External Components

SystemRDL allows some component instances to be defined as “external” elements of an address space definition. In the context of this regblock generator, the implementation of an external component is left up to the designer. When generating the RTL for a regblock, the implementations of external components are omitted and instead a user-interface is presented on the hwif_in/hwif_out i/o structs.

External component signals on the hardware interface closely follow the semantics of the Internal CPUIF Protocol.

Things you should know

By default external hwif_out signals are driven combinationally. An optional output retiming stage can be enabled if needed.
Due to the uncertain access latency of external components, the regblock will always enforce that only one outstanding transaction to an external component at a time. This is enforced even if the CPUIF is capable of pipelined accesses such as AXI4-Lite.

External Registers

External registers can be useful if it is necessary to implement a register that cannot easily be expressed using SystemRDL semantics. This could be a unique access policy, or FIFO-like push/pop registers.

External registers are annotated as such by using the external keyword:

// An internal register
my_reg int_reg;

// An external register
external my_reg ext_reg;

Request

hwif_out..req

When asserted, a read or write transfer will be initiated. Qualifies all other request signals.

If the register is wide (regwidth > accesswidth), then the hwif_out..req will consist of multiple bits, representing the access strobe for each sub-word of the register.

If the register does not contain any readable fields, this strobe will be suppressed for read operations.

If the register does not contain any writable readable fields, this strobe will be suppressed for write operations.

hwif_out..req_is_wr

If 1, denotes that the current transfer is a write. Otherwise transfer is a read.

hwif_out..wr_data

Data to be written for the write transfer. This signal is ignored for read transfers.

The bit-width of this signal always matches the CPUIF’s bus width, regardless of the regwidth.

If the register does not contain any writable fields, this signal is omitted.

hwif_out..wr_biten

Active-high bit-level write-enable strobes. Only asserted bit positions will change the register value during a write transfer.

If the register does not contain any writable fields, this signal is omitted.

Read Response

hwif_in..rd_ack

Single-cycle strobe indicating a read transfer has completed. Qualifies all other read response signals.

If the transfer is always completed in the same cycle, it is acceptable to tie this signal to hwif_out..req && !hwif_out..req_is_wr.

If the register does not contain any readable fields, this signal is omitted.

hwif_in..rd_data

Read response data.

If the register does not contain any readable fields, this signal is omitted.

Write Response

hwif_in..wr_ack

Single-cycle strobe indicating a write transfer has completed.

If the transfer is always completed in the same cycle, it is acceptable to tie this signal to hwif_out..req && hwif_out..req_is_wr.

If the register does not contain any writable fields, this signal is omitted.

External Blocks

Broader external address regions can be represented by external block-like components such as addrmap, regfile or mem elements.

To ensure address decoding for external blocks is simple (only requires simple bit-pruning), blocks that are external to an exported regblock shall be aligned to their size.

Request

hwif_out..req

When asserted, a read or write transfer will be initiated. Qualifies all other request signals.

hwif_out..addr

Byte-address of the transfer.

Address is always relative to the block’s local addressing. i.e: The first byte within an external block is represented as hwif_out..addr == 0, regardless of the absolute address of the block.

hwif_out..req_is_wr

If 1, denotes that the current transfer is a write. Otherwise transfer is a read.

hwif_out..wr_data

Data to be written for the write transfer. This signal is ignored for read transfers.

The bit-width of this signal always matches the CPUIF’s bus width, regardless of the contents of the block.

hwif_out..wr_biten

Active-high bit-level write-enable strobes. Only asserted bit positions will change the register value during a write transfer.

Read Response

hwif_in..rd_ack: Single-cycle strobe indicating a read transfer has completed. Qualifies all other read response signals.
hwif_in..rd_data: Read response data.

Write Response

hwif_in..wr_ack: Single-cycle strobe indicating a write transfer has completed.