DSP core and memory for modem processing
The ADSP-21MOD870-000 is a fixed-point DSP from the ADSP-21xx family, configured as part of the 2181KST 4-modem chipset. Its 52 MHz clock rate drives the arithmetic logic unit for real-time modem signal processing — V.34 bis, V.90, or similar full-duplex data pump algorithms fit within the cycle budget when the code is optimized for the single-cycle MAC. On-chip RAM totals 224 kB, partitioned between program and data memory. That is enough buffer space to hold two 8 kB modem frames plus the protocol stack without thrashing external memory — the chip relies on external non-volatile memory for boot code and firmware storage, so the board must include a serial flash or EEPROM on the host interface or serial port.
Supply rails and temperature grade
Both the core and I/O operate from a single 3.30 V rail — no separate 1.8 V core supply needed. This simplifies the power tree to one 3.3 V regulator, but the 100-LQFP package draws enough dynamic current at 52 MHz that the PCB should place a 10 µF ceramic within 5 mm of each VDD pin pair, with a 0.1 µF right at the pin. The 100-LQFP (14x14 mm) body has a 0.5 mm pitch that demands a 4-layer PCB for reliable fan-out of the inner rows. The exposed pad on the bottom is not a thermal pad — it is a mechanical tie-bar remnant; the datasheet recommends connecting it to ground plane for EMC, not for heat sinking. Junction-to-ambient thermal resistance is typically 40-50°C/W in still air, so the 52 MHz switching losses keep the die well within the 70°C ambient ceiling.
Sourcing and compliance reality
However, it is RoHS non-compliant (leaded solder balls or die-attach), which means it cannot ship into EU RoHS-exempt categories without an explicit exemption letter. For new designs targeting RoHS markets, the ADSP-2189NKST-320V offers a 1.8 V core, 80 MHz clock, and 192 kB RAM in a similar footprint, but the supply voltage difference requires a board spin. The RoHS non-compliant status means this part is typically sourced for legacy repair or military/aerospace BOMs where leaded exemption applies.