PT2399

The PT2399 is a delay/echo integrated circuit by Princeton Technologies. Although digital in nature, it is frequently used to emulate the sound of analogue (BBD- based) delay circuits by means of additional filtering. Its being found more often lately in a lot of DIY delay pedals.

http://www.diyaudiocircuits.com/tutorials/pt2399-digital-delay-analog-echo/

This link has a lot of useful information regarding this chip, and also has a link to the full and unabridged spec sheet, which is surprisingly hard to find online.