Sleep Unit

Source File: rtl/

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 CV32E40X. 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

  • wfe instruction

Table 14 describes the Sleep Unit interface.

Table 14 Sleep Unit interface signals






Core is sleeping because of a wfi or wfe 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 and WFE for details.



Wake-up signal for custom wfe instruction. Positive level sensitive. See WFE for details.

Startup behavior

clk_i is internally gated off (while signaling core_sleep_o = 0) during CV32E40X 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.


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)

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


Figure 13 wfi example


The custom wfe instruction behaves exactly as the wfi instruction, except that a wake-up can additionally be triggered by asserting wu_wfe_i.

The wfe instruction is encoded as a custom SYSTEM instruction with opcode 0x8C00_0073.