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diff --git a/doc/README.t4240qds b/doc/README.t4240qds
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--- a/doc/README.t4240qds
+++ b/doc/README.t4240qds
@@ -96,3 +96,27 @@ The addresses in brackets are physical addresses.
0x0_ffff_f000 (0x0_7fff_fff0) - 0x0_ffff_ffff 4KB Boot page translation for secondary cores
The physical address of the last (boot page translation) varies with the actual DDR size.
+
+Voltage ID and VDD override
+--------------------
+T4240 has a VID feature. U-boot reads the VID efuses and adjust the voltage
+accordingly. The voltage can also be override by command vdd_override. The
+syntax is
+
+vdd_override <voltage in mV>, eg. 1050 is for 1.050v.
+
+Upon success, the actual voltage will be read back. The value is checked
+for safety and any invalid value will not adjust the voltage.
+
+Another way to override VDD is to use environmental variable, in case of using
+command is too late for some debugging. The syntax is
+
+setenv t4240qds_vdd_mv <voltage in mV>
+saveenv
+reset
+
+The override voltage takes effect when booting.
+
+Note: voltage adjustment needs to be done step by step. Changing voltage too
+rapidly may cause current surge. The voltage stepping is done by software.
+Users can set the final voltage directly.