Datasheet

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USI Operation

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14.2.4 I

C Mode

The USI module is configured in I

C mode when USII2C =1, USICKPL = 1, and USICKPH = 0. For I

data compatibility, USILSB and USI16B must be cleared. USIPE6 and USIPE7 must be set to enable the

SCL and SDA port functions.

14.2.4.1 I

C Master Mode

To configure the USI module as an I

C master the USIMST bit must be set. In master mode, clocks are

generated by the USI module and output to the SCL line while USIIFG = 0. When USIIFG = 1, the SCL

will stop at the idle, or high, level. Multi-master operation is supported as described in the Arbitration

section.

The master supports slaves that are holding the SCL line low only when USIDIVx > 0. When USIDIVx is

set to /1 clock division (USIDIVx = 0), connected slaves must not hold the SCL line low during data

transmission. Otherwise the communication may fail.

14.2.4.2 I

C Slave Mode

To configure the USI module as an I

C slave the USIMST bit must be cleared. In slave mode, SCL is held

low if USIIFG = 1, USISTTIFG = 1 or if USICNTx = 0. USISTTIFG must be cleared by software after the

slave is setup and ready to receive the slave address from a master.

14.2.4.3 I

C Transmitter

In transmitter mode, data is first loaded into USISRL. The output is enabled by setting USIOE and the

transmission is started by writing 8 into USICNTx. This clears USIIFG and SCL is generated in master

mode or released from being held low in slave mode. After the transmission of all 8 bits, USIIFG is set,

and the clock signal on SCL is stopped in master mode or held low at the next low phase in slave mode.

To receive the I

C acknowledgment bit, the USIOE bit is cleared with software and USICNTx is loaded

with 1. This clears USIIFG and one bit is received into USISRL. When USIIFG becomes set again, the

LSB of USISRL is the received acknowledge bit and can be tested in software.

; Receive ACK/NACK

BIC.B #USIOE,&USICTL0 ; SDA input

MOV.B #01h,&USICNT ; USICNTx = 1

TEST_USIIFG

BIT.B #USIIFG,&USICTL1 ; Test USIIFG

JZ TEST_USIIFG

BIT.B #01h,&USISRL ; Test received ACK bit

JNZ HANDLE_NACK ; Handle if NACK

...Else, handle ACK

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Universal Serial Interface (USI) SLAU144J–December 2004–Revised July 2013

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