AT89C5122D-RDRUM datasheet(26/208 Pages) ATMEL

26 / 208 page

AT83R5122, AT8xC5122/23

4202F–SCR–07/2008

Using CRAM Memory

The CRAM is a read / write volatile memory that is mapped in the program memory

space. Then when the power is switched off the code is lost and needs to be reload at

each power up. In return, the CRAM enables a lot of flexibility in the code development

as it can be programmed indefinitely. The user code running in the CRAM can perform

read operations in CRAM itself by means of MOVC instructions like any C51 microcon-

troller does. Although the writing operations in CRAM are usually handled by the

bootloader, it is possible for the user code to handle its own writing operations in CRAM

as well. The user code must call API functions provided by the bootloader in the ROM

memory. Refer to bootloader datasheet for further details about the use of these API

functions. These API functions use a mechanism provided by the AT8xC5122 microcon-

troller. When the bit RPS is set in RCON register (Table 8 on page 24), the MOVX

intructions are configured to write in CRAM instead of XRAM memory. However, due to

C51 architecture, it is not possible for the user code to write directly in CRAM when it is

itself running in CRAM. This is why the API functions must be called in order to have the

code executing in ROM while the CRAM is written.

Figure 11.

Read / Write Mechanisms in CRAM Memory

MOVX

API functions

RPS=1

Read operation

Writing operation

User code

CRAM

BOOTLOADER

API Call

MOVC

AT89C5122D-RDRUM 数据表(PDF) 26 Page - ATMEL Corporation