RISC-V Debug System Wrapper Technical Specification
This document specifies the RISC-V Debug System wrapper functionality.
The debug system follows the execution-based debug approach described in the RISC-V Debug Specification 0.13.2 and provides the following features.
- JTAG Test Access Port (TAP)
- Run-control debug features (in cooperation with the CPU core), including breakpoints, single-stepping through code, and reading core registers
- System Bus Access (SBA): Access to arbitrary bus-attached peripherals through JTAG
- Compatible with RISC-V Debug Specification 0.13-compliant debug software, including OpenOCD and GDB
- TileLink Uncached Light (TL-UL) bus interfaces
This module provides a RISC-V Debug Specification-compliant debug system with TileLink Uncached Light bus interfaces. The main functionality is provided by the PULP RISC-V Debug System, which is instantiated by this module. All bus interfaces are converted into TL-UL.
See the PULP RISC-V Debug System Documentation for a full list of features and further design documentation. This document only describes the additional logic provided on top of the PULP RISC-V Debug System.
The debug system is compliant with the RISC-V Debug Specification 0.13.2.
The memory map accessible over the TL-UL device interface is documented in the Debug Memory section of the PULP RISC-V Debug System Documentation. Note this contains a mixture of read only and read-write regions and which is which isn’t documented. The read-write regions are:
0x10c: Halted, Going, Resuming, Exception
0x382: DataAddr (
0x383 - 0x338are not implemented and will read undefined data)
All other regions are read only.
The Debug Module Registers are only accessible via the Debug Module Interface (DMI) accessed via JTAG. There is no access to these via the TL-UL device interface.
All hardware interfaces of the debug system are documented in the PULP RISC-V Debug System Documentation, with the exception of the bus interfaces, which are converted to TL-UL by this wrapper.
Referring to the Comportable guideline for peripheral device functionality, the module
rv_dm has the following hardware interfaces defined.
Other Clocks: none
Bus Device Interfaces (TL-UL):
Bus Host Interfaces (TL-UL):
Peripheral Pins for Chip IO: none
Inter-Module Signals: Reference
This fatal alert is triggered when a fatal TL-UL bus integrity fault is detected.
End-to-end bus integrity scheme.
The life cycle hardware debug enable signal is multibit encoded.
The debug module is enabled with the LC_HW_DEBUG_EN signal. This enablement is implemented by gating / enabling critical blocks with separately buffered copies of the life cycle signal. This comprises the debug module interface (DMI) attached to the TAP, the reset request line, the system bus access module (SBA), the debug request output, the TL-UL adapter for the debug ROM, and the ifetch indicator being fed into the TL-UL adapter for the debug ROM.
The instruction fetch enable signal that is modulated with LC_HW_DEBUG_EN and that feeds into the TL-UL adapter is multibit encoded.
The table below lists other debug module signals.
||Multibit life cycle hardware debug enable signal coming from life cycle controller, asserted when the hardware debug mechanisms are enabled in the system.|
||Non-debug module reset request going to the system reset infrastructure.|
||This signal indicates whether the debug module is active and can be used to prevent power down of the core and bus-attached peripherals.|
||This is the debug request interrupt going to the main processor.|
||This signal indicates to the debug module that the main processor is not available for debug (e.g. due to a low-power state).|
||JTAG input signals|
||JTAG output signals|
Debug system functionality is controlled by the HW_DEBUG_EN function of the life cycle controller.
input lc_ctrl_pkg::lc_tx_t lc_hw_debug_en_i, // Debug module lifecycle enable/disable
The debug system provides a standard JTAG (IEEE Std 1149.1-2013) port for external debug access.
All JTAG logic is clocked with an externally supplied test clock (
The protocol used for this JTAG port is specified in the RISC-V Debug Specification as JTAG Debug Transport Module (DTM).
input logic tck_i, // JTAG test clock pad input logic tms_i, // JTAG test mode select pad input logic trst_ni, // JTAG test reset pad input logic td_i, // JTAG test data input pad output logic td_o, // JTAG test data output pad output logic tdo_oe_o // Data out output enable
The debug system is able to reset the system through its JTAG connection; the non-debug module reset (
ndmreset_req_o) signals this intent.
It is up to the larger system design to specify which parts of the system are actually reset by this signal.
dmactive_o signals that some kind of debugging is ongoing.
Use this signal to prevent the power down of the core and bus-attached peripherals, which might be accessed by the debug system.
output logic ndmreset_o, // non-debug module reset output logic dmactive_o, // debug module is active
Most communication between the core and the debug system is performed through the debug memory.
To enter debug mode due to an external debug request, the debug system provides a
If the core is unavailable to the debug system, e.g. because it is powered down or in a locked-down state, the
unavailable_i signal can signal this condition to the debug system.
output logic [NrHarts-1:0] debug_req_o, // async debug request input logic [NrHarts-1:0] unavailable_i, // communicate whether the hart is unavailable // (e.g.: power down)
The debug system implements execution-based debug according to the RISC-V Debug Specification. Most interactions between the core and the debug system are performed through the debug memory, a bus-exposed memory. The memory needs to be accessible from the core instruction and data interfaces. A full memory map is part of the PULP RISC-V Debug System Documentation.
input tlul_pkg::tl_h2d_t tl_d_i, output tlul_pkg::tl_d2h_t tl_d_o,
Bus-attached peripherals can be accessed through the debug system, a functionality called System Bus Access (SBA) in the RISC-V Debug Specification. It is up to the interconnect fabric to decide which peripherals are actually accessible. The debug system wrapper provides a TL-UL host bus interface for SBA.
// bus host, for system bus accesses output tlul_pkg::tl_h2d_t tl_h_o, input tlul_pkg::tl_d2h_t tl_h_i,