HD6417615 Hitachi Semiconductor, HD6417615 Datasheet - Page 352

• Manufacturer Part Number: HD6417615
• Description: Hardware Manual
• Manufacturer: Hitachi Semiconductor

In the case of a power-on reset, the bus state controller’s registers are initialized, and therefore the
self-refresh state is cleared.
7.6.9
When DRAM is used after the power is turned on, there is a requirement for a waiting period
during which accesses cannot be performed (100 µs or 200 µs minimum) followed by at least the
prescribed number of dummy CAS-before-RAS refresh cycles (usually 8). The bus state controller
(BSC) does not perform any special operations for the power-on reset, so the required power-on
sequence must be implemented by the initialization program executed after a power-on reset.
7.7
Set the BSTROM bit in BCR1 to set the CS0 space for connection to burst ROM. The burst ROM
interface is used to permit fast access to ROMs that have the nibble access function. Figure 7.51
shows the timing of nibble accesses to burst ROM. Set for two wait cycles. The access is basically
the same as an ordinary access, but when the first cycle ends, only the address is changed. The
CS0 signal is not negated, enabling the next access to be conducted without the T1 cycle required
for ordinary space access. From the second time on, the T1 cycle is omitted, so access is 1 cycle
faster than ordinary accesses. Currently, the nibble access can only be used on 4-address ROM.
This function can only be utilized for word or longword reads to 8-bit ROM and longword reads to
16-bit ROM. Mask ROMs have slow access speeds and require 4 instruction fetches for 8-bit
widths and 16 accesses for cache filling. Limited support of nibble access was thus added to
alleviate this problem. When connecting to an 8-bit width ROM, a maximum of 4 consecutive
accesses are performed; when connecting to a 16-bit width ROM, a maximum of 2 consecutive
accesses are performed. Figure 7.50 shows the relationship between data width and access size.
For cache filling and DMAC 16-byte transfers, longword accesses are repeated 4 times.
CKIO
RAS
CASn
Power-On Sequence
Burst ROM Interface
Tp
Figure 7.49 DRAM Self-Refresh Cycle Timing
Trr
Trc1
Trc2
Trc2
Tre
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