National Semiconductor's GX1 is one of the x86 products targeted at this space (Fig. 6). Although the GX1 requires additional chips to reach the PC-compatibility level, the mix-and-match approach allows design flexibility.
A 2D accelerated video adapter is incorporated with an x86 core in the GX1. The memory and PCI bus interface is included on-chip, making it an SoC. A Southbridge chip is necessary if PC compatibility is required because the GX1 lacks PC peripheral support, such as USB and keyboard interfaces.
National Semiconductor uses the GX1 core in the SC1200, SC2200, and SC3200 chips, which are PCoCs. (See "System-On-A-Chip Line Features Pentium Core And 2D Video Acceleration," p. 72.) These include peripheral support like a USB hub or AC'97 audio support. Each of the chips wraps further video support around the GX1 2D accelerated graphics support.
A Version For Set-Top Boxes
For instance, the SC1200 is specifically designed for the TV set-top box market. Its video support includes a video overlay processor, and the chip has a video input port. This configuration allows an application to present information on the television screen with minimal, mostly passive, external components.
The SC2200 is the design closest to a conventional PC. Its video support provides outputs to either an LCD or a VGA monitor. This design is targeted at low-cost, low-power laptop applications and thin-client desktop applications. Also supported by the SC3200 is an LCD, but it's designed for more compact operations, such as those in a handheld device, where a VGA connection is unnecessary.
Another PCoC solution is ZF Linux's MachZ chip. Though it's based on a 486 core, it's wrapped with PCI Northbridge support. This provides external compatibility while retaining low-power performance.
The MachZ includes a full complement of PC peripherals. It doesn't incorporate a video controller, however, so it's at least a two-chipper. But it provides designers with more flexibility.
As with the National Semiconductor solutions, the MachZ PCoC has USB, IDE, PCI, and PC peripheral support. The same is true for interrupt and DMA support. MachZ, though, has some unusual and useful architectural system enhancements.
The first is the FailSafe boot support. Essentially, this is a preboot sequence that checks the system configuration. It utilizes the processor's cache as RAM, so the system will boot even if the external RAM isn't installed or is defective. The boot process checks various systems and can invoke a download from any one of many sources, like a serial port based on configuration settings. This is especially useful when a flash-memory update becomes interrupted. With a conventional system, the system would normally be dead in the water because the flash-based application couldn't run. With FailSafe, the problem is detected and the download is restarted. Dual watchdog timers also help detect problems that FailSafe can then attempt to correct.
The second item is the Z-TAG interface. It provides a high-speed link for downloading information. Typically, it's used for system initialization and is supported by the FailSafe boot feature.
From Rise Technology, the SCX501 is similar to National Semiconductor's SC1200. It incorporates the same kind of peripheral support, including the memory controller and 2D video adapter with television video overlay support. Many of the PC-compatible peripherals are dispensed to simplify the chip. The SCX501 has both ISA and PCI bus support.
STMicroelectronics' STPC line is very much like the SCX501. That's not surprising because Rise and STMicroelectronics have signed a codevelopment agreement. STMicroelectronics provides customization of the product line for applications like Internet appliances. Both companies use the Rise core designed for very low power and high performance.
Enhancements Versus Complexity
Though not specifically related to the x86 architecture, the x86 SoCs and PCoCs represent one avenue of x86 system enhancements, which are tightly coupled with the x86 architecture. Often these enhancements are more important to embedded-system designers than how complex the execution pipeline is or whether or not the underlying system is a VLIW processor.
Additional features will continue to show up in these integrated solutions as chip geometries get smaller and as the demand for more powerful solutions grows. Ethernet and Bluetooth support are just two possible additions to this type of architecture.
The number of new enhancements and the interest in the market make it clear that the x86 architecture still has a lot of growth potential. RISC and now VLIW architectures will continue to compete with the x86 solutions. But the amount of software support, the large number of experienced x86 programmers and designers, and the quality of x86 processor design will continue to give the x86 architecture an edge.