With the help of this MS Excel tool you can count total frequencies of all dinucleotides in the nucleotide sequence as well as its statistical probability and the pattern of dinucleotides distribution between three codon positions.
To make calculation you have to paste your nucleotide sequence into the special cell on the "sequence" list - results will automatically appear on the "gene" list.
Codon and nucleotide usage are also calculated by this simple but useful algorithm.
Khrustalev V.V. HIV1 V3 loop hypermutability is enhanced by the guanine usage bias in the part of env gene coding for it // In Silico Biology. – 2009. – Vol.9. – Number 4. – P.255-269. (Electronic version: Vol.9. – ¹0022).