Universal Serial Interface USI

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Universal Serial InterfaceUSI

• CS423
• Dick Steflik

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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)

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USI Block Diagram
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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

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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

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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