8. Completion of the PCM2702 Product
The development of SpAct brought about success in the product (PCM2702) development as well. The THD+N is less than 0.002%. The PCM2702 distortion characteristics are shown in Figure 18. This is very close to the theoretical minimum of 0.0015% for a 16-bit, 48 kHz, stereo signal, and must be the top level for a USB DAC that tracks at 1 kHz.
Figure 18a. Distortion Performance for the PCM2702/2703
Figure 18b. Distortion Performance for the PCM2702/2703
Figure 18c. Distortion Performance for the PCM2702/2703
Figure 18d. Distortion Performance for the PCM2702/2703
Figure 18e. Distortion Performance for the PCM2702/2703
Figure 18f. Distortion Performance for the PCM2702/2703
Figure 18g. Distortion Performance for the PCM2702/2703
Figure 18h. Distortion Performance for the PCM2702/2703
The A-weighted dynamic range reaches the theoretical value of 100 dB, and the signal-to-noise ratio is over 105 dB. The appropriate FFT is shown in Figure 19.
Figure 19. FFT Measured from PCM2702
Figure 20. PCM2702 Application Circuit Example
Also, Figure 20 shows the basic application circuit. As seen from the figure, the PCM2702 requires only a 12 MHz crystal and a few capacitors and resistors as external circuitry.
Take Care in Laying Out the Power, Ground, and Crystal
In order to achieve a low-distortion, high SNR design, please take extra care in laying out the analog power and ground, and the crystal oscillator circuit. First, a +5V analog power supply is needed. By using a separate regulator from that used in the +3.3 digital power supply, the interference can be minimized.
Take care to avoid a common impedance on the ground. Use short, fat traces for ground. It's a simple circuit so shouldn't be too hard. As for the area around the crystal oscillator, it is best to use short fat traces here too. It is well known that interference tuning of C1 and C2 in Figure 20 will be required, depending upon the crystal oscillator.
While this may be redundant, the gain of the oscillator depends upon the ratio of C2/C1. It is easy to design a high gain, but if it goes to far, 3rd, 5th, and higher harmonics are created, and this can be a cause of unusual oscillations. So try to make C1 a little larger than C2.
Damping resistors R2 and R3 (22ohm) are needed to prevent ringing between the device and the USB connector. Also, on D+ a pull-up resistor, R1 (1.5 kohm) is needed; according to the USB specification, it must be pulled up to 3.3V. This is not VBUS (the power line available at the USB connector), so please exercise caution.
PCM2702 Operation 1 - The Interface Protocol
Figure 21 shows the interface protocol.
Figure 21a. Connecting to USB after power up
Figure 21b. When the power is turned on after USB attach
Figure 21c. End of playback, USB detach
(1) About 1,024fs (approximately 23 ms) after the power is connected, when Vdd gets above 2.0V, all device resets go low. At this time the analog output does a level shift to bipolar zero and the ZERO terminal changes from '0' to '1'. At this time the PLYBCK terminal goes low, and the SSPND terminal goes high (they are both low asserted).
(2) When the USB connector is connected to the PC, the VBUS terminal goes to 5V which is detected by the PCM2702. Then the PC (host) detects that the D+ terminal has the 1.5 kW pull-up resistor connected, and begins sending start of frame (SOF) packets at 1 ms intervals.
The operating system might send a reset (Single Ended Zero). We know when this happens because the SSPDN terminal blinks. Of course the PCM2702 will work fine whether or not the reset is sent.
(3) When the PCM2702 detects that the SOF is being sent, it sends SSPND low. After this, the PC issues the PCM2702 a control packet and recognizes PCM2702 as an audio device.
Here the OS comes to know that the connected chip is a Burr-Brown PCM2702.
(4) When the PCM2702 is first connected to the PC (when the first ID is made), the OS prepares the standard, built-in driver. The PCM2702 is designed to work with the standard, built-in drivers in Windows 98, Windows 2000, and Mac OS. There is no need to install a special driver. The user need only follow the instructions from the OS and click 'OK'.
By double clicking the 'Multi-Media' option under 'Control Panel' and selecting 'USB Audio,' the sound that had been coming from a Sound Blaster card or elsewhere, will now come through the PCM2702. When an audio data packet arrives, the PCM2702 awaits the next SOF packet and then begins playing back the audio. At this time the PLYBCK terminal is asserted. The PCM2702 is a high-performance DAC. The output terminal drive capabilities are low by design in order to control noise. In the case where PLYBCK and SSPND need to drive LEDs, a 74HC04 is used as a buffer.
(5) When audio data packets stop arriving from the OS, the PCM2702 sends PLYBCK low. At this time the DAC output goes to BPZ (bipolar zero), and, in 1,024 fs (23 ms) the ZERO terminal is asserted. Also, if SOF packets stop arriving for 4 ms, because the PC itself has been put into suspend mode, for example, the PCM2702 goes into suspend mode and asserts SSPND.
And here it concludes
By integrating an Interface, a DAC, and a PLL on a single chip we were able to free the user from the trouble of designing a difficult clock circuit. It started as a chip design with a low-cost target, and concluded with the bonus that the user doesn't have to worry about the DAC system clock. By simply connecting the USB connector, a high-performance DAC can be realized.
Although the development can run into a variety of difficulties, semiconductor devices are tiny little things that are naturally expected to work. And, of course, there are few chances to emphasize these tiny devices.
In this case I just happened to get a chance to write. Furthermore, I made it into a story. It wanted readers to enjoy (?) hearing about the struggles of the development process. And I wanted students to know that there are people energetically developing semiconductor devices, even in Japan.
Finally, the Professor Kobayashi who made an appearance herein was my professor at the university. It's been twenty years since I met him. But we still enjoy frequent and friendly communication. Clamor about Industry-University Cooperation has been heard loudly and long. And large cooperative projects are fine. But lately I have come to think that these small but deep relationships with research labs might be even more important. And I feel that industry needs to have more concern for how students, who are Japan's future, will be trained.
References:
1) Fuminori Kobayashi et al., " Time-optimal PLL Using the High-Speed Null Method,)," Journal of the Society of Instrument and Control Engineers, Vol. 16, No. 4, 1980, pp. 573-578
2) Fuminori Kobayashi et al., "A Scheme of PLL with Finite Impulse Responses," IEEE Transactions on Circuits and Systems - I: Fundamental Theory and Applications, Vol. 43, No. 4, April 1996, pp. 340-343
3) Fuminori Kobayashi et al., "High-Speed PLL Frequency Synthesizer for Low Frequencies," IEEE Transactions on Circuits and Systems, Vol. CAS-31, No. 10, October 1984.
4) Fuminori Kobayashi et al., "Efficient Digital Techniques for Implementing a Class of Fast Phase-Locked Loops (PLL's)," IEEE Transactions on Industrial Electronics, Vol. 43, No. 6, December 1996.
5) Universal Serial Bus Specifications Revision 1.1, USB Implementers Forum.
6) Kuwano, Masahiro, "KORE DAKE WA SHITTEOKITAI USB KISOCHISHIKI (Just What You've Always Wanted to Know about USB Fundamentals)," Interface, CQ Publications, March, 2000.
http://www.usb.org
http://www.burr-brown.com
http://www.bbj.co.jp
Author's Biography
Hitoshi Kondoh became a ham operator in junior high, and makes an appearance on the airwaves night after night. His specialty is fast keying, and he holds a First Class Amateur Radio qualification. After becoming enchanted with Micon, he enrolled in Nagaoka University of Technology and met Professor Kobayashi (cited frequently in this article). Later he passed the First Class Information Technology Engineers examination. He went to graduate school to study image processing but became interested in parallel processing. He received the First Class Technical High School teaching certificate and returned to his Alma Mater, Tokuyama College of Technology, as a teacher. Then he transferred to the Tokyo Institute of Technology, took a PhD in multi-processing, and made the switch to private industry. After being involved in such pursuits as LSI development for image processing, he made a career change and accepted his present position at Burr Brown. E-mail: kondoh_hitoshi@ti.com
