GR740-UM-DS, Nov 2017, Version 1.7
290
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GR740
In 8-bit mode, the PROM devices should be connected to the MSB byte of the data bus (PROMIO_-
DATA[15:8]). The LSB address bus should be used for addressing (PROMIO_ADDR[25:0]). In 16-
bit mode, PROMIO_DATA[15:0] should be used as data bus, and PROMIO_ADDR[26:1] as address
bus. EDAC protection is not available in 16-bit mode.
19.5
8- and 16-bit I/O access
Similar to the PROM area, the IO area can also be configured to 8- or 16-bits mode. However, the I/O
device will NOT be accessed by multiple 8/16 bits accesses as the memory areas, but only with one
single access just as in 32-bit mode. To access an IO device on an 8-bit bus, only byte accesses should
be used (LDUB/STB instructions for the CPU). To accesses an IO device on a 16-bit bus, only half-
word accesses should be used (LDUH/STH instructions for the CPU).
19.6
Burst cycles
To improve the bandwidth of the memory bus, accesses to consecutive addresses can be performed in
burst mode. Burst transfers will be generated when the memory controller is accessed using an AHB
burst request. These includes instruction cache-line fills, double loads and double stores. The timing
of a burst cycle is identical to the programmed basic cycle with the exception that during read cycles,
the idle cycle will only occurs after the last transfer. Burst cycles will not be generated to the IO area.
Only word (32-bit) bursts of incremental type is supported. Note that the processors can access the
PROM area using larger accesses. The AHB/AHB bridge connecting the Processor AHB bus to the
Slave I/O AHB bus will split larger accesses into bursts of 32-bit accesses. Note that all accesses to
this memory controller traverses over the bridge connecting the Processor AHB bus to the Slave I/O
Figure 30.
8-bit memory interface example
CS
OE
WE
A
D
PROM
PROM_CEN[0]
PROMIO_OEN
A
D
MEMORY
PROMIO_ADDR[27:0]
PROMIO_DATA[15:8]
D[15:8]
A[25:0]
PROMIO_WEN
8-bit PROM
CONTROLLER
Figure 31.
16-bit memory interface example
CS
OE
WE
A
D
PROM
PROM_CEN[0]
PROMIO_OEN
A
D
PROMIO_ADDR[27:0]
PROMIO_DATA[15:0]
D[15:0]
A[26:1]
PROMIO_WEN
16-bit PROM
MEMORY
CONTROLLER
