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I2C Protocol Subtleties, Half 2: Lacking START


That is the 2nd in a collection of articles on the extra complicated features of the I2C protocol (which cowl TWI and SMBus implementations as nicely). Within the earlier article, we mentioned the essential {hardware} and software program approaches to implementing help for I2C, as a Grasp or as a Slave. This text will focus on the Lacking Begin error situation.

What’s a “regular” START?

Per the I2C specification, START is outlined as a falling SDA line whereas SCL stays excessive. This might theoretically happen at any level throughout a transmission, and will reset all I2C Slave gadgets (and Masters as nicely, in multi-master environments) to be prepared to just accept a Slave Deal with transmission. After all, having this incidence below circumstances apart from a Bus Idle situation or whereas awaiting a STOP is probably going an error (Surprising START); this can be mentioned in a later article.

What’s a “lacking” START?

A Lacking Begin situation might happen if the present bus state permits both a START or a REPEATED START as the following state, however as an alternative sees a falling SCL (with SDA being both excessive or low, however usually it might be excessive). The precedent situations would come with: (a) Bus Idle, adopted by a START; (b) a NAK having been despatched by a Slave system after a Learn operation, adopted by a Repeated START, or (c) a NAK having been despatched by the Grasp after a Write operation, adopted by a Repeated START. Within the case of an ACK having been despatched by both the Grasp or a Slave, it isn’t attainable to detect a 'lacking' START since a START or Repeated START shouldn’t be required below these circumstances.

By definition, a Bus Idle situation can solely be arrived at after a STOP is issued (though at startup an Idle situation could often be assumed after the Grasp is initialized). Throughout Bus Idle, SDA and SCL are outlined as each being excessive. Producing a Lacking Begin error on this case requires SCL to fall whereas SDA stays excessive. The query could possibly be requested, Is it attainable to recuperate legitimate information even on this state of affairs? In I2C, a falling SCL signifies that the Transmitter (both Grasp or Slave) can now advance to the following information bit and replace the SDA line. Nevertheless, there was no earlier legitimate information bit. Slave gadgets must be anticipated to disregard SCL exercise till a legitimate START is seen, because the Slave mustn’t settle for a Slave Deal with with out a legitimate START precedenting it.

Following a NAK situation, the following bus state must be both a STOP or a Repeated START. If exercise is seen on SCL after a NAK, it may be assumed {that a} Repeated START is lacking, since a STOP was not issued (and there earlier than the bus stays below the management of the present Grasp). Slave gadgets seemingly is not going to deal with the primary byte following the NAK as a Slave Deal with, since a START or Repeated START is required to organize the Slave system to just accept a brand new deal with. The query of how the current-active Slave responses to extra clocking after a NAK is device-dependent, however any information offered ought to usually be seen as invalid.

The following article within the collection will give attention to the Lacking Cease error. Thanks for studying!

Supply by Robert Fries

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