LH79524/LH79525 User’s Guide
External Memory Controller
Version 1.0
7-27
7.4.4 Data Mask Signals
Depending on the external memory system width and the operand size, one or two mem-
ory cycles may be required for operand transfer. The Data Mask signals (DQM[3:0]) select
the data phase for each cycle, as shown in Table 7-9.
• For 32-bit wide memory systems, only one memory cycle is required for any data trans-
fer width, with the DQM bits configured on write cycles to disable bytes unaffected by
the transfer.
• For 16-bit wide memory systems, DQM[1] is used as the memory system upper data
mask (UDQM) and DQM[0] is used as the lower data mask (LDQM).
• For 32-bit transfers in 16-bit wide memory systems, two memory data phases are
required to complete the memory cycles. Half word (16-bit) and byte-width transfers
complete in one data phase.
Table 7-9. Memory System Examples
MEMORY SYSTEM
SIZE
DATA BUS
AHB PHYSICAL
ADDRESS
BYTE ENABLES
16M by 16-bit
32MB
D[15:0]
A[24:1]
DQM[1:0]
16M by 32-bit
64MB
D[31:0]
A[25:2]
DQM[3:0]
64M by 16-bit
128MB
D[15:0]
A[26:1]
DQM[1:0]
64M by 32-bit
256MB
D[31:0]
A[27:2]
DQM[3:0]