Eric Miller ECE 441, Group 3 11/21/04. Controlling an ATA Hard Drive with a Microcontroller

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1 Eric Miller ECE 441, Group 3 11/21/04 Controlling an ATA Hard Drive with a Microcontroller

2 The AT Attachment (ATA) interface is designed for transferring data between a storage device and a computer (or microcontroller). It defines everything from the physical interface, such as the cables and voltages, to the command protocols, such as the way data is read and written to the device. Figure 1, below, may help explain where the ATA interface is located. Fig. 1: Block Diagram describing the location of the ATA Interface. As you can see, the ATA interface governs the signals between the computer/microcontroller and the hard drive. Specifically this paper addresses only the most essential aspects of the ATA interface. The ATA interface can be broken up into the following categories, which this paper will cover: Pins: General descriptions. Registers: Utilizing the hard drive s registers. Commands: How to Read, Write, and Seek Pins: General descriptions. There are a total of 40 pins for the standard ATA interface: some are used for transferring raw file data, others are used to send commands and return status flags. Below, Figure 2 illustrates an ATA/IDE (Integrated Device Electronics) connector. The picture shows the orientation of the input/output lines; pin 1 can be seen in the lower left and pin 40 in the upper right. The power 1

3 connector is also defined by the ATA standard. Additionally, jumper settings allow the user to select different modes of operation that this paper does not discuss. Notice that pin 30 of the input/output lines is missing; this prevents the ATA/IDE ribbon cable from being plugged in upside-down. Fig. 2: The backside of a hard drive, showing a power connector, jumper settings, and I/O lines. Below are descriptions of the more important pin sets, the ones that must be used for basic operation. Chip Select (CS0:1) - Selects the command block or control block registers. Device Address (DA0:2) - Selects the storage data or one of the registers. I/O Read (DIOR) - Reads data from the storage device. I/O Write (DIOW) - Writes data to the storage device. Reset (RESET) - Resets the device. Device Data (DD0:15) - Bidirectional data bus for reading from or writing to. Each of these pins will need to be connected to a port of the microcontroller. Most other pins can be grounded as they are unnecessary. Software will be necessary to control these pins correctly. Keep in mind each of these pins has signal timing requirements that will not be mentioned in this paper; consult the ATAPI Standard for this information. 2

4 Registers: Utilizing the hard drive s registers. A hard drive has numerous registers that allow the host computer/microcontroller to send commands and access different bits of information. For example, the host computer may wish to change sectors of a hard drive; to do this the host must write the sector number into the Sector Count Register. Below are brief descriptions of the more important registers. Command Register (W) Takes a command from the host and executes it. The most basic commands are discussed in the next section. Data Port/Data Register (R/W) This is the 16 bit data line. All files are read from the hard drive and wrote to the hard drive through this line. Device Register (R/W) & Features Register (W) Holds information specific to the command being used at the time. Device Control Register (W) Allows the host to reset the hard drive and enable/disable the interrupt signal. Sector Count Register (R/W) The host can write to this register Status Register (R) Contains the hard drives current status which includes the Busy bit (BSY), Device Ready bit (DRDY), and Error bit (ERR). The Device Ready bit indicates when the hard drive is ready to accept a command, the Busy bit shows when the device is waiting for a command to complete and the error bit shows when a command has failed. Commands: How to Read, Write, and Seek? Commands are an important aspect of the ATA interface. Some of the commands allow the host to read, write, seek (move to a location on the hard disk), sleep, and idle. However, optional commands like sleep and idle are not discussed in this paper. To issue a command, the host must first load the 3

5 appropriate registers with the correct values/parameters and then send the command. For example, the Read Multiple command takes several inputs including the number of sectors to read, the address to read at, and the device to read from. The command has several outputs as well including all of the bits in the Status Register mentioned earlier (e.g. the BSY, DRDY, and ERR bits). Below are descriptions of some of the more common commands. Read Multiple Transfers data from the hard drive to the host over the 16 bit data bus until the device has reached the specified sector count. o Inputs Sector count, Address, Device. o Outputs Data Status register, Data bus. Write Multiple Writes the number of specified sectors to the hard disk. o Inputs Sector count, Address, Device, Data bus. o Outputs - Data Status register. Seek Moves the drive header to the specified location on the hard disk, allowing the host to provide advanced notification in anticipation of a particular request for data. This can help speed up reading and writing in many cases. o Inputs Address, Device. o Outputs - Data Status register. As you can see the parameters between Read, Write, and Seek do not change very much. Each of these commands requires an address pointing to a location on the hard drive and the device number. The read/write commands also require a sector count number specifying the amount of data to read or write. Each command returns a status flag, signaling whether the device is busy, done, or has failed. 4

6 Conclusion: By now you should understand the basics of the ATA interface which included the following: What the different pins do. How to utilize the hard drive s registers. How to use the read, write, and seek commands. With this information you should now know how to control an ATA hard drive with a microcontroller. More detailed information on the ATA interface, such as other registers and commands, can be found in the ATAPI standard. 5

7 References 1) Davis, L. (2004). Ide, ata bus interface description and pinout. retrieved Nov. 22, 2004, from interfacebus Web site: 2) M.T., LoBlue. "Surveying today's most popular storage interfaces." Dec 2002: ) McLean, P. (2001). AT Attachment with Packet Interface. 6 ed. Longmont, CO: T13. 4) Tyson, J. (2001). How ide controllers work. retrieved Nov. 22, 2004, from Howstuffworks Web site: 6

University of California at Berkeley College of Engineering Department of Electrical Engineering and Computer Science

University of California at Berkeley College of Engineering Department of Electrical Engineering and Computer Science EECS 150 Fall 2001 Mike Lowey Project Checkpoint 2 CD-ROM Drive Control ATA/ATAPI CD-ROM