Most likely, your parents opted out. But look at your birth cert, is your Chinese name written there in Chinese characters? If yes, then you can have Hanyu Pinyin name on your IC and passport. In those days, without Chinese word processing system, those characters are written by hand on the birth cert. My father and I were always invited by friends and relatives to KK birth cert department to write for them. (There's a red packet for that, especially big if we were also requested to name the baby.) In duplicate, as one copy would be filed by government and the other copy the family retains.
Usually, we were requested to write in Chinese brush or fountain pen instead of ballpoint, and in traditional Chinese instead of simplified Chinese. However, the name in printed Chinese characters on IC has been standardized to simplified Chinese only.