HI-6120 HOLTIC [Holt Integrated Circuits], HI-6120 Datasheet - Page 111

• Manufacturer Part Number: HI-6120
• Description: MIL-STD-1553 Remote Terminal ICs
• Manufacturer: HOLTIC [Holt Integrated Circuits]

Three single-byte SPI commands modify the current ad-
dress pointer value in register 15:
The “Add 4” command may be useful when sequential-
ly accessing the same word (for example, the Control
Word) in a series of 4-word Descriptor Table entries. The
“Add 2” command might be useful for reading the Inter-
rupt Log Buffer, comprised of 2-word log entries. In both
cases, the Add command would be probably followed
by Read command 0x40 to read the location addressed
by the current pointer value. Similarly, Write command
0xC0 writes the location addressed by the current point-
er value. Two command bytes cannot be “chained”; CE
must be negated after the Add command, then reassert-
ed for the following Read or Write command.
The memory address pointer is not affected by fast-
access read/writes to registers 0-14 because fast-
access SPI commands use a separate, internal pointer
not directly accessible to the host.
Just two single-byte SPI commands use the current
address pointer value in register 15 without first loading
or otherwise modifying it:
Either of these commands can be used to read or write
a single location, or may be used when starting a multi-
word read or write by using the pointer’s auto-increment
feature.
Two single-byte SPI commands increment the current
address pointer value in register 15, then perform a read
or write:
Command Write Operation
Command Write Operation
Command Address Pointer Operation
Command Read Operation
Command Read Operation
0xC0
0xC8
0xD0
0xD8
0xE0
0x40
0x48
write location addressed by pointer value
add 1 to pointer then write addressed
location
add 1 to the current pointer value
add 2 to the current pointer value
add 4 to the current pointer value
read location addressed by pointer value
add 1 to pointer then read addressed
location
HOLT INTEGRATED CIRCUITS
HI-6120, HI-6121
111
15.2.5. Data Prefetch for SPI Read Cycles
Data prefetch is a technique used by the HI-6121 to
speed up host multi-word read access to registers or
RAM. Prefetching occurs when HI-6121 logic accesses
data before it is actually needed. Because register or
RAM locations are often read sequentially, performance
improves when data is prefetched in address sequence
order. For any SPI read cycle, the HI-6121 first fetches
the addressed location, then increments the memory
address pointer and prefetches the following address,
to speed up access in the likely event that the following
word will be read next. For the HI-6121, read cycle
prefetch allows the SPI host to read sequential locations
back-to-back, continuing as long as the host asserts
chip select and provides SPI clock. This is described as
the Memory Address Pointer “auto-increment” feature.
There is an exception: read cycle prefetch is blocked
when the next RAM address is a Control Word in the
Descriptor Table. If allowed, pre-fetch (like any other
read) would reset the Control Word DBAC status bit. To
preserve DBAC status bit function, prefetch is disabled
when reading Control Words within Descriptor Table
address range. The table base address (set by the value
in register 0x0005) and every fourth word thereafter is a
Control Word. This consists of table addresses having
these offsets from the table start address: 0, 4, 8, 0xC
through and including 0x1F8 and 0x1FC. See further
information in Section 15.2.7.
These two commands can be used to read or write a
single location, or may be used to start a multi-word read
or write that uses the pointer’s auto-increment feature.
15.2.6. Special Purpose Commands
Several other HI-6121 SPI commands load or otherwise
modify the primary address pointer before initiating a
read or write process. These commands were tailored
to the specific needs of HI-6121 Remote Terminal host
software.
Using a single-byte SPI command, the address pointer
can be directly loaded with the memory address for the
descriptor table Control Word corresponding to the last
completed MIL-STD-1553 command. The Control Word
is then read.
This command can be used to read just the current
Command Read Operation
0x50
Copy Current Control Word Address
register 13 to address pointer register
15. Read the location addressed by the
new pointer value.