7.2.2.3 Advanced Encryption Standard (AES) Cryptography Tables
AES is a strong encryption method with reasonable performance and size. AES is fast in both
hardware and software, is fairly easy to implement, and requires little memory. AES is ideal for
applications that can use pre-arranged keys, such as setup during manufacturing or configuration.
Four data tables used by the XySSL AES implementation are provided in the ROM. The first is the
forward S-box substitution table, the second is the reverse S-box substitution table, the third is the
forward polynomial table, and the final is the reverse polynomial table. See the Stellaris® ROM
User’s Guide for more information on AES.
7.2.2.4 Cyclic Redundancy Check (CRC) Error Detection
The CRC technique can be used to validate correct receipt of messages (nothing lost or modified
in transit), to validate data after decompression, to validate that Flash memory contents have not
been changed, and for other cases where the data needs to be validated. A CRC is preferred over
a simple checksum (e.g. XOR all bits) because it catches changes more readily. See the Stellaris®
ROM User’s Guide for more information on CRC.
7.2.3 Flash Memory
At system clock speeds of 50 MHz and below, the Flash memory is read in a single cycle. The Flash
memory is organized as a set of 1-KB blocks that can be individually erased. An individual 32-bit
word can be programmed to change bits from 1 to 0. In addition, a write buffer provides the ability
to concurrently program 32 continuous words in Flash memory. Erasing a block causes the entire
contents of the block to be reset to all 1s. The 1-KB blocks are paired into sets of 2-KB blocks that
can be individually protected. The protection allows blocks to be marked as read-only or execute-only,
providing different levels of code protection. Read-only blocks cannot be erased or programmed,
protecting the contents of those blocks from being modified. Execute-only blocks cannot be erased
or programmed and can only be read by the controller instruction fetch mechanism, protecting the
contents of those blocks from being read by either the controller or by a debugger.
Caution – The Stellaris Flash memory array has ECC which uses a test port into the Flash memory to
continually scan the array for ECC errors and to correct any that are detected. This operation is
transparent to the microcontroller. The BIST must scan the entire memory array occasionally to ensure
integrity, taking about ve minutes to do so. In systems where the microcontroller is frequently powered
for less than ve minutes, power should be removed from the microcontroller in a controlled manner
to ensure proper operation. This controlled manner can either be through entering Hibernate mode or
software can request permission to power down the part using the USDREQ bit in the Flash Control
(FCTL) register and wait to receive an acknowledge from the USDACKbit prior to removing power. If
the microcontroller is powered down using this controlled method, the BIST engine keeps track of
where it was in the memory array and it always scans the complete array after any aggregate of ve
minutes powered-on, regardless of the number of intervening power cycles. If the microcontroller is
powered down before ve minutes of being powered up, BIST starts again from wherever it left off
before the last controlled power-down or from 0 if there never was a controlled power down. An
occasional short power down is not a concern, but the microcontroller should not always be powered
down frequently in an uncontrolled manner. The microcontroller can be power-cycled as frequently
as necessary if it is powered-down in a controlled manner.
7.2.3.1 Prefetch Buffer
The Flash memory controller has a prefetch buffer that is automatically used when the CPU frequency
is greater than 50 MHz. In this mode, the Flash memory operates at half of the system clock. The
prefetch buffer fetches two 32-bit words per clock allowing instructions to be fetched with no wait
states while code is executing linearly. The fetch buffer includes a branch speculation mechanism
July 24, 2012306
Texas Instruments-Production Data
Internal Memory
OBSOLETE: TI has discontinued production of this device.
