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TCAN1145-Q1: Readback value of reserved register bits

Isao Takigasaki
Intellectual 2530 points
Part Number: TCAN1145-Q1

Hi,
Could you answer for the following questions related to reserved register bits?

1. If the readback value of reserved register bits is different from the reset value, can we  judge that any failure occurred in communication with the device?  
The readback value of the register bits with R of type should be always fixed value and the register bits with RH of type can be changed, because the each bits can be set or reset by hardware.  So, I think we can judge by reading the register bits with R if any failure is occurred.

2. Several register bits which works as a variable can have reserved value.  For example, MODE_SEL[2:0] is one of them and the values than 001(Sleep),  100(Standby), 101(Listen), and 111(Normal) are reserved. 
2-1. If users try to write the reserved values, the written value is ignored and register value is kept the value, so that the readback value should be previous one after the register write.  Is my understanding correct?
2-2. And if so, we can judge any failure is occurred when reserved value is read back.  Is the understanding correct?

Best Regards, Taki

  • 0
    TI__Mastermind 48215 points

    Hi Taki-san,

    Thanks for your patience while we were on break.

    1. This is not necessarily true. Several registers that are classified by "Set or cleared by hardware" are expected to have their values changed during normal operation. For example, the global interrupt register INT_GLOBAL is "RH". This register will reflect the status of other interrupt registers. Having any of these values change from their reset value is expected for several interrupt types that do not necessarily represent a failure. 

    2. If reserved values are writing to the MODE_SEL bits for example, the invalid write will be ignored and the original register value will be maintained. During this scenario, a readback of the register will reflect the old value, so readback checking is a valid way to screen for such communication errors. 

    Overall, it sounds like the desire here is to use a readback method to validate any SPI writes to the device. This is a valid way to screen for data errors in a SPI write. Examples of this method are included in the TCAN114x sample drivers using the TCAN114X_CONFIGURE_VERIFY_WRITES definition. Please send me an email if you need access to this sample. 

    Let me know if you have any more questions.

    Regards,
    Eric Schott