Googling, i can find a lot of solution about a Time Base (i.e. 1Hz) crystal based:
http://www.efymagonline.com/pdf/circuit-2.pdf
http://www.electro-tech-online.com/atta ... png.42724/
http://circuit-labs.blogspot.it/2008/11 ... ebase.html
The frequency of the crystal can by as low as 32768Hz or...