I2C: clear ERROR flag before checking it

kaysievers · kaysievers · commit 289a27209373 · 2026-02-05T17:58:07.000+01:00
Commit ccfc7db introduced checks for INTFLAG::ERROR. From the datasheet: Bit 7 – ERROR Error This flag is cleared by writing '1' to it. This bit is set when any error is detected. Errors that will set this flag have corresponding status bits in the STATUS register. These status bits are LENERR, SEXTTOUT, MEXTTOUT, LOWTOUT, ARBLOST, and BUSERR. Writing '0' to this bit has no effect. Writing '1' to this bit will clear the flag. Also restore the change before commit 34d15d0, as the missing clear of the flag appears to cause the problem.
diff --git a/cores/arduino/SERCOM.cpp b/cores/arduino/SERCOM.cpp
@@ -562,6 +562,7 @@ bool SERCOM::startTransmissionWIRE(uint8_t address, SercomWireReadWriteFlag flag
   }
 
   // Send start and address
+  sercom->I2CM.INTFLAG.bit.ERROR = 1;
   sercom->I2CM.ADDR.reg = SERCOM_I2CM_ADDR_ADDR(address) | 
                           ((sercom->I2CM.CTRLA.bit.SPEED == 0x2) ? SERCOM_I2CM_ADDR_HS : 0);
 
@@ -611,13 +612,16 @@ bool SERCOM::startTransmissionWIRE(uint8_t address, SercomWireReadWriteFlag flag
 bool SERCOM::sendDataMasterWIRE(uint8_t data)
 {
   //Send data
+  sercom->I2CM.INTFLAG.bit.ERROR = 1;
   sercom->I2CM.DATA.bit.DATA = data;
 
   //Wait transmission successful
   while(!sercom->I2CM.INTFLAG.bit.MB) {
     // If a data transfer error occurs, the MB bit may never be set.
     // Check the error bit and bail if it's set.
-    if (sercom->I2CM.STATUS.bit.BUSERR) {
+    // The data transfer errors that can occur (including BUSERR) are all
+    // rolled up into INTFLAG.bit.ERROR from STATUS.reg
+    if (sercom->I2CM.INTFLAG.bit.ERROR) {
       return false;
     }
   }

Original file line number	Diff line number	Diff line change
`@@ -562,6 +562,7 @@ bool SERCOM::startTransmissionWIRE(uint8_t address, SercomWireReadWriteFlag flag`
`562`	`562`	`}`
`563`	`563`
`564`	`564`	`// Send start and address`
	`565`	`+ sercom->I2CM.INTFLAG.bit.ERROR = 1;`
`565`	`566`	`sercom->I2CM.ADDR.reg = SERCOM_I2CM_ADDR_ADDR(address) \|`
`566`	`567`	`((sercom->I2CM.CTRLA.bit.SPEED == 0x2) ? SERCOM_I2CM_ADDR_HS : 0);`
`567`	`568`
`@@ -611,13 +612,16 @@ bool SERCOM::startTransmissionWIRE(uint8_t address, SercomWireReadWriteFlag flag`
`611`	`612`	`bool SERCOM::sendDataMasterWIRE(uint8_t data)`
`612`	`613`	`{`
`613`	`614`	`//Send data`
	`615`	`+ sercom->I2CM.INTFLAG.bit.ERROR = 1;`
`614`	`616`	`sercom->I2CM.DATA.bit.DATA = data;`
`615`	`617`
`616`	`618`	`//Wait transmission successful`
`617`	`619`	`while(!sercom->I2CM.INTFLAG.bit.MB) {`
`618`	`620`	`// If a data transfer error occurs, the MB bit may never be set.`
`619`	`621`	`// Check the error bit and bail if it's set.`
`620`		`- if (sercom->I2CM.STATUS.bit.BUSERR) {`
	`622`	`+ // The data transfer errors that can occur (including BUSERR) are all`
	`623`	`+ // rolled up into INTFLAG.bit.ERROR from STATUS.reg`
	`624`	`+ if (sercom->I2CM.INTFLAG.bit.ERROR) {`
`621`	`625`	`return false;`
`622`	`626`	`}`
`623`	`627`	`}`