Program Memory
Overlay Memory
64 Byte
0x00001000
1 kByte
0x00024400
1 kByte
0x00000000
64 Byte
0x00000400
Module Operation
674
SPNU503C – March 2018
Copyright © 2018, Texas Instruments Incorporated
Parameter Overlay Module (POM)
18.2.1 Decode Regions
There are 32 decode regions. Regions are defined by a start address and a region size. The start address
is 23 bits wide to cover the maximum 8 MByte address space defined for program memory. The region
size ranges from 64 Bytes to 256 kBytes with a step size of power of 2. If a region size of 0 is selected,
the region is disabled. To support overlay memory that is not the same size as the program memory, an
overlay start address that is 23-bits wide can be specified. The size of the overlay region is the same as
the program memory region. The start address of both program memory and overlay memory region have
to be a multiple of the programmed region size. When the address of the access falls into one of the
programmed regions, the POM will start requesting the data from the overlay memory address, based on
the overlay start address. If the address falls into multiple overlapping regions, the region with the lowest
number has highest priority and only one read request from overlay memory will be initiated. This avoids
that regions with the same program memory address, but different overlay memory addresses, request
different data.
Figure 18-2. Region Definition Example
18.2.2 Bus Errors on Accesses via POM
In case a read by the POM cannot be completed within 32 HCLK cycles due to other transactions ongoing
or due to other bus errors, a timeout mechanism is implemented. This timeout mechanism is disabled by
default and must be enabled by writing Ah to the enable timeout (ETO) field of the POM global control
register (POMGLBCTRL).
An abort is generated to the host CPU when this timeout occurs. The host CPU responds by either taking
the prefetch abort or the data abort exceptions depending on the nature of the access that caused the
timeout.
The abort handler can check if the TO flag is set in the POM module's flag register (POMFLG). If this flag
is set, then the application can assume that the timeout is caused by the POM access not completing.
This flag must be cleared so that any further aborts are not misinterpreted as having been caused by
timeout on a POM access.