3.3.3. Direct Command Register

The direct_cmd Register characteristics are:

Purpose

Initializes and updates the external memory devices by sending the following commands:

  • NOP

  • PRECHARGEALL

  • AUTO REFRESH

  • MODEREG

  • DPD.

Usage constraints

Only accessible in Config state.

Configurations

Available in all configurations of the DMC.

Attributes

See the register summary in Table 3.1.

The direct_cmd Register therefore enables any initialization sequence that an external memory device might require. The only timing information associated with the direct_cmd Register are the command delays that are programmed in the timing registers. Figure 3.2 shows the timing registers. Therefore, if an initialization sequence requires additional delays between commands, they must be timed by the master driving the initialization sequence.

Figure 3.9 shows the direct_cmd Register bit assignments.

Figure 3.9. direct_cmd Register bit assignments


Table 3.5 shows the direct_cmd Register bit assignments.

Table 3.5. direct_cmd Register bit assignments

Bits

Name

Function

[31:23]-Undefined, write as zero
[22]ext_mem_cmdExtended memory command, see Table 3.6
[21:20]chip_nmbrBits mapped to external memory chip selects, cs_n[MEMORY_CHIPS-1:0] or dfi_cs_n[MEMORY_CHIPS-1:0]
[19:18]memory_cmdSelects the command required, see Table 3.6
[17:16]bank_addrBits mapped to external memory bank address bits, ba[1:0] or dfi_bank[1:0], when DMC issues a MODEREG command
[15:14]-Undefined, write as zero
[13:0]addr_13_to_0Bits mapped to external memory address bits, add[13:0] or dfi_address[13:0], when DMC issues a MODEREG command

Table 3.6 shows the memory command encoding from the setting of the ext_mem_cmd and memory_cmd bits.

Table 3.6. Memory command encoding

ext_mem_cmdmemory_cmdCommand
0b00PRECHARGEALL.
0b01AUTO REFRESH.
0b10MODEREG or Extended MODEREG access.
0b11

NOP.

A NOP command asserts all chip selects that are set as active_chips when the chip_nmbr is set to 0. The active_chips field is in the Memory Configuration Register.

If chip_nmbr is set to:

  • 1, only cs_n[1] is asserted

  • 2, only cs_n[2] is asserted

  • 3, only cs_n[3] is asserted.

1b00DPD.
1b01- [a]
1b10- [a]
1b11- [a]

[a] Illegal combination that might cause undefined behavior.


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