Sleep Unit

Source File: rtl/cv32e40p_sleep_unit.sv

The Sleep Unit contains and controls the instantiated clock gate, see Clock Gating Cell, that gates clk_i and produces a gated clock for use by the other modules inside CV32E40P. The Sleep Unit is the only place in which clk_i itself is used; all other modules use the gated version of clk_i.

The clock gating in the Sleep Unit is impacted by the following:

  • rst_ni

  • fetch_enable_i

  • wfi instruction (only when PULP_CLUSTER = 0)

  • cv.elw instruction (only when PULP_CLUSTER = 1)

  • pulp_clock_en_i (only when PULP_CLUSTER = 1)

Table 10 describes the Sleep Unit interface.

Table 10 Sleep Unit interface signals

Signal

Direction

Description

pulp_clock_en_i

input

PULP_CLUSTER = 0: pulp_clock_en_i is not used. Tie to 0.

PULP_CLUSTER = 1: pulp_clock_en_i can be used to gate clk_i internal to the core when core_sleep_o = 1. See PULP Cluster Extension for details.

core_sleep_o

output

PULP_CLUSTER = 0: Core is sleeping because of a wfi instruction. If core_sleep_o = 1, then clk_i is gated off internally and it is allowed to gate off clk_i externally as well. See WFI for details.

PULP_CLUSTER = 1: Core is sleeping because of a cv.elw instruction. If core_sleep_o = 1, then the pulp_clock_en_i directly controls the internally instantiated clock gate and therefore pulp_clock_en_i can be set to 0 to internally gate off clk_i. If core_sleep_o = 0, then it is not allowed to set pulp_clock_en_i to 0. See PULP Cluster Extension for details.

Note

The semantics of pulp_clock_en_i and core_sleep_o depend on the PULP_CLUSTER parameter.

Startup behavior

clk_i is internally gated off (while signaling core_sleep_o = 0) during CV32E40P startup:

  • clk_i is internally gated off during rst_ni assertion

  • clk_i is internally gated off from rst_ni deassertion until fetch_enable_i = 1

After initial assertion of fetch_enable_i, the fetch_enable_i signal is ignored until after a next reset assertion.

WFI

The wfi instruction can under certain conditions be used to enter sleep mode awaiting a locally enabled interrupt to become pending. The operation of wfi is unaffected by the global interrupt bits in mstatus.

A wfi will not enter sleep mode, but will be executed as a regular nop, if any of the following conditions apply:

  • debug_req_i = 1 or a debug request is pending

  • The core is in debug mode

  • The core is performing single stepping (debug)

  • The core has a trigger match (debug)

  • PULP_CLUSTER = 1

If a wfi causes sleep mode entry, then core_sleep_o is set to 1 and clk_i is gated off internally. clk_i is allowed to be gated off externally as well in this scenario. A wake-up can be triggered by any of the following:

  • A locally enabled interrupt is pending

  • A debug request is pending

  • Core is in debug mode

Upon wake-up core_sleep_o is set to 0, clk_i will no longer be gated internally, must not be gated off externally, and instruction execution resumes.

If one of the above wake-up conditions coincides with the wfi instruction, then sleep mode is not entered and core_sleep_o will not become 1.

Figure 9 shows an example waveform for sleep mode entry because of a wfi instruction.

../_images/wfi.svg

Figure 9 wfi example

PULP Cluster Extension

CV32E40P has an optional extension to enable its usage in a PULP Cluster in the PULP (Parallel Ultra Low Power) platform. This extension is enabled by setting the PULP_CLUSTER parameter to 1. The PULP platform is organized as clusters of multiple (typically 4 or 8) CV32E40P cores that share a tightly-coupled data memory, aimed at running digital signal processing applications efficiently.

The mechanism via which CV32E40P cores in a PULP Cluster synchronize with each other is implemented via the custom cv.elw instruction that performs a read transaction on an external Event Unit (which for example implements barriers and semaphores). This read transaction to the Event Unit together with the core_sleep_o signal inform the Event Unit that the CV32E40P is not busy and ready to go to sleep. Only in that case the Event Unit is allowed to set pulp_clock_en_i to 0, thereby gating off clk_i internal to the core. Once the CV32E40P core is ready to start again (e.g. when the last core meets the barrier), pulp_clock_en_i is set to 1 thereby enabling the CV32E40P to run again.

If the PULP Cluster extension is not used (PULP_CLUSTER = 0), the pulp_clock_en_i signal is not used and should be tied to 0.

Execution of a cv.elw instructions causes core_sleep_o = 1 only if all of the following conditions are met:

  • The cv.elw did not yet complete (which can be achieved by witholding data_gnt_i and/or data_rvalid_i)

  • No debug request is pending

  • The core is not in debug mode

  • The core is not single stepping (debug)

  • The core does not have a trigger match (debug)

As pulp_clock_en_i can directly impact the internal clock gate, certain requirements are imposed on the environment of CV32E40P in case PULP_CLUSTER = 1:

  • If core_sleep_o = 0, then pulp_clock_en_i must be 1

  • If pulp_clock_en_i = 0, then irq_i[] must be 0

  • If pulp_clock_en_i = 0, then debug_req_i must be 0

  • If pulp_clock_en_i = 0, then instr_rvalid_i must be 0

  • If pulp_clock_en_i = 0, then instr_gnt_i must be 0

  • If pulp_clock_en_i = 0, then data_rvalid_i must be 0

  • If pulp_clock_en_i = 0, then data_gnt_i must be 0

Figure 10 shows an example waveform for sleep mode entry because of a cv.elw instruction.

../_images/load_event.svg

Figure 10 cv.elw example