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Playback adapter for CD-ROM drives Ever wanted to use an old CD-ROM drive as a CD player for audio playback? Now you can do it, with this nifty CD-ROM Playback Adapter. It can control one or two CD-ROM drives and has an infrared remote control. A 16x2 line LCD screen provides track information and other data. W E HAVE OFTEN been asked how to interface a hard drive or computer CD-ROM drive to a microcontroller. This is an interesting question, since there are countless old CD-ROM drives out there that are still in perfect functioning order but they are “obsolete”. Instead of letting them end up in landfill, you could do your bit and build this project. 40  Silicon Chip As well, this project will be good experience for those readers who wish to learn more about the ATA interface and who want to use hard drives and CD-ROM drives in their own projects. The interface can be easily modified to suit any other micro and only requires a few I/O ports and a reasonably fast processing core. The main features of the Playback Adapter are listed below: (1) Can connect up to two ATAPI CDROM drives. (2) Auto detection of up to two connected drives. (3) Plays your favourite CDs. (4) Random play and repeat modes. (5) Controls volume (16 levels) and balance digitally. (6) Remote control with user-selectable key definitions. (7) Works with any RC5 remote control. (8) ISP (in-system programmable) if you wish to experiment with the firmware. (9) The CD is automatically locked when playing (10) LCD screen. Accessing an ATAPI device The CD-ROM Playback Adapter siliconchip.com.au By MAURO GRASSI /CS1 /CS0 A2 A1 A0 1 0 0 0 0 Data /RD Data /WR 1 0 0 0 1 Error Features 1 0 0 1 0 Sector Count Sector Count 1 0 0 1 1 Sector Number Sector Number 1 0 1 0 0 Cylinder Low Cylinder Low 1 0 1 0 1 Cylinder High Cylinder High 1 0 1 1 0 Device/Head Device/Head 1 0 1 1 1 Status Command 0 1 1 1 0 Alt Status Device Control Table 1: the ATAPI register file. All ATA and ATAPI devices are controlled by reading and writing to these registers. Interfacing to an ATAPI device is simple because most of the work is done inside the drive. In effect, it acts as a black box. It conforms to a standard and the internal implementation is left to the manufacturer. That is why the standard was originally called IDE (integrated drive/device electronics). It just means that a lot of the complexity of the interface is in the drive and the drive responds to a uniform set of commands. It speaks well of the design that one of the easiest parts of a computer to get working is the hard drive or CDROM/DVD drive. Plug any drive into any motherboard and it will usually work first time. Overview of the ATA interface presented here lets you connect one or two drives and control each independently using a standard RC5 remote control. CD-ROM drives that conform to the parallel ATA (AT attachment) standard can be used with the adapter and most old drives fall in this category. In fact, most CD-ROMs will be ATAPI devices, which is a superset of ATA. It just means they support the packet interface, a feature that was added to the original ATA interface. The resulting protocol was renamed ATAPI, with the ‘PI’ standing for packet interface. Most CD-ROMs, as well as DVD drives, are ATAPI devices, although others conform to different standards, like SCSI and SATA (serial ATA). These have a different connector and are not compatible with this project. siliconchip.com.au The ATA interface is register-based. There are essentially two banks of eight registers, although only one of the eight registers in the second bank is ever used. The interface consists of two chip select lines, called /CS0 and /CS1 which are active low. There are three address lines designated A0, A1 & A2, as well as the read and write control lines. The latter are designated /RD and /WR respectively and are also active low. In order to access a register, one sets the register address given by [A2:A0] and then brings either /CS0 or /CS1 low (but not both). Then it is a matter of bringing either /RD or /WR low and reading or writing the data through data port D7:D0. Note that the data bus width is actually 16 bits but for accessing the ATA registers, only the lower eight bits are used. However, the full width of the bus is used for data transfers. Note also that the name of the register sometimes changes depending on whether you are reading from or writing to the specific address. For example, at address 110b and with / CS1 low and /CS0 high, reading will give the Alternate Status register (a read only register), while writing will affect the Device Control Register (a write only register). All commands to control the drive are sent through the register file (ie, the set of ATA registers). For example, the Command Register can be written with the opcode for a particular operation – eg, “SLEEP” - and the drive will respond by going into power saving mode, barring any errors. Note: the order in which you assert the control lines on the ATAPI/IDE bus is important. For example, you would think that you could assert / RD or /WR first and then bring /CS1 or /CS0 low. However, this approach does NOT work on all drives. The correct procedure is to assert / CS1 or /CS0 first, then to assert either /RD or /WR. Of course, because we are using a general-purpose micro and these pins are on different ports, it is impossible to assert them simultaneously. This is not required however, but would be closer to a native IDE/ ATAPI port interface. Low level drivers It is relatively simple to write to an ATAPI device. As explained, you first prepare the data and the address, bring the chip select line low and then apply either the read or write signal. This is the minimum you would need to interface to an ATA device like a hard drive. It would not be the fastest interface possible – you’d have to get November 2007  41 1 3 5 A3 37 39 A1 33 A0 35 29 31 27 21 23 25 D2 13 D1 15 D017 19 D5 7 D4 9 D3 11 1 D7 3 D6 5 D4 11 D613 15 D2 9 D0 7 A  40 34 36 A2 38 A4 30 32 26 28 22 24 12 D12 14 D13 16 D14 18 D15 20 10 D11 2 4 D8 6 D9 8 D10 16 12 D5 14 D7 8 D1 10 D3 4 6 2 IRD1 CON1 CON2 2  3 +5V 8 14 1 5 8 10 F 100 (A0–4) (D8–D15) IC2: 74LS00N IC3: 74LS04N (D0–D7) 51 8 S3 1k +5V 9 IC3d Vdd 40 12 17 4 3 2 1 4 3 2 1 CON4 CON5 PD2/INT0 PD7 Vss 20 +5V +12V +5V +12V XTAL1 XTAL2 19 18 7 6 9 29 11 10 8 31 30 100nF PD5 14 PD4 D7 32 PA7 D6 33 PA6 D5 34 PA5 D4 35 PB7/SCK PA4 D3 36 PA3 PD0/RxD D2 37 PA2 D1 38 PD1/TxD PA1 D0 39 PA0 16 PD6 13 PD3/INT1 D15 28 PC7 RESET D14 27 PC6 D13 26 PC5 IC1 D12 25 PC4 ATMEGA8515 D11 24 PC3 D10 23 PC2 D9 22 PC1 D8 21 PC0 A4 5 PB4 A3 4 PB3 PB5/MOSI A2 3 PB2 A1 2 PB1 A0 1 PB6/MISO PB0 15 100nF 2 IC3a 470  LED3 +5V 22pF X1 7.3728MHz +5V A K 3 47 F 2 1 22pF 1 7 IC2a 14 6 3 K  A 4 K  A 1 F 1 F 1 F 1 F GND OUT IN REG1 7805 7 IC3b 470 LED2 S1 5 LED1 470 47 F IC3c 100nF 15 D1 T2o 7 T1o 14 R2in 8 R1in 13 5 SC CD-ROM 2007 100nF S2 10 T2in 11 T1in 9 R2o 12 R1o 3 4 1 IC4 MAX232 6 16 T2o 7 T1o 14 2 10 T2in 11 T1in R2in 8 9 R2o 5 4 6 R1in 13 15 IC5 MAX232 16 12 R1o 3 1 2 CON6 1 F 1 F 9-12V DC  5 4 3 2 1 CON3 DB9F K 9 8 7 6 +5V A K OUT 1 2 3 IRD1 LEDS GND IN 7805, 7812 470 LED4 PLAYER ADAPTOR 1 F 1 F +5V 1 F 1 F A Fig.1: the circuit uses an Atmel ATmega8515 microcontroller (IC1) to interface to the CD-ROM drives (via CON1) and the LCD module (via CON2). IC4, IC5 and CON3 are optional, to provide in-circuit programming for the microcontroller. +5V 470 LED5 LCD CONTRAST VR1 10k K 42  Silicon Chip siliconchip.com.au Fig.2: the Error Screen. The numbers give information about the state of the program and the drive when the error occurred. involved in DMA (direct memory access) for that – but it would work. With ATAPI devices like CD-ROM drives, most operations are initiated by writing packets rather than single byte commands. A packet is a string of 12 or sometimes 16 bytes that are sent to the drive in sequence. In order to send packets, a more involved algorithm than just writing to the register file is needed. Here you have to worry about bus timings and whether the drive is busy or requesting data. There is a well-defined protocol for PIO (peripheral input output) access to an ATAPI device. Feedback is provided by the bits BSY (bit 7) and DRQ (bit 3) in the Status register, which can be polled to determine the current state of the drive. When the drive shows BSY=1 it does not respond to commands and reading any register except the Status register is undefined. In other words, the only valid information that can be read from the drive is bit 7 (BSY) of the status or alternate status registers (when it is busy). As an example, the packet to open the tray of the drive is given by the 12-byte string: 0x1B, 0x00, 0x00, 0x00, 0x02, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00. To send this packet, you first send the PACKET command 0xA0 to the command register and then follow the packet protocol, as outlined in Flowchart 1. The protocol begins with a packet being written to the drive. Optionally, there may follow a data read or write transfer, depending on the packet written to the drive. The CoD (command/data) and IO (input/output) bits are in the Sector Count ATA register (also known as the Interrupt Reason Register in ATAPI devices). CoD is bit 0 and IO is bit 1. When CoD is 0, data is being transferred and when it is 1, a command (packet) is being transferred. The IO bit indicates the direction of transfer. When IO is 0 the host writes to the drive and when it is 1 the host siliconchip.com.au Pin Name Description 1 /RESET A low level on this pin resets all connected drives. 2, 19, 22, 24, 26, 30, 40 GND All these pins are connected to the common ground plane. 3, 5, 7, 9, 11, 13, 15, 17 [D7:D0] 3=D7, 5=D6 . . . . . . 15=D1, 17=D0 These are the eight least significant bits of the data bus. 4, 6, 8, 10, 12, 14, 16, 18 [D15:D8] 4=D8, 6=D9 . . . . . . 16=D14, 18=D15 These are the eight least significant bits of the data bus. This pin is not connected and is used to prevent the cable being connected the wrong way around. 20 KEY 21 DDRQ 23 /WR Write strobe, active low. 25 /RD Read strobe, active low. 27 /IOREADY 28 ALE Address latch enable – not used in this project. 31 IRQ Interrupt request – not used in this project. 32 IO16 Obsolete since ATA-3 – not used in this project. 33, 35, 36 [A2:A0] Data request pin – not used in this project. Device ready pin, active low, not used in this project. Used to slow a controller if it is too fast for the drive. 35=A0; 33=A1; 36=A2. Address bus 37 /CS0 Chip select 0 38 /CS1 Chip select 1 39 /ACT A low level on this pin indicates that the drive is working. It can be connected directly to a LED to show drive activity. Table 2: this table shows the pin-outs of the ATAPI interface. Note that this project leaves many pins usused, as they are unnecessary for PIO transfer. reads from the drive. When the above packet has been successfully processed by the drive, it will respond by opening the tray. This packet does not require any extra data transfer but other commands, such as reading the CD TOC (table of contents), do require reading from the drive. Other packet commands such as setting the volume require both reading and writing to the drive (refer to the ATAPI specification for the relevant packet codes). The firmware The main component of this project is the firmware, as the hardware is little more than a Atmel microcontroller. The firmware is responsible for interfacing to the drives, decoding the remote control signals, auto detecting the connected drives and controlling playback and volume, among other things. All this is done with only 512 bytes of RAM! The firmware size is approximately 7.2KB and fits inside the micro’s 8KB flash memory. ATA and ATAPI commands are either “Mandatory”, “Optional” or not supported. To make sure that the CDROM Playback Adapter works with just about any ATAPI device, we’ve used only “Mandatory” commands as per the specification (rev 2.6 1996). Note, however, that we cannot guarantee that it will work correctly with all ATAPI devices. Some are buggy and the standard covers a period of many years. We’ve also come across drives that don’t conform to the standard in every detail. In our case, we tested the adapter with seven different ATAPI devices, including both CD-ROMs and DVD drives, and it worked correctly with six of these. The seventh drive had a problem in that it was not detected by the firmware and closer inspection and debugging revealed that the micro was unable to write to the drive’s register file. Thus it failed the first test of the auto detect subroutine, as explained below. Basically, if a particular CD-ROM or DVD drive is not detected by the firmware on start-up, it will not be functional with the adapter. In that case, try using a different drive. Conversely, if the drive is correctly detected, there November 2007  43 Flowchart 1: this is the packet writing routine used in the firmware. The interrupt signal INTRQ, intended for PCI buses on computers, is not used and a method of polling for DRQ and BSY is used in its place. 44  Silicon Chip siliconchip.com.au is a high chance that it will work correctly with this adapter. How it works Refer now to Fig.1 for the circuit details. The circuit is essentially just an Atmel ATmega8515 microcontroller (IC1) with its general IO pins configured to read and write to up to two drives. It also controls the LCD screen and reads the remote control sensor. Add in a power supply and a few support chips and that’s about it. ICs4 & 5 are MAX232 line drivers and are used to interface the microcontroller to the serial port of a computer (RS-232). These devices are optional and are only needed if you are planning to experiment by writing your own firmware. Basically, they allow the board to be connected to a PC’s serial port so that the microcontroller can be programmed in-circuit. The software to use for this job is called “Pony Prog 2000” and is free for download from www.lancos.com/ ppwin95.html ICs2 & 3 are simple logic gates, used here as “glue logic” for the interface. These devices are 74LS00 and 74LS04 quad NAND gates and hex NOT gates respectively but only one NAND gate and four NOT gates are used from these devices. Infrared receiver IRD1 is an infrared receiver module, containing a photodiode, amplifier, filter and demodulator all in a compact package. It accepts a modulated infrared signal on a 38kHz carrier and outputs a demodulated TTL level serial stream. This stream is fed to pin 12 of IC1 and is decoded by the firmware in the microcontroller. Note that IRD1’s output is usually high (around +5V) and varies as a square wave when an infrared input is received. S3 is used to select the remote control setup option at boot time. For normal operation it is open and this allows the signal from IRD1 to pass to the microcontroller for decoding the remote control signals. Conversely, when S3 is pressed, it temporarily pulls this line low via a 1kW resistor to allow remote control set-up. There are five indicator LEDs on the board. LED4 (red) is the power LED, while LED3 (orange) lights when the micro is being programmed or is in the reset state. This state can be entered siliconchip.com.au CD-ROM drives have three sets of jumper pins at the back to configure the drive. If you have just one drive, it can be configured as either a master (MA) or a slave (SL) using the jumper link. However, if you are using two drives, then one must be configured as a master and the other as a slave, as shown here. using switch S1. LED1 (green) shows the activity of the currently selected drive. Finally, LED2 & LED5 make a pair. Only one will be lit at any one time. LED5 (green) indicates that the MASTER device is being controlled, while LED2 (red) indicates that the SLAVE device is being controlled. If you have two drives connected, you may toggle between them using the Line-In button on the remote. Power supply In order to power the drives, you will need a power supply capable of delivering +12V at 2A and +5V at 2A (eg, a computer power supply). By contrast, the board requires a +5V supply and draws just 200mA. Basically, you’ve got two choices when it comes to the power supply. The first option is to power the PC board directly from a 9-12V plugpack supply and power the drives separately. In this case, the board supply is fed in via CON6 and is regulated to +5V using 3-terminal regulator REG1. Diode D1 provides reverse polarity protection, in case the supply is connected the wrong way around. The second option is to plug a +12V/+5V supply into either CON4 or CON5 on the PC board. The board will then be directly powered from this supply, while the supply for the drives can then be taken from the unused connector. Note that you will need a Y-splitter cable if there are two drives. In this case, you can use a surplus computer power supply to power both the boards and the drives. This will simply plug straight into either CON4 or CON5. Another option is to use a ready-made adapter like the Jentec JTA0202Y (from Taiwan). This unit supplies +12V and +5V at 2A each, which is enough to power two drives and the PC board. It also comes with the proper plug, so all you need then is a Y-splitter cable. Setting up the drives The two drives must be configured before being installed. Specifically, if you wish to connect only one drive, it can be configured as either a slave or master device. Usually this is accomplished by a jumper setting on the back of the drive. The drive will usually have a label indicating the appropriate position of the jumper. If you wish to use two drives, however, make sure that one is configured as a master while the other is configured as a slave. It doesn’t matter which is which as long as they are not both slaves or both masters. How auto-detection works Let’s now see how the micro detects any connected ATAPI devices at boot up. First, a simple test is done. The miNovember 2007  45 What’s A Finite State Machine? A finite state machine (also known as a finite state automaton) is a set of states together with a transition table and a designated state that is the “initial state”. The transition table can be thought of as a table with three columns and a finite number of rows. The first column corresponds to the current state, the second column corresponds to the input and the third column corresponds to the next state. These triplets (X, I, Y) are interpreted as follows: if the machine is in state X and an input I is received, it moves to state Y. While there is no input, the machine stays in its current state. For example, in our case, if the firmware is in the neutral state, and the user presses the Play key on the remote control, then the transition table dictates that the machine moves to the Playing state. This “rule” would be written as the triplet (“Neutral”, “PLAY”, “Playing”). The user interface of this playback adapter is simply implemented as a finite state machine, meaning there are a number of rules that make up the transition table. The machine begins in the “neutral” state, after a short initialisation. Flowchart 2 shows the finite state machine implemented in the firmware. The transitions correspond to arrows, while the blue blocks are the possible states. cro writes a known value to an ATA register and then attempts to read that value. If the value read is the same as that written, the auto detect subroutine goes to the next stage. Conversely, if the drive fails this test, it is assumed to be absent. Instead, 0xFF is returned as the value read due to internal pull-ups on all inputs (which incidentally, is not the value that is written). The next stage of auto-detection involves searching for the signature that all ATAPI devices are required to have (according to the standard). In fact, all ATAPI devices have a unique signature of 0x14 and 0xEB (notice that 0x14 + 0xEB = 0xFF) in the Count Low and Count High registers on start up. The inquiry command of the ATA interface, while mandatory for ATA devices, is actually aborted by ATAPI devices. Instead, the effect of this ATA command on ATAPI devices is to put the ATAPI signature word in the Count Low and Count High registers. What is mandatory for ATAPI devices is to support the ATAPI inquiry command. The algorithm for detecting the drives is as follows: (1) Perform a simple read-write test. Abort if this test fails, otherwise continue. (2) Select the drive by writing to the drive/head register. 46  Silicon Chip (3) Issue an ATA identify device command. (4) If the signature 0x14 0xEB in the Count Low and Count High registers is present, go to step 5. (5) If this signature is not present, the device is either absent or it is not an ATAPI device. Therefore, we may assume that no ATAPI device is present and terminate. (6) If the ATAPI signature is detected, we issue an ATAPI inquiry command to get further information about the drive and conclude that an ATAPI device is connected. The test is then terminated. Firmware operation Flowchart 2 shows the structure of the firmware. After initialisation, the program can optionally jump to a subroutine to set-up the remote control. This should be done at least once, preferably the first time the program is run. Once the remote control has been successfully set up, the adapter is ready to be used. The next stage in the firmware is the auto detection of the drives. Up to two drives can be connected and they should be configured correctly as master or slave as detailed previously. The firmware then enters a “finite state machine” by going to the neutral (or initial) state. It then listens for activity on the infrared port and responds to the remote control commands. There are three playing modes: (1) the default mode, (2) the repeat mode and (3) the random mode. In default mode, the adapter will play the current track and when that is done, will jump to the next track. In repeat mode, the adapter plays the current track and then repeats it over and over. This mode is indicated by the digit ‘1’ appearing as the last character of the first line of the display in playing mode. Finally, in random mode, the adapter will play the current track and then select the next track randomly. This mode is indicated by the letter ‘R’ appearing as the last character of the first line of the display in playing mode. In operation, the user can scroll between the default, repeat and random modes by pressing the ‘Record’ button on the remote control during play mode. The volume is controlled by the Volume Up and Volume Down buttons on the remote. Up to 16 levels ranging from muted (0) to full volume (15) can be selected. The ‘Mute’ button has the usual effect of storing the current volume and then setting the volume to 0. If pressed again when the volume is 0, the original volume level is restored. The percentage balance of the right and left audio channels can be modified by the user, by pressing the Channel Up and Channel Down buttons on the remote. The percentages range from 0-100% in steps of 5%. The volume for each channel is then calculated in terms of the balance using a simple formula: (1) Volume (Left) = (Balance Left)/100 x Volume Level (2) Volume (Right)= (Balance Right)/100 x Volume Level In playing mode, there are the usual control options like going to the next track (pressing the Fast Forward button) or to the previous track (pressing the Rewind button). You can also pause playing (by pressing Pause) or stop playing (by pressing Stop). The 20+ button on the remote can be used to either eject the CD or close the tray (depending on whether the tray is already closed or open). The LineIn button is used to switch between master and slave devices, if two drives siliconchip.com.au Flowchart 2: this flowchart shows the “finite state machine” implemented by the firmware. After a short initialisation which includes the automatic detection of connected drives, the firmware goes into the neutral state. From there it starts accepting remote control commands that change the state of the machine. Typical display readouts corresponding to each state are also shown. are connected and have been correctly detected by the firmware. The 0-9 number buttons are used to select a particular track number to play. Simply press the correct number siliconchip.com.au (which will be shown on the screen) and then press Play to play the selected track number. As you can see, the user interface has been kept deliberately simple and intuitive. By the way, you can use virtually any RC5-compatible remote control since you can assign the buttons during the set-up procedure (more SC on this next month). November 2007  47