The use of single-chip systems has become increasingly popular, allowing design engineers to produce devices such as cell phones and coffee makers at reduced cost and space. Along with the advantages they bring, however, these single-chip implementations have also presented distinct challenges. Because of their complexity and the fact that they combine digital and analog signals, they are difficult to reproduce and debug. Engineers are abandoning the use of in-circuit emulators (ICE's), which has resulted in inaccurate reproductions, in favor of the embedded emulation module, which allows the SOC (system-on chip)-based application to emulate itself.

Before embedded emulation, microcontroller-unit-style ICE's were commonly used in the development of mixed-signal SOC devices featuring programmable processors. The ICE utilized a special system that emulated the device being developed as accurately as possible. Using this replication of the target device, the ICE would then reproduce the required functions. Via cable, the ICE is connected to the end application, which affords a crucial but momentary window into the target device's periphery and embedded-processor capabilities. Another cable links the ICE to a PC or workstation. Using this system, designers can evaluate and update software code in real time. After debugging the software, engineers create a prototype to be field-tested and later released to the silicon vendor, which then produces the final factory mask of the chip.

ICE limitations abound, however. This system fails to eliminate behavioral disparities between the emulation scheme and the actual production silicon. Furthermore, the ICE cannot keep up with the increasing sophistication of today's single-chip options, which require reduced voltage, tighter packaging, and greater-precision analog. In such highly sensitive applications, it cannot prevent noise from such sources as crosstalk on the ribbon cabling. Lastly, the ICE is unable to differentiate crucial application parameters, such as EMI and power consumption. Because of its shortcomings, this system may require design engineers to run expensive repeat trials with prototype silicon to approximate the application under final operating conditions.

Engineers are now turning to embedded emulation to develop SOC devices. The SOC itself contains the emulation module, a tiny kernel of debugging logic. The module has a serial interface that lets the device emulate itself. Through the interface, engineers can view and direct the SOC device's internal capabilities. The most prevalent serial interface is JTAG. With JTAG, engineers do not have to use up other system resources that may be needed by the application. Data moves in and out of the target device synchronously and non-intrusively. JTAG-accessed embedded emulation has actually been used for over a decade, but has only recently become prevalent with advancements in flash-memory technology. Embedded flash memory is not only inexpensive but can be easily developed using the JTAG-emulation-module method with full code flexibility. With this approach, engineers can access and control digital and analog signals, processor resources and the application code in the flash memory. Engineers can develop embedded-processor code in an application that precisely mirrors the electrical characteristics of the production product.

The limitations of embedded emulation are few and easy to overcome. One concern is that it needs at least four dedicated JTAG signals. This can pose a problem for devices with few pins but can be addressed by multiplexing the signals with an on/off switch. Another issue is the silicon cost of implanting the module on every device. However, the emulation module only requires a minimal increase in silicon area because the JTAG interface is already present on the chip for production testing. An enhanced emulation module (EEM) has also been produced to enable the development of extremely complicated devices with sophisticated digital and analog peripherals. In short, the advantages of embedded emulation are tremendous. It has virtually perfected emulation, allowing engineers to use the same device during development and production. With flash memory, they can also update the application code frequently, up until the product's shipment date. Remote upgrades, whether scheduled or not, are also an option. Finally, with this method, engineers can cut total product-development time by as much as 60 percent and with an industry-standard JTAG interface, support new devices by simply updating a script file.

Source: Embedded Emulation Shortens the Mixed-Signal-SOC Design Cycle
Mark Buccini
EDN, May 16, 2002
http://www.e-insite.net/ednmag/index.asp?layout=article&articleId=CA216161