One Night at 10
The Developer Awaits that Moment of Ultimate Happiness...
So-called "engineering samples" are created when wafers come from the fab and the die are attached to lead frames at the assembly plant. Next we fire up the device to check performance. This is the most nerve-racking, heart-throbbing time for a designer.
When the chip has been soldered to the previously-prepared test board, we confront our feelings and, in a prayerful mood, flip the switch.
The confidence that the device will definitely work, which is amassed through extensive simulations and tests prior to tape out gives way to uncertainty as the mind has fleeting thoughts of worst-case scenarios. Even if the device does function, we don't believe our eyes. We flip the switch and continually check the power supply and traces on the board because it could be functioning improperly, which means we can't really say that it is functioning at all. But after verifying that it really is functioning, we feel like we have gone to heaven. This is the moment of ultimate happiness for a designer.
However, even if a single portion of the device is not functioning (the usual case!), the designer sinks to the depths of despair.
The Non-functioning Chip
The PCM2702 didn't work at all for 2 days after receiving the engineering samples. Furthermore, one of us was scheduled to take the board to the US a couple of days after the samples arrived. Saturday passed and it was now Sunday at 10 PM and still no response from the chip. Three of the responsible parties arrived early and worked all day, even on the weekend. At some point, we were staring at a single point (one of the engineering samples) and saying, "This is weird. The gate-level simulations were perfect. We did ample verification using the large-scale PLD (Programmable Logic Device). And we even took a board with the PLD installed to the US and passed the USB compliance test..."
Part 1: The USB Compliance Test
The Development of a D-to-A Converter for Digital Speakers
We (Burr-Brown) were building a USB controller with a D-to-A converter (DAC) inside. The application for such a device is clear - digital speakers.
USB has been standardized as a digital serial interface and, in the PC world, is replacing RS-232-C, which had been the industry standard. Furthermore, it appears that USB will also be used for various applications outside the PC. Applications that were unimagined at the beginning of USB development, such as MD/CD radio cassette systems with USB audio adapters and USB ports, seem likely to appear.
USB can connect not only input devices such as a keyboard or a mouse, but also output devices such as a printer or speakers. We focused on these USB speakers. Sound information is an extremely important medium for human beings. If the digital packetized information streaming across USB can be converted to analog with high fidelity, high quality sound can be delivered. We thought that there would be many applications for a one-chip USB interface and DAC if it could be provided at a low cost.
But if a USB chip is to be developed, it is necessary to have a certification. So let's begin with the story of the USB compliance test.
The Plan for the USB Compliance Test
The USB compliance test is sponsored by the USB Implementers Forum (USB-IF) and is held approximately four times per year at irregular intervals. Only those chips that have passed this test are allowed to use the USB logo, and product ID's of passing devices are included in the Implementers List that is published by the USB-IF.
As mentioned earlier, the engineering samples we created to formally undergo this test didn't work. I say formally since we actually had a dry run test using a large-scale PLD board implementation.
There were two reasons for undergoing the dry run test:(1) To assure that there was nothing we had missed. In other words, this test acted as our test bench to assure that we had done sufficient debugging prior to taking the design to silicon.
(2) Since this was our first, we wished to see just what is involved in this compliance test.
From there our plan was to take the design to silicon and be ready for the next compliance test.
This test has not recently been held in Japan, it is only held about once every three months on irregular intervals. Therefore, thinking that you can't win if you don't play, we put together an aggressive plan: hoping for a first-pass success, we would put the engineering samples on the evaluation board as soon as they had arrived from assembly, and go to the US for the test. After making careful preparations, from getting the test board ready to getting hotel and flight reservations, we waited eagerly for the samples to arrive.
The Silent Samples
But the engineering samples didn't work.
It was a shock to us designers because we had confidence (although not 100%) that it should work. To back that up we had done ample testing on the PLD implementation. Of course, since we were aware that the circuits were not exactly the same in the PLD as in actual device, we knew there was some chance that the samples would not work. If they didn't, nothing could be done immediately (it takes time to make changes to the silicon), so we just planned to wait for the next test (give up with a smile) if necessary.
But even though we had planned that way, what was to become of all the overtime effort and expectation? Looking for anything that might help, we checked the circuit board traces until they were nearly worn out, and used an oscilloscope to verify the operation of the external clocks... Finally we decided to probe the chip's internal circuits directly.
An Engineering Sample on the Dock
After using chemicals to remove part of the plastic package we put the chip under an electron microscope and use a remote control apparatus to apply an oscilloscope probe to specific signals. This job is one that requires great skill - it is realm of a great craftsman who can use a manually controlled lever to maneuver at the micron level.
Photo 1. Large Scale PLD Board: The right side is the PLD evaluation portion; the left side is the structure for the actual chip
Even as we marveled at this probing process, we designers were half sobbing because a bug at this level requires a change at the wafer level. In this situation, we might as well be doing all of this after taking some time off; but not a single person went home.
The process continued to the point of finally checking the master clock, which, it was discovered, was operating at the wrong frequency. At this frequency there was no possibility that the USB bus could communicate. Prepared for the worst, we double-checked the source file.
And found the bug!
An Engineering Sample Gets Outpatient Surgery
Could we call it happiness amid unhappiness? It wasn't a problem with the USB core or the DAC core, but with the control circuit for the integrated phase locked loop (PLL). And it was determined that it could be repaired by using a laser to sever a particular connection.
An electron microscope was used to locate the spot and a laser scalpel was applied.
Now the chip with the hole in it was again put on the board.
And the device was once again prayerfully fired up. This time it worked!!
Photo 2. Probing of the engineering sample
Personal Sidebar A:
The USB-DAC Development Strategy
I am employed at an American company (Burr-Brown) with a long history in the business of providing high-performance operational amplifiers for industrial and audio applications. I am in the Japanese branch, which has responsibilities for development of a variety of chips. It may not be well-known, but a large number of devices are actually developed in Japan. In particular, our emphasis is on DACs and PLLs. In Japan devices have been developed for use in high-performance audio products, of course, as well as for CD-ROMs for PCs and well-known electronic games. In addition, we also produce extremely high-performance DACs that require individual trimming and cost several thousand yen each (1000 yen is approximately $10).
I myself have not so much changed jobs, but changed occupations, and have not been with Burr-Brown for long enough to know the company very well; but I have found it to be very strict about the quality of analog products. One of the first things I heard on arriving is "We must maintain this performance level!"
However, Burr-Brown also has great interest in the massive PC market since we expect that IT business will become more popular, and that the distribution of music over networks will catch on. Both Windows 98 and the Mac OS support USB, so to provide a single chip USB interface plus DAC solution, with the kind of DAC performance that we have provided in the past, was the first step in our plan of attack.
By only connecting the USB connector, audio line out level is available directly from the chip. This chip is an interface between a USB terminal and an analog audio amplifier. Of course it is important that this be simple, but, on the other hand, since the data coming from the USB terminal is distortion-free digital audio, if the DAC performance is like what we have offered previously, the story is that even the PC can become a superior audio device.
The IDs that can be returned by the PCM2702 are shown in Table A.
Table A. PCM2702 Device Descriptors
Photo A. A Packaged PCM2702 (a 100 Yen coin is near the size of a US quarter)
The Sample Makes its Flight
We three who had just been delivered from the jaws of death used the electron microscope and the laser cutter to prepare a spare device. By the time we finished it had become Sunday.
The device with a hole in its package (somewhat embarrassing) caught its flight, went to the US, and passed the second compliance test without incident.
With the feeling of having hit the game-winning home run after having been down in the count with two outs in the bottom of the ninth, today's beer was the best ever! In cases like this we can make the necessary design change in the best of moods. From there, after living through the trials of six months of rigorous reliability and manufacturability tests, and having completed the design change, a new device was completed - the USB-DAC, PCM2702. (See Sidebar A, earlier.)
Part 2. The Pitfalls of USB Isochronous Audio Data Transfer
The Audio Capability of the PC
Although it makes a nice story to say that as long as a good DAC is used, great audio can come from the PC, there is the problem that the PC is a haven for digital noise. Not only that, but there are bothersome problems with real-time digital transmission that are not even limited to USB.
The former can be alleviated through use of the USB cable, which allows the power supply and analog circuitry to be separated into a different box. But, without a lot of forethought about the latter, it can become impossible to reach the desired audio quality level.
There are four USB transmission modes (please see Table 1). The two of those that are used for sending large quantities of data are
(1) Isochronous Mode - A fixed number of packets is guaranteed to be sent and received. This mode is used with multimedia data such as images and audio.
(2) Bulk Mode - A fixed quantity of data is sent at one time. If for some reason some of the data, is lost it is resent.
For data storage or printer applications the bulk mode is best because speed is of utmost importance and, through retransmission, data errors will be eliminated. But for audio data, real-time transfer is even more important than occasional missing data. (Noise is more tolerable than interruptions in the data. Of course pops and clicks are intolerable, but even more unpleasant is an intermittence of the data.) In this case, the isochronous mode is used. In other words, a real-time transmission scheme, with no re-sending of packets, is used for audio data, which streams from the PC in an RS-232-C-like manner.
Photo 3a. USB Connectors (Series A)
Photo 3b. USB Connectors (Series B)
USB is Clockless, Differential, Serial Transmission
It is not our intent to fully discuss the USB specification here. (Please refer to sister publication, Interface, March 2000.) USB 1.1 is a 12 Mbps bi-directional, serial bus, which is connected with 4 conductors as shown in Figure 1.
Figure 1. Conductors within the USB Cable
Two of those conductors are power (VBUS) and ground (GND). Information is transferred through the other two:
Since there are only two data lines, only four states can be used for data transmission.
To prevent noise and corruption due to the cable and connectors from affecting the receiver's detection, which is accomplished by thresholding, a basic differential signaling scheme is used:
When D+='1', D-='0'
When D+='0', D-='1'
This signaling scheme uses these two states to send data (please refer to Figures 2 and 3). In addition, a special meaning is ascribed to the state where D+ and D- are both zeros (SE0: Single Ended Zero). The state where both D+ and D- are both '1' is not used.
Figure 2. Arrangement of USB Connector Pins
Figure 3. USB Pull-ups and Pull-downs
The NRZI Method
USB data transfer is basically a two-conductor signaling method wherein, in the case where the data is '1', the signal does not change, and when the data is '0', the signal does change. This is the so-called Non Return to Zero Inverted (NRZI) method (please refer to Figure 4).
Figure 4. NRZI Modulation
It follows that there is no explicit clock on the USB cable (this compounds the problem). Rather, the signal is restored based upon the intervals between edges of the data. In this type of digital communication, if the sender uses a perfect clock to create the signal, and the receiver uses a perfect clock to interpret the data, the original data can be reconstructed. Since NRZI reconstruction is possible if there is a clock that is four times the bit rate, it can be accomplished if both the sender and receiver both have 48 MHz clocks (the transmission rate is 4 times 12 Mbps).
However, when viewing this from the standpoint of an audio device, the very fact that the sender and receiver both have local clocks becomes a stumbling block.
The Evil of Clocklessness
The fact that there is no clock line within the USB cable leads to a thinner cable which is an advantage. But, no matter how good the crystal oscillators are at the send and receive ends, there will always be some difference between the two. For example, if the sender is sending audio data at a rate of 48.001 MHz and the receiver is receiving at 47.999 MHz, the receiver is reconstructing data slightly slower than the transmission rate. When a large quantity of audio data is sent under these conditions, the buffer will soon overflow, resulting in lost data (please see Figure 5).
Figure 5. When there is Clock Frequency Error
On the other hand, if the receiver is running faster, an underflow will occur resulting in a discontinuity in the audio data. In a CD player, angular control can be used to control the motor such that it will synchronize with the playback data rate. But the USB receiver cannot control the sender. The resulting missing data can be digitally compensated (using a smoothing filter, please see Figure 6), but our company's development philosophy does not allow for such deception! (As an aside, there is no problem at all if the data is reconstructed with the receiver's clock after it has all been sent.)
Figure 6. Compensating For Missing Data
Proceed to Part 2...
*A version of this article entitled, "Story of the Development of USB D-A Converter," appeared in the Japanese publication, Design Wave Magazine, June, 2000, pp. 28 - 47.