We've all heard the line about having more processing power in your palm than companies had years ago in multiple computer rooms. This certainly holds true for the Hewlett-Packard iPAQ 4355, jam-packed with features and sporting a qwerty keyboard, a 3.5-inch QVGA display, Bluetooth, 802.11b and IrDA communications, and the Microsoft PocketPC operating system-all built around a 400-MHz Intel Xscale microprocessor.
I was pleasantly surprised with all I could do with this little computer, which comes in at 5.4 x 2.9 x 0.6 inch and weighs 5.8 ounces. It's hard to call it a PDA, given all that it offers. I'm accustomed to a PDA that stores my calendar and address book, and that's abut it. But I found myself surfing the Web and corresponding via e-mail with the 4355.
Note that all this functionality comes with a price. The keyboard extends the length of the platform to slightly beyond what I would prefer, and when I had the Wi-Fi enabled, there was a significant drain on the battery. Unfortunately, at least in my case, the PDA was mostly used when I was on the road, so that meant carrying another charger (in addition to the chargers for my cell phone and laptop). I didn't have this problem with my older, albeit less-functional, PDA.
Some of the advanced functionality can be attributed to the memory device that's used in the PDA: 64 Mbytes of SDRAM and 32 Mbytes of flash ROM. The flash is a DiskOnChip G2 device, from M-Systems, and it's used for both boot and code storage.
Help from the OS
The flash is optimized by the Pocket PC 2003 operating system, which implements a paging-on-demand function. This means that it accepts data in the boot mode, like a binary file system. The Microsoft file system that's employed is responsible for the loading and removing of files in the memory. In other words, when the system boots up, only specific parts of the OS are loaded. Then as you require different pieces of the OS, they are swapped in and out of the SDRAM. The older version of the OS required a larger memory footprint because the entire OS would be shadowed, somewhere in the neighborhood of 20 Mbytes.
"If you have 32 Mbytes and over 20 is filled by the OS, there's little left for the user," said Arie Tal, marketing director for mobile systems at M-Systems. "When you try to run multiple applications simultaneously, the performance can be dramatically reduced."
Another option would have been to employ 32 Mbytes of NOR flash. At the time this system was designed, however, that memory would have cost about twice as much as the DiskOnChip part.
Today, the two options are about equal in cost. But the PocketPC 2003 OS is not optimized to run from NOR-based memory, so there would be a performance hit there as well.
A potential third option might have been to use raw NAND memory. But the Xscale processor doesn't have a NAND interface, so there's no physical way to connect the raw NAND directly to the CPU.
"We were pleased that HP was willing to take the architectural risk of leaving NOR and moving to NAND," Tal said. "This is generally the biggest issue with customers. It's an easier decision, however, now that Microsoft has officially embraced NAND and provided the benefits of the boot architecture, thereby eliminating some of the DRAM."
One of the obstacles the 4355's design team had to overcome with respect to the flash has to do with the boot process, because the software must be customized for the specific hardware. While Microsoft supplies a generic driver, customization will increase the performance.
Designers today also have an option that was not available at the time the 4355 was built: the DiskOnChip G3 device, which has twice the capacity of the G2.
The Xscale CPU that resides in the 4355 is the PXA255. It could even be considered a system-on-chip, since it includes an LCD controller to support QVGA, a memory controller and some other peripherals. The 255 is a replacement for the 206-MHz StrongARM 1110 (SA1110), which was used in the previous-generation iPAQ.
The PXA255 significantly increased performance over the SA1110, but not necessarily the 2x boost that comes from doubling the operating frequency (a common misconception). There are other system issues that must be addressed, such as memory bottlenecks.
The 255, which supports version 5 of the ARM instruction set, has a seven-stage pipeline, and 32 kbytes of instruction and data cache. It also moves from a 0.35-micron process down to 0.18 microns.
"As you push performance on the processor core and the memory buses, you must be careful about how you route your board, doing it in as few layers as possible to minimize costs," said Todd Trammell, Intel Corp.'s market development manager. "But you have to be careful with your signal integrity because these devices are running at such high frequencies, and just a little bit of noise can have a large impact."
Not such a hot design
Thankfully, the Intel CPU designers were fully aware of the heat issues that could negatively affect a PDA design. There's obviously no room (or power) in the budget for a fan. And the space constrictions make air flow difficult. But the power efficiency of the Xscale family makes the thermal consideration almost a nonissue.
The power-management architecture is somewhat complex because of the processor's multiple power planes, with up to five for the PXA255. That includes one for the core, one for the memory bus, one for legacy peripherals like USB, one for the internal SRAM (which must remain powered) and one for a real-time clock. That complexity extends to the software to control the various power planes.
The 270 also has a USB host port as well as the client contained on the 255. The generation beyond the 270 is now in development. It'll contain Intel's wireless MMX2 technology to improve video encode and decode performance further.
Talking without wires
Texas Instruments Inc. gets credit for both the Bluetooth and Wi-Fi communications subsystems in the 4355. The Wi-Fi is handled by the TW1100B, a media-access control baseband 802.11b chip with external interfaces and power amplifiers. TI claims the part was the first Wi-Fi device designed specifically for handheld applications.
"We worked directly with HP to understand what needed to happen for wireless LAN to fit better in the mobile space," said Matt Kurtz, a senior business development manager at TI. "We worked hard on the low-power aspect, particularly getting in and out of low-power modes. The standard wasn't necessarily written with low power in mind. So it was an accomplishment to achieve the level that we did."
The goal was to be below 3 milliamps for the nontransmitting (standby) mode. Other devices with similar functionality were pulling in 20 to 25 mA. Another key feature is that the device is housed in a 12 x 12-mm package.
While the power levels achieved by the TW1100B may have been the best-in-class at the time, TI has since released three generations. The latest is the WiLink 4.0 for mobile platforms, which comes in a package measuring 6 x 6 mm. The target application for the device is mobile handsets.
The Bluetooth part, the BRF6100, was TI's first single-chip Bluetooth device and is based on the company's digital RF-processing technology. Like the 130-nanometer Wi-Fi device, the 150-nm 6100 is a first-generation part. Both families have since migrated to 90 nm.
One challenge for HP's system designers was the coexistence of Bluetooth and Wi-Fi. "The 6100 was the first device available that could coexist between Bluetooth and wireless LAN," Kurtz said. "It essentially multiplexes the usage so you don't get into a race condition where the Wi-Fi starts getting fallback interference and takes longer to transmit, potentially taking you down to 1 Mbit/second on the WLAN."
All in all, if you look at the timing of this PDA, it really is an exceptional design.
Rich Nass (firstname.lastname@example.org)
, site editor for CommsDesign and WirelessNet DesignLine