What is SMBus free timeout? What is the behavior of SMBus after the free timeout occurs?
The SMBus specification stipulates that if the SCL and SDA lines remain high for more that 50 µs, the bus is designated as free.
The SMBus Free Timeout detection on 8-bit MCUs can be enabled by setting the SMBFTE (or SMBnFTE) bit. When this bit is set to logic 1, the bus will be considered free if SCL and SDA remain high for more than 10 SMBus clock source periods. A clock source which is determined by the SMBus clock setting (the SMBCS bit field in SMB0CF ) is required for free timeout detection, even in a slave-only implementation.
When a Free Timeout is detected, the interface will respond as if a STOP was detected. For a matched slave address, the Free Timeout sets STO flag and generates an interrupt, the interrupt must be cleared by firmware.
If the Free Timeout condition happens after STOP detection but before servicing the STOP status flag, then STOP bit gets cleared by the Free Timeout, but the SI interrupt flag is not affected.
If the Free Timeout condition happens after STOP detection and servicing, then the STOP status flag will not be set by the Free Timeout condition.