AN4035 datasheet(32/42 Pages) STMICROELECTRONICS

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Nexus Read/Write access block

AN4035

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Doc ID 022669 Rev 2

4.2

Single memory write access

The steps to perform a single memory write access via the Nexus R/W access block are:

Initialize RWA with the address to be written using Nexus register index 0x9.

Initialize RWCS using Nexus register index 0x7.

Access Control (AC) = 0b1 (to indicate start access)

Map Select (MAP) = 0b000 (primary memory map)

Access Priority (PR) = 0b00 (lowest priority)

Read/Write (RW) = 0b1 (write access)

Word Size (SZ) = 0b000 (8-bit) or 0b001 (16-bit) or 0b010 (32-bit)

Access Count (CNT) = 0b00_0000_0000_0000 or 0b00_0000_0000_0001 (single

access)

Burst Control (BST) = 0b0 (burst disabled)

Initialize RWD using Nexus register index 0xA with the data to be written to the address

in RWA. The endianess of the data needs to be right-justified little endian.

- 8-bit value of 0xDE to be written to memory: RWD = 0x000000DE

- 16-bit value of 0xDEAD to be written to memory: RWD = 0x0000ADDE

- 32-bit value of 0xDEADBEEF to be written to memory: RWD = 0xEFBEADDE

The Nexus block then arbitrates for the system bus and transfer the data value from

RWD to the memory mapped address in RWA. When the access has completed

without error, then RWCS[ERR] = 0 and RWCS[DV] = 0. This indicates that the device

is ready for the next access. Nexus also asserts the RDY pin when the transaction has

completed without error. The external tool can use this as an alternative to polling the

RWCS status bits.

4.3

Burst block memory write access

The steps to perform a burst block memory write access via the Nexus R/W access block

are:

Initialize RWA with the first address to be written using Nexus register index 0x9. The

address needs to be aligned on an 8-byte boundary. RWA[2:0] are ignored on a burst

write.

Initialize RWCS using Nexus register index 0x7.

Access Control (AC) = 0b1 (to indicate start access)

Map Select (MAP) = 0b000 (primary memory map)

Access Priority (PR) = 0b00 (lowest priority)

Read/Write (RW) = 0b1 (write access)

Word Size (SZ) = 0b011 (64-bit)

Access Count (CNT) = 0b00_0000_0000_0100 (four double-words)\

Burst Control (BST) = 0b1 (burst enabled)

Write all 32 bytes of data to be burst to RWD using Nexus register index 0xA, 32-bits at

a time, starting with the first 32-bit word to be written to the address in RWA. This data

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