA hard drive controller to life with the 2.5" Hitachi drives and converter boards. This may sound funny, but when I was formatting the 3.5" 850-MB drives, I was hoping that a few of them would show some errors. I wanted to verify that the hard drive controller could detect them, and then show you what they looked like. I really didn’t expect them to be trashed so badly. So, the drive error examples will come from the 3.5" drives, and the good data examples will feature the smaller Hitachi drives.

The ATA hard drive controller requires a single 5-VDC power source. Also, the Hitachi drives require only 5 VDC. However, the larger 3.5" drives need 12 VDC in addition to the 5 VDC. The original spin of the hard drive controller used a 2-mm, 44-pin hard drive I/O attachment point. The extra four pins on the 2-mm connector provided 5 VDC and ground for the 2.5" drives right at the hard drive I/O connector. In this spin, the 44-pin, 2-mm pin set is replaced with the standard 40-pin 0.1" pin set, and there isn’t a power supply outlet at the hard drive I/O connector.

The hard drive controller is equipped with a standard 4-pin floppy drive power plug. As you might have figured out, the inclusion of a standard PC power connector on the hard drive controller allows you to power the 3.5" drive and the hard drive controller’s electronics from a common off-the-shelf PC power supply.

If the 2.5" drives are used, you’ll have to provide an attachment to supply power to the extra I/O-based power pins on the drive. That’s where the 2.5" to 3.5" drive I/O adapters come in. The adapters I purchased have a 3.5" drive power connector that has only the 5-VDC lines tapped into the 44-pin, 2-mm drive connector. The drive converter board allows you to use the smaller 2.5"² drives with the standard 40-pin 0.1" cables and a PC power supply. Although using a commodity power supply is the easiest way to go, any other suitable power supply method will work just as well. Photo 2 is a shot of an Hitachi 2.5" drive and its associated converter board attached to the ATA hard drive controller.

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Photo 2—The 540-MB drive formats quickly and is easy to handle even with the converter boards attached. This is made for quick turnarounds in the initial development stages when I was experimenting, debugging, and doing a lot of hard drive formatting.