Wikipedia
Tamil-Brahmi, or Tamili, is a variant of the Brahmi script used to write the Tamil language. These are the earliest documents of a Dravidian language, and the script was well established in the Chera and Pandyan states, in what is now Tamil Nadu, Kerala, Andhra Pradesh, and northern Sri Lanka. Inscriptions have been found on cave beds, pot sherds, Jar burials, coins, seals, and rings. The language is Archaic Tamil, and led to classical Sangam literature.
Tamil Brahmi differs in several ways from Ashokan Brahmi. It adds several letters for sounds not found in Prakrit: ṉ ṟ ṛ ḷ. Secondly, in many of the inscriptions the inherent vowel has been discarded: A consonant written without diacritics represents the consonant alone, whereas the Ashokan diacritic for long ā is used for both ā and short a in Tamil Brahmi. This is unique to Tamil Brahmi and Bhattiprolu among the early Indian scripts. This appears to be an adaptation to Dravidian phonotactics, where words commonly end in consonants, as opposed to Prakrit, where this never occurs. According to Mahadevan, in the earliest stages of the script the inherent vowel was either abandoned, as above, or the bare consonant was ambiguous as to whether it implied a short a or not. Later stages of Tamil Brahmi returned to the inherent vowel that was the norm in India.