AMBA Design Kit Technical Reference Manual - ARM Information ...

AHB ComponentsDownsizer transfersThe following are options for downsizer transfers:Downsizer not selectedWhen the HSELS signal of the downsizer module is LOW, thetransfer is passed to the 32-bit AHB and HSELM is driven LOW.The 32-bit slaves must ignore the transfers by monitoringHSELM and HADDRM. HREADYS from the 64-bit bus isoutput to HREADYM. All 32-bit devices connected to the 32-bitAHB must monitor this HREADYM signal to determine the endof the current transfer and the start of the next.Narrow transfers, downsizer selectedIf the downsizer module is selected, and the transfer is 32 bits orless, the downsizer module passes the transfer through. All of thecontrol signals and responses from the slave are left unmodified.In this case, the only function of the downsizer is to route theappropriate half of the wide master write data bus on to the narrowslave data bus for write transfers.Read transfers require even less control and the narrow slave readdata is replicated across the wide master bus.Table 3-41 shows the handling of narrow transfers.Table 3-41 Narrow transfer handlingTransfer on64-bit AHB32, 16, or 8-bittransferTransfer on32-bit AHB32, 16, or 8-bittransferAddressHADDR pass through.If HADDR[2] = 0 then HWDATAS[31:0] pass through else HWDATAS[63:32] passthrough. HRDATAS = HRDATAM, HRDATAM.For 32, 16, and 8-bit transfers, HWDATA is selected by bit [2] ofthe transfer address. If this bit is set to 0, HWDATA[31:0] isrouted to the 32-bit AHB. If this bit is set to 1, bits [63:32] arerouted.If an ERROR, SPLIT, or RETRY response is received from the32-bit slave, the downsizer module automatically terminates thecurrent transfer by passing the response to the 64-bit bus. If thecurrent transfer request on the 64-bit bus is a valid transfer(NON_SEQ or SEQ), it is captured by the registers in the3-86 Copyright © 2003, 2007 ARM Limited. All rights reserved. ARM DDI 0243C