Note: The v1.0 Ultra96 board definition files (BDF) embedded in Vivado 2018.1 and 2018.2 have a bug. The latest BDF is on the Avnet GitHub here: https://github.com/Avnet/bdf . The article below describes the parameters included in the v1.2 board definition file.
The Avnet Ultra96 board from Avnet has 2 GB of LPDDR4 RAM that is interfaced to the Zynq UltraScale+ MPSoC's Processing System (PS) DDR Controller. This versatile controller is described in Chapter 17 of the ZU+ Technical Reference Manual (UG1085). The DDR subsystem supports DDR3, DDR3L, LPDDR3, DDR4, and LPDDR4. In order to support all these memories at many different interface speeds, it is critical that the hardware designer enter correct parameters into the Vivado Block Design.
Ultra96 uses Micron MT53B512M32D2NP-062 WT, which is a 16 Gbit component consisting of two 8 Gbit die. This -062 rating stands for 625 ps clock period, which is 1600 MHz. Because it is double data-rate, this is considered a 3200 Mbps device.
The MPSoC on Ultra96 is the XCZU3EG-1SBVA484E. This cost-optimized package supports an LPDDR4 maximum interface rate of 1066 Mbps. Therefore, the ideal clock rate is 533 MHz. In version 1.0 of the BDF, Vivado is instructed to automatically calculate the PLL parameters, which results in an actual output clock frequency of 525 MHz. The v1.2 BDF explicitly sets the PLL parameters to produce 533.3333 MHz. Therefore, the clock period tCK = 1.875 ns.
In Vivado Block Design for Zynq UltraScale+, the DDR Configuration dialog expects several parameters in the DDR Memory Options section. These parameters can be found using the Micron LPDDR4 datasheet, knowing what the memory clock period tCK (1.875 ns) is for this particular design.
- RAS to CAS Delay (cycles), abbreviated tRCD.
- tRCD (ns) = MAX(18ns, 4nCK)
- 4nCK = 4 * tCK = 7.5ns
- MAX(18ns, 4nCK) = 18ns
- 18ns/tCK = 9.6, so round up to 10
- tRCD (cycles) = 10 cycles
- tRCD (ns) = MAX(18ns, 4nCK)
- Precharge Time (cycles), abbreviated tRP
- Micron provides both an All Bank precharge (tRPab) and a Per Bank precharge (tRPpb). We will find the maximum.
- tRPab (ns) = MAX(21ns, 3nCK)
- 3nCK = 5.715ns
- MAX(21ns, 3nCK) = 21ns
- tRPpb = MAX(18ns, 3nCK)
- 3nCK = 5.715ns
- MAX(18ns, 3nCK) = 18ns
- tRPab (ns) = MAX(21ns, 3nCK)
- tRP (ns) = MAX(tRPab, tRPpb) = 21ns
- 21ns/tCK = 11.2, so round up to 12
- tRP (cycles) = 12 cycles
- Micron provides both an All Bank precharge (tRPab) and a Per Bank precharge (tRPpb). We will find the maximum.
- Row active time, abbreviated tRAS (ns)
- tRAS = MAX(42ns, 3nCK) = 42ns
- ACTIVATE-to-ACTIVATE command period (same bank), abbreviated tRC (ns)
- tRC (ns) = MAX{[tRAS + tRPab (with all-bank precharge)], [tRAS + tRPpb (with per-bank precharge)]}
- tRAS + tRPab = 42ns + 21ns = 63ns
- tRAS + tRPpb = 42ns + 18ns = 60ns
- tRC (ns) = 63ns
- Four-bank activate window, abbreviated tFAW (ns)
- tFAW (1066 Data Rate) = 40ns
- DRAM IC Bus Width (per die)
- 32 bits, based on the Part Number MT53B512M32D2
- DRAM Device Capacity (per die, so use 16 instead of 32)
- 512 Meg * 16 bits = 8192 Mbits
- Also shown in Table 2 as "Density per die"
- Row Address Count (bits)
- Table 2, x16 Row addresses for 512M32 (16Gb) device = R[14:0] = 15
- Dual Rank
- Yes, checked, since Table 2 Configuration for 512M32 (16Gb) device shows "32Mb x 16 DQ x 8 banks x 2 channels x 2 ranks"
The resulting preset provides an LPDDR4 configuration in the block design that looks like this in Vivado:
