An Inside Look At The iPaq 4355

This little system is jam-packed with features. We've all heard the line about having more processing power in your palm than they had in multiple computer rooms of years ago. There's no doubt in my mind that that's true in this case. The HP iPAQ 4355 packs a QWERTY keyboard, a 3.5-in. QVGA display, Bluetooth, 802.11b, and IrDA communications, and the Microsoft PocketPC operating system, all built around a 400-MHz XScale microprocessor (Fig. 1).

1. The HP iPAQ 4355 PDA integrates many of the functions you'd expect in a desktop PC.

I was pleasantly surprised with all I could do with this little computer, which comes in at 5.4 by 2.9 by 0.6 in., and weighs 5.8 oz. It's hard to call it a PDA, given all that it offers. I'm accustomed to a PDA that stores my calendar, address book, and that's about it. But I found myself surfing the Web, corresponding over e-mail, etc., 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 4355, 64 Mbytes of SDRAM and 32 Mbytes of flash ROM (Fig. 2). The flash is a DiskOnChip G2 device, from M-Systems, and it's used for both boot and code storage.

2. Most of the logic fits on the pcb's front side, including the XScale processor, the flash memory, and the Wi-Fi baseband.

Help from the OS
The flash is optimized thanks to the Pocket PC 2003 OS, 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," says 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 memory. However, at the time this system was designed, 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. However, the XScale processor doesn't have a NAND interface, so there's no physical way to directly connect the raw NAND to the CPU.

"We were pleased that HP was willing to take the architectural risk of leaving NOR and move to NAND," says Tal. "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—a higher capacity DiskOnChip device, the G3, which is twice the capacity of the G2.

2X the frequency
The XScale CPU that resides in the 4355 is the PXA255. It could even be considered an SoC, as 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 the 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," offers Todd Trammell, Intel'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 quite cognizant of the heat issues that could negatively impact 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 non-issue.

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 that must remained powered, and one for a real-time clock. That complexity extends to the software to control the various power planes.

Looking ahead, designers can take advantage of the next-generation XScale, the PXA270, which adds Intel's wireless MMX technology.

"We took a subset of the MMX instruction set from the Intel architecture on the PC side and offloaded some of the additional instructions for optimal video encode and decode performance," claims Trammell. "That's what makes up wireless MMX, which is closely coupled with the XScale core. The whole idea is to run multimedia applications more efficiently."

The 270 also has a USB host port in addition to the client contained on the 255. The generation beyond the 270 is now in development. It'll contain the next generation of wireless MMX, aptly named wireless MMX2.

Talking without wires
Texas Instruments claims responsibility for both the Bluetooth and Wi-Fi communications subsystems in the 4355. The Wi-Fi is handled by the TW1100B, a MAC baseband 802.11b chip with external interfaces and power amplifiers. TI claims this was the first Wi-Fi device designed specifically for handheld applications.

"We worked directly with HP to understand what needs to happen for wireless LAN to fit better in the mobile space," says 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 mA for the non-transmitting (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- by 12-mm package.

New and improved Wi-Fi
While the power levels achieved by the TW1100B may have been the best-in-class at the time, there was definitely room for improvement. The TI has since released three generations, with the latest being the WiLink 4.0. It's a completely top-to-bottom optimized solution for mobile platforms. The package measures 6 by 6 mm. The target application for this device is mobile handsets.

The Bluetooth part, the BRF6100, was TI's first single-chip Bluetooth device, which is based on the company's digital RF processing (DRP) technology. Since this release, TI has produced various devices based on its DRP technology. Like the 130-nm Wi-Fi device, the 150-nm 6100 is a first-generation part. Both families have since migrated to 90 nm.

One of the challenges that the HP system designers had to deal with was the Bluetooth-Wi-Fi coexistence. "The 6100 was the first device available that could co-exist between Bluetooth and wireless LAN," claims Kurtz. "It essentially multiplexes the usage so you don't get into a race condition where the Wi-Fi starts getting fall-back interference and takes longer to transmit, potentially taking you down to 1 Mbit/s on the wireless LAN."

All in all, if you look at the timing of this PDA, it really is an exceptional design. Some of the ICs that have been announced since the 4355 was designed will have immediate performance improvements. But for it's day, it really was (and continues to be) a winner.