I think At89c51 doesn't have i2c , spi, and ads ?Some 8051's have I2C and some have SPI, but as most don't I have witten a tutorial here.https://www.electro-tech-online.com/articles/basic-8051-tutorial-6.714/ There are one or two 8051 tutorials in the article section..
In what case we need hardware ic ? like 0804There are many 8051 derivatives that have ADC I2C and SPI in hardware, but its easier if you only have an AT89C51 to do it in software... The P89LP952 and P89LP954 have usart SPI I2c and ADC
by reading a lot, confusedDid you look at my 8051 articles? I did an ADC with the MCP3008 (SPI) and a DAC MCP4921 (SPI)..All this with a basic 8051...
There are several SPI RTC units, which will use the same interface..