In the age of the IoT (Internet of Things), a random number generator plays an important role of generating encryption keys and authenticating a piece of an embedded equipment. The random numbers are required to be uniformly distributed statistically and unpredictable. To satisfy the requirements, a physical true random number generator (TRNG) is used. In this paper, we implement a TRNG using an SR latch on 40 nm CMOS ASIC. This TRNG generates the random number by exclusive ORing (XORing) the outputs of 256 SR latches. We evaluate the random number generated using statistical tests in accordance with BSI AIS 20/31 and using an IID (Independent and Identically Distributed) test, and the entropy estimation in accordance with NIST SP800-90B changing the supply voltage and environmental temperature within its rated values. As a result, the TRNG passed all the tests except in a few cases. From this experiment, we found that the TRNG has a robustness against environmental change. The power consumption is 18.8 mu W at 2.5 MHz. This TRNG is suitable for embedded systems to improve security in IoT systems.