SPRUI30H November 2015 – May 2024 DRA745 , DRA746 , DRA750 , DRA756
The device DDR2/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 DDR2/DDR3 memory and negative terminal connected to a source of reference voltage.
To work properly, a reference voltage must be provided to the device DDR2/DDR3 input buffers. There are two Vref-generation cells per memory channel in the device intended to supply this internal reference voltage.
Figure 18-11 shows the Vref-generation cells in the device.
Both the Vref-generation cells associated with EMIF1 are powerd through the vdds_ddr1 ball. Both the Vref-generation cells associated with EMIF2 are powerd through the vdds_ddr2 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 and CTRL_CORE_CONTROL_DDRIO_1 registers. There are VREF_x_TAP[1:0] control bits which set the output drive capability of the Vref cells. Table 18-23 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 and CTRL_CORE_CONTROL_DDRIO_1 registers, as specified in Table 18-24.
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 and CTRL_CORE_CONTROL_DDRIO_1 registers. These cells can be disabled (for leakage improvement and when not in use) by clearing the same VREF_x_EN bits.
Table 18-25 shows the Vref-generation cells control bits and the DDR2/DDR3 pads used as receivers to which the corresponding Vref cell supplies reference voltage.
Vref-generation Cell Control Bits for Memory Channel 1 | DDR2/DDR3 Vref Cell Associated Pads Used as Receivers (pads for memory channel 1) |
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 | |
Vref-generation Cell Control Bits for Memory Channel 2 | DDR2/DDR3 Vref Cell Associated Pads Used as Receivers (pads for memory channel 2) |
CTRL_CORE_CONTROL_DDRIO_1[26] DDRCH2_VREF_DQ0_INT_CCAP0 | ddr2_d[7:0], ddr2_d[15:8] |
CTRL_CORE_CONTROL_DDRIO_1[25] DDRCH2_VREF_DQ0_INT_CCAP1 | |
CTRL_CORE_CONTROL_DDRIO_1[24] DDRCH2_VREF_DQ0_INT_TAP0 | |
CTRL_CORE_CONTROL_DDRIO_1[23] DDRCH2_VREF_DQ0_INT_TAP1 | |
CTRL_CORE_CONTROL_DDRIO_1[22] DDRCH2_VREF_DQ0_INT_EN | |
CTRL_CORE_CONTROL_DDRIO_1[21] DDRCH2_VREF_DQ1_INT_CCAP0 | ddr2_d[23:16], ddr2_d[31:24] |
CTRL_CORE_CONTROL_DDRIO_1[20] DDRCH2_VREF_DQ1_INT_CCAP1 | |
CTRL_CORE_CONTROL_DDRIO_1[19] DDRCH2_VREF_DQ1_INT_TAP0 | |
CTRL_CORE_CONTROL_DDRIO_1[18] DDRCH2_VREF_DQ1_INT_TAP1 | |
CTRL_CORE_CONTROL_DDRIO_1[17] DDRCH2_VREF_DQ1_INT_EN |