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AR# 14583

MicroBlaze - What should I do with unused OPB signals when creating a custom OPB slave peripheral?


General Description: 

I am creating a custom MicroBlaze OPB slave peripheral, and I am not using the signals "Sl_errACK", "Sl_toutSup", "Sl_retry", "OPB_seqADDR", or "OPB_BE(0:3)". What should I do with these signals?


The signals that are required on a Xilinx OPB slave peripheral in order for a MicroBlaze system to properly connect them to the OPB are shown in Table 2 in the "OPB Usage in Xilinx FPGA" chapter of the MicroBlaze Hardware Reference Guide. The signal-naming convention and other information about the byte-enable subset of OPB that Xilinx supports are also discussed in this document.  


MicroBlaze is fully compatible with the byte-enable version of OPB. MicroBlaze does not support "legacy" signals such as "OPB_hwAck", "OPB_fwAck", and "OPB_dwAck", as doing so would necessitate the support of conversion cycles on the OPB, which is an expensive use of FPGA resources.  


Thus, a port on the slave is needed in order for these signals to be connected to the OPB properly. Anything that is not used will be trimmed in the synthesis or map processes. Unused input signals from the OPB to the slave peripheral may be left floating; unused output signals must be tied to zero. 


This signal set for OPB is standardized so that all OPB devices (master, slaves, and combination master/slaves) present a common interface to customers and the MicroBlaze system generation tools. If these signals are present and a correct MPD file is generated (see the "Microprocessor Peripheral Definition Format" chapter in the MicroBlaze Hardware Reference Guide), the MicroBlaze system generation tools will correctly stitch the peripheral into the OPB.  


The MicroBlaze Hardware Reference Guide is located at http://support.xilinx.com/ipcenter/processor_central/microblaze/doc/hwref.pdf

AR# 14583
Date 05/14/2014
Status Archive
Type General Article