CD00001237 datasheet(14/105 Pages) STMICROELECTRONICS

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ST6215C ST6225C

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MEMORY MAP (Cont’d)

3.1.6.2 Data ROM Window memory addressing

In cases where some data (look-up tables for ex-

ample) are stored in program memory, reading

these data requires the use of the Data ROM win-

dow mechanism. To do this:

1. The DRWR register has to be loaded with the

64-byte block number where the data are located

(in program memory). This number also gives the

start address of the block.

2. Then, the offset address of the byte in the Data

ROM Window (corresponding to the offset in the

64-byte block in program memory) has to be load-

ed in a register (A, X,...).

When the above two steps are completed, the

data can be read.

To understand how to determine the DRWR and

the content of the register, please refer to the ex-

ample shown in Figure 6. In any case the calcula-

tion is automatically handled by the ST6 develop-

ment tools.

Please refer to the user manual of the correspod-

ing tool.

3.1.6.3 Recommendations

Care is required when handling the DRWR regis-

ter as it is write only. For this reason, the DRWR

contents should not be changed while executing

an interrupt service routine, as the service routine

cannot save and then restore the register’s previ-

ous contents. If it is impossible to avoid writing to

the DRWR during the interrupt service routine, an

image of the register must be saved in a RAM lo-

cation, and each time the program writes to the

DRWR, it must also write to the image register.

The image register must be written first so that, if

an interrupt occurs between the two instructions,

the DRWR is not affected.

Figure 6. Data read-only memory Window Memory Addressing

DATA

PROGRAM SPACE

DATA SPACE

0000h

0400h

0421h

07FFh

64 bytes

OFFSET

000h

040h

061h

07Fh

OFFSET

21h

0FFh

DRWR

DATA address in Program memory : 421h

DRWR content : 421h / 3Fh (64) = 10H data is located in 64-bytes window number 10h

64-byte window start address : 10h x 3Fh = 400h

10h

DATA

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