des device drivers, signaling stacks, debugging and testing tools; and reference designs for modems and network interface cards (NICs) using the Atom chip sets. The Atom chips' on-board intelligence will enable firms to design xDSL modem cards or NICs that require little or no external software.
A Periodic Table of ASICs
ATML's Atom Accelerator suite is
aimed at developers of equipment at all three of these levels: the central office (CO), the distributed loop carrier (DLC), and the customer premises. The first ATOM ASIC to ship is a multifunction endpoint chip called Hydrogen, which supports ATM25, Ethernet, and a PCI controller. It will soon be followed by Helium, which adds multiple line drivers, and Oxygen, which implements a universal switching fabric. The Hydrogen chip can drive the xDSL modems required in the home and at the DLC. Helium is intended for building Digital Subscriber Line access multiplexers (DSLAMs) used at local switching centers. The Oxygen switch chip will be used in both DLC and CO switching equipment.
ATML developed Hydrogen in collaboration with Cirrus Logic, which is fabricating the chip. Atom is available for evaluation at $75 in small quantities, and the price falls to under $30 in large volumes. The low price is achievable thanks to the tiny ARM core, which permits a small overall die size. The chip is fabricated in 0.5-
micron CMOS and comes packaged as a 208-pin quad flat pack. The pricing makes Atom viable for very low-cost consumer equipment and DSLAMs, which are price-sensitive items.
Hydrogen is a universal endpoint chip suitable for video, voice, and data devices such as NICs, modems, cable modems, and set-top boxes that require a very low chip count. For example, you could build a PC-based internal xDSL modem with just three chips: a Hydrogen, one SIMM, and an xDSL line driver. The main function units, shown in
"Hydrogen's Microarchitecture"
above, are: a 32-bit ARM7 CPU core (which has 4 KB of in-core SRAM instead of a cache) clocked at 32 MHz; 8 KB of on-chip SRAM outside the CPU core; a controller for up to 32 MB of external EDO DRAM; a PCI bus interface capable of acting as master and slave; an I/O expansion bus that can connect an ISA peripheral such as an Ethernet controller; two ATM interfaces (one for direct ATM25 and one for 8-bit UTOPIA); a segmentation and reassembly unit (SAR)
for manipulating ATM packets; and an IrDA- and MIDI-compliant serial port.
Software Nucleus
ATMOS, the microkernel-based real-time operating system supplied with the Hydrogen chip, is a very simple multithreaded OS that supports a single processor and a flat memory model. It's completely modular, consisting only of the kernel and an arbitrary number of modules, which run as separate processes. All modules get complete access to the hardware, and so device drivers are no different than any other module. The kernel handles interrupts, process scheduling, and interprocess communications using a proprietary lightweight message protocol. The kernel's scheduler supports time-slicing, thread blocking and unblocking, and dynamic context switching in response to external interrupts.
The ATMOS kernel needs around 32 KB of memory and runs entirely off-chip. It requires 1 MB minimum of external memory to run simple applications (e.g., a PC-based ATM/Ethernet NIC), while up to 8 MB is necessary for a
n ADSL application that does routing and bridging. Hydrogen's on-chip memory is for use by time-critical nonkernel tasks: For example, the fastest 4 KB of in-core memory is typically occupied by the rate-pacing and cell-reception routines of the ATM driver.
ATMOS comes with device drivers for Ethernet and ATM, and various stacks for signaling, IP routing, Ethernet bridging, remote management via SNMP, Telnet and serial port management, and ATM Forum LAN emulation. The ATM driver supports all the required quality of service (QoS) levels: constant bit rate, variable bit rate, available bit rate, and unspecified bit rate. ATMOS supports the ATM Adaptation Layers 0, 3/4, and 5, which enable other network protocols to run on top of ATM. It also supports proper rate pacing for ATM cells.
Some older ATM controllers achieve a particular cell rate by sending bursts of cells at maximum line speed for a while, then pausing briefly until the average throughput is the desired rate. This simplifies the circuitr
y but means that cells are not equally spaced in time. The Atom driver sends evenly spaced cells at a software-determined rate, and this rate can be changed for different virtual circuits. This lets developers write sophisticated rate-based tariff schemes in software. It's possible for your Atom-based modem to be simultaneously hooked up to three different ISPs via three different virtual circuits, and all of them would still charge you for the correct number of cells used.
The development environment for ATMOS runs on a Sun workstation and is based on the GNU C and C++ tools that generate ARM code. There are simple post-mortem debugging facilities built into ATMOS itself (to inspect registers and memory contents after a crash), and ARM's EmbeddedICE debugging system is available as an option.
ATML hopes that the Atom Accelerator architecture will be attractive to xDSL developers because purchasing a single-chip ATM controller, OS, and development software from one vendor ensures that they all wo
rk together smoothly--which is too often not the case when you buy them from separate sources.
Where to Find
Advanced Telecommunications Modules Ltd.
Cambridge, England
Phone: +44-1223-566919
Internet:
http://www.atminc.com/
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