4.8.2. External interfaces

The primary external interface to the system is an AHB-Lite master port that supports:

The AHB-Lite master port does not support:

Table 4.8 shows the other AHB-AP ports.

Table 4.8. Other AHB-AP ports

dbgenInputEnables AHB-AP transfers if HIGH. Access to the AHB-AP registers is still permitted if dbgen is LOW, but no AHB transfers are initiated. If a transfer is attempted when dbgen is LOW, then the DAP bus returns dapslverr HIGH.
spidenInputPermits secure transfers to take place on the AHB-AP. If spiden is HIGH, then hprot[6] can be asserted as programmed into the SProt bit in the CSW Register. See Chapter 3 Programmers Model.

HPROT encodings

hprot[6:0] is provided as an external port and is programmed from the Prot field in the CSW register with the following conditions:

  • hprot[4:0] programming is supported.

  • hprot[5] is not programmable and is always LOW. Exclusive access is not supported, and therefore hprot[2] is not supported.

  • hprot[6] programming is supported. hprot[6] HIGH is a non-secure transfer. hprot[6] LOW is a secure transfer. hprot[6] can be asserted LOW by writing to the SProt field in the CSW Register. A secure transfer can only be initiated if spiden is HIGH. If SProt is set LOW in the CSW Register to perform a secure transfer, but spiden is LOW, then no AHB transfer takes place.

See Chapter 3 Programmers Model for values of the Prot field.


hresp[0] is the only RESPONSE signal that the AHB-AP requires:

  • AHB-Lite devices do not support SPLIT and RETRY and therefore hresp[1] is not required. It is still provided as an input, and if not present on any slave it must be tied LOW. Any hresp[1:0] response that is not 0b00, OKAY, is treated as an ERROR response.

  • hresp[2] is not required because exclusive accesses are not supported in the AHB-AP.

HBSTRB support

hbstrb[3:0] signals are automatically generated based on the transfer size hsize[2:0] and haddr[1:0]. Byte, half-word, and word transfers are supported. It is not possible for you to directly control hbstrb[3:0].

Unaligned transfers are not supported. Table 4.9 shows an example of the generated hbstrb[3:0] signals for different-sized transfers.

Table 4.9. Example generation of byte lane strobes

Transfer descriptionhaddr[1:0]hsize[2:0]hbstrb[3:0]
8-bit access to 0x10000b000b0000b0001
8-bit access to 0x10030b110b0000b1000
16-bit access to 0x10020b100b0010b1100
32-bit access to 0x10040b000b0100b1111

AHB-AP transfer types and bursts

The AHB-AP cannot initiate a new AHB transfer every clock cycle because of the additional cycles required to serial scan in the new address or data value through a debug port. The AHB-AP supports two htrans transfer types, IDLE and NONSEQ:

  • When a transfer is in progress, it is of type NONSEQ.

  • When no transfer is in progress and the AHB-AP is still granted the bus, the transfer is of type IDLE.

The only unpacked hburst encoding supported is SINGLE. Packed 8-bit transfers or 16-bit transfers are treated as individual NONSEQ, SINGLE transfers at the AHB-Lite interface. This ensures that there are no issues with boundary wrapping, to avoid additional AHB-AP complexity.

A full AHB master interface can be created by adding an AHB-Lite to AHB wrapper to the output of the AHB-AP, as provided in the AMBA Design Kit.

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