AHB and AXI protocol response types

1. What are different response type for AHB and AXI and give use of the same?

Both supports 2 bit response signals.

AXI :

Total four response type is there  :

1) OKAY : 

Normal response used for indication that Normal access is succesfully completed and also used to indicate exclusive access has failed.

2) EXOKAY : 

Indicates exclusive access is successful. Or indiicates that either read or write portion of exclusive access has been successful. 

3)SLVERR : 

Indicates that request has been successfully reached to slave but slave has given some error to the request. 

4) DECERR : 

Indicates that interconnect is not able to find any slave that supports requested transaction address.

From above errors only three error are given by SLAVE AXI. While DECERR response should be given by Interconnect if it is not able to find suitable SLAVE for requested address from master device. Spec recommends to keep one default slave to which arbiter route request if it is not able to decode it and that default slave give DECERR response anytime it is requested. 

 AHB 

Four responses are there same as AXI  with different in indication. 

OKAY :
Indicates transfer is going normally and there is not error as long as this response comes from salve with HREADY signal high. This response indicates transfer has completed successfully.

ERROR :
This response indicates that last transfer was unsuccessful due to some ERROR. Master do not required to repeat the transaction if ERROR response arrived from slave/Arbiter.

RETRY : Indicates transfer can not completed immediately and master should request again the same data to slave. 

SPLIT : Indicates transfer is not completed immediately and master should request same data to slave when slave is ready for data transaction.


Difference between SPLIT and RETRY: 
--> Retry : Slave asks master to request again after sometime arbiter will not remove master from arbitration logic. 
--> SPLIT : Slave tell master that I will tell you when I am ready to serve you request till now arbiter will keep you out of arbitration logic.
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