
____________________________________________________ DS34T101, DS34T102, DS34T104, DS34T108
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Table 10-1. CPU Data Bus Widths
Value
Data Bus
Width
Access to
Chip Internal
Resources
Access to
SDRAM
Data Bus
Bits
MSB
Pins Used
1
32 bits
32 bit only
8, 16, 32 bit
3:0
0
16 bits
16 bit only
8, 16 bit
1:0
Burst accesses are not supported. The device uses the big-endian byte order, as explained in section
11.1.
The CPU starts an access to the device by asserting the
H_CS_N signal (active low), accompanied by the desired
(for a write access) on the
H_D[31:0] pins. In response, the device asserts
H_READY_N to indicate that the access
has been carried out. The ready assertion indicates that data from the CPU has been written into the device
register or external SDRAM (for write access) or that valid data from register/SDRAM is present on the data bus
(for read access). In response to
H_READY_N assertion, the CPU de-asserts
H_CS_N. This causes the chip to
de-assert
H_READY_N, and thereby finish the CPU access.
In order to make CPU operation more efficient, the device immediately asserts
H_READY_N during a write access.
On successive accesses (write or read)
H_READY_N is asserted only after the previous write has been completed.
write byte enables, replacing the functionality of
H_AD[0] in the address bus. These signals enable byte-resolution
write access to the external SDRAM.
When performing a write access to internal chip resources, all
H_WR_BE pins should be asserted since write
access to device registers must be done at the full bus width only.
Examples of read and write accesses on 32- and 16-bit buses are shown in the figures below.
Figure 10-2. Write Access, 32-Bit Bus
DAT_32_16_N[0]
H_CS_N[0]
H_AD[24:1]
H_R_W_N[0]
H_READY_N[0]
[0]
H_D[31:24]
H_D[23:16]
H_D[15:8]
H_D[7:0]
H_WR_BE3_N[0]
H_WR_BE2_N[0]
H_WR_BE1_N[0]
H_WR_BE0_N[0]
cpu_addr[[1]='don't care'
cpu_addr[1]='don't care'
data ignored
valid
data ignored
valid
data ignored
valid
data_ignored
data ignored
valid
SDRAM WRITE ACCESS
32 bit data bus
INTERNAL
Figure 10-2 shows two write accesses to the SDRAM, one to a byte (at address 2) and the other to a word (at
addresses 0 and 1), followed by a write access to the internal chip resources.