33.10.1 Control A

Name: CTRLA
Offset: 0x00
Reset: 0x00000000
Property: PAC Write-Protection, Enable-Protected, Write-Synchronized

Bit 3130292827262524 
  LOWTOUTINACTOUT[1:0]SCLSM SPEED[1:0] 
Access R/WR/WR/WR/WR/WR/W 
Reset 000000 
Bit 2322212019181716 
 SEXTTOENMEXTTOENSDAHOLD[1:0]   PINOUT 
Access R/WR/WR/WR/WR/W 
Reset 00000 
Bit 15141312111098 
          
Access  
Reset  
Bit 76543210 
 RUNSTDBY  MODE[2:0]ENABLESWRST 
Access R/WR/WR/WR/WR/WR/W 
Reset 000000 

Bit 30 – LOWTOUT SCL Low Time-Out

This bit enables the SCL low time-out. If SCL is held low for 25ms-35ms, the Host will release its clock hold, if enabled, and complete the current transaction. A stop condition will automatically be transmitted.

INTFLAG.SB or INTFLAG.MB will be set as normal, but the clock hold will be released. The STATUS.LOWTOUT and STATUS.BUSERR status bits will be set.

This bit is not synchronized.

ValueDescription
0 Time-out disabled.
1 Time-out enabled.

Bits 29:28 – INACTOUT[1:0] Inactive Time-Out

If the inactive bus time-out is enabled and the bus is inactive for longer than the time-out setting, the bus state logic will be set to idle. An inactive bus arise when either an I2C Host or Client is holding the SCL low.

Enabling this option is necessary for SMBus compatibility, but can also be used in a non-SMBus set-up.

Calculated time-out periods are based on a 100kHz baud rate.

These bits are not synchronized.

ValueNameDescription
0x0 DIS Disabled
0x1 55US 5-6 SCL cycle time-out (50-60µs)
0x2 105US 10-11 SCL cycle time-out (100-110µs)
0x3 205US 20-21 SCL cycle time-out (200-210µs)

Bit 27 – SCLSM SCL Clock Stretch Mode

This bit controls when SCL will be stretched for software interaction.

This bit is not synchronized.

ValueDescription
0 SCL stretch according to Figure 33-4.
1 SCL stretch only after ACK bit, Figure 33-5.

Bits 25:24 – SPEED[1:0] Transfer Speed

These bits define bus speed.

These bits are not synchronized.

ValueDescription
0x0 Standard-mode (Sm) up to 100 kHz and Fast-mode (Fm) up to 400 kHz
0x1 Fast-mode Plus (Fm+) up to 1 MHz
0x2 High-speed mode (Hs-mode) up to 3.4 MHz
0x3 Reserved

Bit 23 – SEXTTOEN Client SCL Low Extend Time-Out

This bit enables the Client SCL low extend time-out. If SCL is cumulatively held low for greater than 25ms from the initial START to a STOP, the Host will release its clock hold if enabled, and complete the current transaction. A STOP will automatically be transmitted.

SB or MB will be set as normal, but CLKHOLD will be release. The MEXTTOUT and BUSERR status bits will be set.

This bit is not synchronized.

ValueDescription
0 Time-out disabled
1 Time-out enabled

Bit 22 – MEXTTOEN Host SCL Low Extend Time-Out

This bit enables the Host SCL low extend time-out. If SCL is cumulatively held low for greater than 10ms from START-to-ACK, ACK-to-ACK, or ACK-to-STOP the Host will release its clock hold if enabled, and complete the current transaction. A STOP will automatically be transmitted.

SB or MB will be set as normal, but CLKHOLD will be released. The MEXTTOUT and BUSERR status bits will be set.

This bit is not synchronized.

ValueDescription
0 Time-out disabled
1 Time-out enabled

Bits 21:20 – SDAHOLD[1:0] SDA Hold Time

These bits define the SDA hold time with respect to the negative edge of SCL.

These bits are not synchronized.

ValueNameDescription
0x0 DIS Disabled
0x1 75NS 50-100ns hold time
0x2 450NS 300-600ns hold time
0x3 600NS 400-800ns hold time

Bit 16 – PINOUT Pin Usage

This bit set the pin usage to either two- or four-wire operation:

This bit is not synchronized.

ValueDescription
0 4-wire operation disabled.
1 4-wire operation enabled.

Bit 7 – RUNSTDBY Run in Standby

This bit defines the functionality in standby sleep mode.

This bit is not synchronized.

ValueDescription
0 GCLK_SERCOMx_CORE is disabled and the I2C Host will not operate in standby sleep mode.
1 GCLK_SERCOMx_CORE is enabled in all sleep modes.

Bits 4:2 – MODE[2:0] Operating Mode

These bits must be written to 0x5 to select the I2C Host serial communication interface of the SERCOM.

These bits are not synchronized.

Bit 1 – ENABLE Enable

Due to synchronization, there is delay from writing CTRLA.ENABLE until the peripheral is enabled/disabled. The value written to CTRL.ENABLE will read back immediately and the Synchronization Enable Busy bit in the Synchronization Busy register (SYNCBUSY.ENABLE) will be set. SYNCBUSY.ENABLE will be cleared when the operation is complete.

This bit is not enable-protected.

ValueDescription
0 The peripheral is disabled or being disabled.
1 The peripheral is enabled.

Bit 0 – SWRST Software Reset

Writing '0' to this bit has no effect.

Writing '1' to this bit resets all registers in the SERCOM, except DBGCTRL, to their initial state, and the SERCOM will be disabled.

Writing '1' to CTRLA.SWRST will always take precedence, meaning that all other writes in the same write-operation will be discarded. Any register write access during the ongoing reset will result in an APB error. Reading any register will return the reset value of the register.

Due to synchronization there is a delay from writing CTRLA.SWRST until the reset is complete. CTRLA.SWRST and SYNCBUSY.SWRST will both be cleared when the reset is complete.

This bit is not enable-protected.

ValueDescription
0 There is no reset operation ongoing.
1 The reset operation is ongoing.