Feature

Prying apart a portable audio player

Prying Eyes: A look inside Sandisk's Sansa M250 reveals platform minded design decisions, surprising flash-memory choices, and potential hacks.

By Brian Dipert, Senior Technical Editor -- EDN, 9/4/2008

Free after $65 rebate, with free shipping. That's the deal that routed a refurbished Sandisk Sansa M250 from Newegg to my front door last summer. I suspected it'd make a fine Prying Eyes patient, and as it turns out I was right. Let's see what's inside, shall we?

Removing a small Philips screw at one end of the backside battery compartment, along with jimmying loose a series of plastic tabs along both sides of the unit, enabled me to extract the double-sided PCB inside. This was a far easier task than it appears a dissection of Apple's first-generation iPod shuffle will be...nonetheless, I also have one of these Sandisk competitors queued up for future Prying Eyes consideration.

Cushioning foam on top of the IC package, with adhesive on one side, protects the system's 'brains' (an ARM9-based and USB2 support-inclusive Telechips TCC770) from collisions with the plastic battery compartment that's directly above it once the unit is fully assembled. The TCC770 claims to support the Ogg Vorbis audio codec, this particular capability isn't advertised in Sandisk's promotional materials. Conversely, Sandisk claims that the Sansa M250 plays back Audible spoken-word content, yet Telechips' website makes no mention of ACELP codec or Audible DRM support.

The CPU's ADPCM (adaptive differential pulse code modulation) audio codec support is likely harnessed by the Sansa M250's built-in microphone for voice recording (and subsequent playback). The Telechips TCC770 also advertises limited-codec image decoding support; JPEG pictures, along with MPEG-4 Simple Profile video clips. The Sansa M250 collateral makes no mention of image file capabilities; then again, the unit's limited-resolution (128×64-pixel) monochrome LCD wouldn't really do them justice, anyway.

Underneath another piece of protective foam hides an NXP Semiconductors TEA5767HN FM radio IC for playback-only purposes (i.e. the Sandisk unit offers no support for live recording and later listening). The lack of a discrete antenna embedded within the Sansa M250's plastic case probably indicates the use of the headphone wire for this particular function. Curiously, European versions of the Sansa M200 series had FM radio functionality disabled, although clever hacks got around the limitation. And speaking of headphones, Texas Instruments' TLV320AIC23B two-channel codec (i.e. ADC and DAC) and headphone amplifier is another notable IC in this design; however, the chip's 24-bit maximum per-channel sample size and 96 kHz peak sample rate aren't fully harnessed here.

From a semiconductor memory standpoint, the Sansa M250 embeds a single battery-backed Elite Semiconductor M12S16161A 16 Mbit (1Mbit × 8 bits × 2 banks) SDRAM, supplementing the 64 kbytes of SRAM integrated within the Telechips TCC770. I suspect that, among other functions, the SDRAM acts as a "shadow" for the direct execution of system code stored in NAND flash memory. And speaking of NAND flash memory, above the SDRAM (and an intermediary piece of cushioning foam) are two Samsung K9K8G08U0M 1-Gbyte (1 Gbit × 8 bit) devices, housed on a double-sided daughter card. This modular arrangement gives Sandisk flexibility to leverage a common primary PCB design across multiple Sansa M200 family capacity proliferations (512-Mbyte, 1-Gbyte, and 4-Gbyte product variants are also offered) as well as to source NAND flash memories in multiple IC density, architecture, and supplier flavors.

To wit, the choice of Samsung NAND flash memory is interesting given Sandisk's longstanding close business relationship with Toshiba, and in light of the fact that Sandisk's Sansa M200 Series press release claims that Sandisk's own flash memory is being used in the players. And somewhat surprisingly, given the substantial bill-of-materials cost pressure in this low-end portable audio player and that the Telechips TCC770 supports ECC (error-correction codes) for MLC (multi-level cell) two-bit-per-cell flash memory applications, the K9K8G08U0M is a SLC (single-level cell) one-bit-per-cell NAND device.