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Feature
Piecing together a Pocket PC
A deeply cycled, defunct battery provided a good excuse to crack open a classic: Compaq/HP's popular late-2001 iPaq 3835.
By Brian Dipert, Senior Technical Editor -- EDN, 11/23/2006
Back in mid-2002, I grumbled about the nonremovable batteries that were beginning to appear in some manufacturers' consumer electronics devices; PDAs, iPods and other MP3 players. While this battery design decision might make the system slightly lighter and slimmer than a more flexibly powered alternative, it also limited the system's life (a flaw that, in many cases, I suspect was intentionally introduced by manufacturers keen on selling you a new widget every year or two). The integrated battery (like any battery) is capable of a noninfinite number of recharge cycles and, once defunct, cannot easily be replaced. And lithium-ion batteries, once subjected to a deep discharge, no longer exhibit even a reasonable approximation of their original charge-storage capacity.
In 2002, the deep-discharge dead-battery bug bit me with the iPaq 3135 I used to own. And unfortunately, it recently bit me again with my iPaq 3835, which Compaq (now HP) unveiled in late 2001. A Dell Axim X5 is actually my primary Pocket PC nowadays, but I've held onto the 3835 a) because thanks to the subsequent pace of technology innovation, it sells on Ebay for only a slim fraction of its original sales price, and b) for audio recording use.
While my wife and I were away on a road trip this past summer, a power outage (endemic to Sacramento, CA that time of year) hit our home. Our house-sitter, in order to quiet the din of multiple beeping alarms, turned all of my UPSs (uninterruptible power supplies) off....including the one the iPaq 3835 was plugged into. When I returned home, I discovered that the Pocket PC would remain "alive" while on battery power for only around 5 seconds.
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iPaq 38xx models incorporated a then-state of the art SD (Secure Digital) card socket, not found in prior-generation iPaq models. Unfortunately, the socket didn't support SDIO mode (for SD connector-equipped Bluetooth, W-iFi, camera and other adapters) and was therefore useable only with memory cards. To the right of the SD socket is a Philips audio codec, the UDA1341TS, which handled the microphone input and headphone jack output functions.
iPaq 31xx, 36xx, 37xx and 38xx product variants all employed a common processor foundation; the StrongARM SA1110 CPU, running at 206 MHz, that Intel acquired from Digital Equipment Corporation (a company which, ironically, was later acquired by Compaq) in 1997. To the left of the CPU is a HTC-labeled (High Tech Computer, a Taiwan-based developer of Pocket PCs and Smartphones) ASIC which likely acts as glue logic between the CPU and the system's memory and various input and output modules; touchscreen display, buttons and joystick pad, expansion buses, infrared transceiver, etc. HTC iPaq 39xx Pocket PCs switched to Intel's first follow-on Xscale ARM-based CPU, the 400 MHz PXA250.
Below the CPU you'll find two memory devices; a ROM-code-stickered 32-Mbyte NOR flash memory on the right and a 64-Mbyte DRAM on the left. Memory allocation, incremented in lockstep both with the included operating system version and with Compaq's asumptions as to a typical user's number of installed applications and size of contact database and other data partitions, is one notable differentiation between iPaq variants. The 36xx family (with color screens; 31xx devices were hardware-identical with the exception of a switch to a monochrome screen) ran the Pocket PC 2000 O/S and combined 16 Mbytes of nonvolatile memory and between 16 and 64 Mbytes of DRAM. 3700 variants were based on Pocket PC 2002 and, correspondingly, bumped up the nonvolatile allocation to 32 Mbytes, as did 38xx devices. The 39xx series ran Pocket PC 2003 and included either 32 or 48 Mbytes of ROM space.
All of these product families direct-executed the OS and factory-installed applications from nonvolatile memory. Unused flash memory blocks found use as a user-accessible read-and-writeable "iPaq File Store" embedded drive (which, note, a firmware upgrade would wipe clean), and user-installed applications went into system RAM. The latest-generation Windows Mobile 5 OS migrates to a different memory model which broadens the number of flash memory options (NAND, for example) available to designers. Flash-memory contents, including user-installed applications and data files formerly found only in RAM, get copied to and executed from RAM upon system power-up. This transition is at the root of many users' frustrations with Windows Mobile 5 updates sold to them by Pocket PC vendors (for example, the Dell Axim X50 and X50V upgrade debacle); the hardware, originally designed for prior-generation Pocket PC O/Ss, contains insufficient DRAM and/or flash memory to smoothly and reliably run the newer OS and application suite.
Not shown is the passive touchscreen display on the backside of the system board. 36xx and 37xx family generations employed a 12-bit color, 240×320 pixel display, while as previously mentioned the 31xx models used a less eye-catching, but also lower-power, monochrome LCD. 38xx and 39xx devices upgraded to a 16-bit color LCD. At the time of its introduction, the iPaq was notable for the ability to view its front-lit reflective LCD in bright sunlight; competitors' PDAs employed backlit transmissive LCDs. Modern Pocket PCs incorporate transreflective (also known as transflective) LCDs which combine the best attributes of both prior-generation technologies: an adjustable-intensity backlight for dim ambient light use, and front illumination for high ambient lighting environments.
Note, as in past Prying Eyes dissections, the significant percentage of total board space devoted to analog and passive components. Although the iPaq 3835 offered infrared connectivity, it didn't provide Bluetooth (included in the 3870 and 3970), IEEE 802.11, or any of the other functions we now take for granted. For example, compare the 3835 to my i-Mate SP5m Smartphone. At 0.69 inches, the SP5m is admittedly slightly thicker. However, at 4.23 inches tall by 1.82 inches wide (due, in part, to the Smartphone's much smaller screen), and weighing 3.74 ounces, the SP5m much more comfortably slips into my pocket. It runs a newer OS, has a faster CPU, contains twice as much flash memory, and includes quad-band GSM telephony, Bluetooth, Wi-Fi, GPRS and EDGE data connectivity, and a 1.3-megapixel camera. All this in only four years' separation between the two systems' unveilings; an amazing testimonial of the power of Moore's Law integration (which, unfortunately, analog circuitry cannot exploit to the same degree as its digital brethren).
Although iPaq 3xxx Pocket PCs' integration capabilities pale in comparison to today's devices, they were still highly flexible, courtesy of a bottom-edge connector that accommodated a series of Compaq/HP- and third party-supplied expansion packs. Accessory options included PCMCIA and CompactFlash adapters, cameras, keyboards, and other flavors; some expansion packs even embedded batteries that supplemented the one integrated within the Pocket PC.
Editor's note
The above is an extended version of an article that appeared in the print edition of EDN. This PDF file shows the print edition of the article.
