SPRUHZ7K August 2015 – April 2024 AM5706 , AM5708 , AM5716 , AM5718 , AM5718-HIREL
The device DDR3 input buffers work in so-called Vref-based receiver mode. In this mode, the buffers act like differential comparators with positive terminal connected to a device pad which receives signals from DDR3 memory and negative terminal connected to a source of reference voltage.
To work properly, a reference voltage must be provided to the device DDR3 input buffers. There are two Vref-generation cells in the device intended to supply this internal reference voltage.
Figure 18-12 shows the Vref-generation cells in the device.
Figure 18-12 Vref-Generation Cells and Their ControlsBoth the Vref-generation cells associated with EMIF1 are powerd through the vdds_ddr1 ball. For more information, see the device data manual.
The control bits for the Vref-generation cells reside in the CTRL_CORE_CONTROL_DDRIO_0 register. There are VREF_x_TAP[1:0] control bits which set the output drive capability of the Vref cells. Table 18-26 lists the possible options for selection of load current sourced from the output of each Vref-generation cell.
| VREF_x_TAP1 | VREF_x_TAP0 | Description |
|---|---|---|
| 0 | 0 | 2-μA load current |
| 0 | 1 | 4-μA load current |
| 1 | 0 | 8-μA load current |
| 1 | 1 | 32-μA load current |
According to the noise environment, the user can choose to filter the supplied reference voltage. Two coupling capacitors internal to each Vref-generation cell are available and configurable through the VREF_x_CCAP[1:0] control bits in the CTRL_CORE_CONTROL_DDRIO_0 register, as specified in Table 18-27.
| VREF_x_CCAP1 | VREF_x_CCAP0 | Capacitor |
|---|---|---|
| 0 | 0 | No capacitor connected. |
| 0 | 1 | One capacitor connected between Vbias and ground.(1) |
| 1 | 0 | One capacitor connected between Vbias and Vdds.(2) |
| 1 | 1 | One capacitor connected between Vbias and ground and one capacitor connected between Vbias and Vdds. |
The Vref-gereration cells can be enabled by setting to 0x1 the VREF_x_EN bits in the CTRL_CORE_CONTROL_DDRIO_0 register. These cells can be disabled (for leakage improvement and when not in use) by clearing the same VREF_x_EN bits.
Table 18-28 shows the Vref-generation cells control bits and the DDR3 pads used as receivers to which the corresponding Vref cell supplies reference voltage.
| Vref-generation Cell Control Bits | DDR3 Vref Cell Associated Pads Used as Receivers |
| CTRL_CORE_CONTROL_DDRIO_0[19] DDRCH1_VREF_DQ0_INT_CCAP0 | ddr1_d[7:0], ddr1_d[15:8] |
| CTRL_CORE_CONTROL_DDRIO_0[18] DDRCH1_VREF_DQ0_INT_CCAP1 | |
| CTRL_CORE_CONTROL_DDRIO_0[17] DDRCH1_VREF_DQ0_INT_TAP0 | |
| CTRL_CORE_CONTROL_DDRIO_0[16] DDRCH1_VREF_DQ0_INT_TAP1 | |
| CTRL_CORE_CONTROL_DDRIO_0[15] DDRCH1_VREF_DQ0_INT_EN | |
| CTRL_CORE_CONTROL_DDRIO_0[14] DDRCH1_VREF_DQ1_INT_CCAP0 | ddr1_d[23:16], ddr1_d[31:24], ddr1_ecc_d[7:0] |
| CTRL_CORE_CONTROL_DDRIO_0[13] DDRCH1_VREF_DQ1_INT_CCAP1 | |
| CTRL_CORE_CONTROL_DDRIO_0[12] DDRCH1_VREF_DQ1_INT_TAP0 | |
| CTRL_CORE_CONTROL_DDRIO_0[11] DDRCH1_VREF_DQ1_INT_TAP1 | |
| CTRL_CORE_CONTROL_DDRIO_0[10] DDRCH1_VREF_DQ1_INT_EN |