Exam zone
Universal Serial Interface USI
CS423 Dick Steflik
What is it for?
Basic hardware needed for serial comm:
SPI (4-wire bus)
MMC/SD Ethernet, UARTs, USARTs, JTAG (IEEE 1149.1)
I2C/TWI (Inter-Integrated Circuit) (2/3-wire bus)
appropriate for peripherals where simplicity and low manufacturing cost are more important than speed
Accessing DACs and ADCs Interfacing OLEDs/LCDs Computer diagnistic sensors (fans, overtemp)
USI Block Diagram
USI Data Register
8-bit, parallel load/read shift register
SFR I/O8 (0x0F)
Port B
PB5 - DI/SDA
PB6 - DO
PB7 – UCSK/SCL
Must set DDR for Port B appropriately Data must be read from USIDR immediately as the register is not buffered
USI Status register
7 – USISIF – Start Cond. Interrupt Flag
6 – USIOIF – Counter Overflow Interrupt Flag
5 – USIPF – Stop Cond. Interrupt Flag
4 – USIDC – Data Output Collision
3 – USICNT3 – 4-bit Counter MSB
2 - USICNT2
1 - USICNT1
0 – USICNT0 – 4-bit Counter LSB
USI Control Register
7 – USISIE – Start Cond. Interrupt Enable
6 – USIOIE – Counter Overflow Interrupt Enable
5 & 4 - USWWM1..0 – Wire Mode
3 & 2 - USICS1..0 – Clock Select
1 – USICLK -
0 – USITC – USI Timer/Counter Strobe bit