As a hardware design engineer, till now I have interfaced following items and I must learn all of them again:
- UART
 - RS232
 - RS485
 - RS422
 - USB
 - Ethernet
 - GSM SIM
 - SDRAM
 - DDR 3
 - Parallel Nor Flash
 - Nand Flash
 - SPI Nor Flash
 - EEPROM
 - Temperature Sensor IC
 - External Crystal
 - Decoupling capacitors
 - High sped digital lines routing
 - mPCIe interface
 - Zigbee
 - GSM
 - FPGA
 - Slave FIFO interface
 - Pullups and pull down resistors
 - Stitching capacitors
 - PCB design
 - Analog and digital ground planes
 - Terminations
 - RC delay for reset Pins and IO pins
 - ESD protection
 - Reset switch interface
 - Schematics presentation - Pur cosmetics
 - Capacitors
 - Resistors
 - Mosfets
 - LEDs
 - Push to on Push to off circuit
 - Choosing ADC
 - ADC interface
 - DAC interface
 - SPI to UART IC
 - SPI interface
 - I2C interface
 - Three wire interface
 - Two wire interface
 - EMI precautions for digital interfaces
 - Booting of Blackfin Processor
 - eMMC interface
 - Open collector/Open drain
 - Series RLC Circuit
 - Routing clock lines
 - list will be updated..