AR# 47564

Zynq-7000 SoC, DDR - I/O Buffers do not Support External VREF

Description

DDR I/O buffers (DDRIOB) use differential input receivers for which one input to the receiver is connected to the data input and the other is connected to a voltage reference pin called VREF (set to half of the DDR VCC I/O voltage).

VREF needs to be supplied from the internal voltage source because the external VREF feature for DDRIOB is not supported.

Solution

Impact:	Minor. Use the internal VREF reference.
Work-around:	Use the internal VREF reference, see the Solution section for more information.
Configurations Affected:	Systems that use the PS DDR memory controller.
Device Revision(s) Affected:	Refer to (Xilinx Answer 47916)

Use the Internal VREF feature. To enable internal VREF:

Set DDRIOB_DDR_CTRL.VREF_EXT_EN to 00 (disconnect I/Os from external signal)
Set DDRIOB_DDR_CTRL.VREF_SEL to the appropriate voltage setting depending on the DDR standard. (VREF = Vcco_ddr / 2)
Set DDRIOB_DDR_CTRL.VREF_INT_EN to 1 to enable the internal VREF generator

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AR# 47564
Date	05/25/2018
Status	Active
Type	Design Advisory
Devices	Zynq-7000

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AR# 47564

Zynq-7000 SoC, DDR - I/O Buffers do not Support External VREF

Description

Solution

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