ATtiny2313A/4313

19. Self-Programming

19.1Features

•Self-Programming Enables MCU to Erase, Write and Reprogram Application Memory

•Efficient Read-Modify-Write Support

•Lock Bits Allow Application Memory to Be Securely Closed for Further Access

19.2Overview

The device provides a self-programming mechanism for downloading and uploading program code by the MCU itself. Self-Programming can use any available data interface and associated protocol to read code and write (program) that code into program memory.

19.3Lock Bits

Program memory can be protected from internal or external access. See “Lock Bits” on page 178.

19.4Self-Programming the Flash

Program memory is updated in a page by page fashion. Before programming a page with the data stored in the temporary page buffer, the page must be erased. The temporary page buffer is filled one word at a time using SPM and the buffer can be filled either before the Page Erase command or between a Page Erase and a Page Write operation:

1.Either, fill the buffer before a Page Erase:

a.Fill temporary page buffer

b.Perform a Page Erase

c.Perform a Page Write

2.Or, fill the buffer after Page Erase:

a.Perform a Page Erase

b.Fill temporary page buffer

c.Perform a Page Write

If only a part of the page needs to be changed, the rest of the page must be stored (for example in the temporary page buffer) before the erase, and then be re-written.

The temporary page buffer can be accessed in a random sequence.

It is essential that the page address used in both the Page Erase and Page Write operation is addressing the same page.

The SPM instruction is disabled by default but it can be enabled by programming the SELFPRGEN fuse (to “0”).

19.4.1Addressing the Flash During Self-Programming

The Z-pointer is used to address the SPM commands.

173

8246B–AVR–09/11

Since the Flash is organized in pages (see Table 21-1 on page 184), the Program Counter can be treated as having two different sections. One section, consisting of the least significant bits, is addressing the words within a page, while the most significant bits are addressing the pages. This is shown in Figure 19-1, below.

Figure 19-1. Addressing the Flash During SPM

PAGEEND

Variables used in Figure 19-1 are explained in Table 19-1, below.

Note that the Page Erase and Page Write operations are addressed independently. Therefore it is of major importance that the software addresses the same page in both the Page Erase and Page Write operation.

174 ATtiny2313A/4313

8246B–AVR–09/11

ATtiny2313A/4313

Although the least significant bit of the Z-register (Z0) should be zero for SPM, it should be noted that the LPM instruction addresses the Flash byte-by-byte and uses Z0 as a byte select bit.

Once a programming operation is initiated, the address is latched and the Z-pointer can be used for other operations.

19.4.2Page Erase

To execute Page Erase:

•Set up the address in the Z-pointer

•Write “00000011” to SPMCSR

•Execute an SPM instruction within four clock cycles after writing SPMCSR

The data in R1 and R0 is ignored. The page address must be written to PCPAGE in the Z-regis- ter. Other bits in the Z-pointer are ignored during this operation.

If an interrupt occurs during the timed sequence above the four cycle access cannot be guaranteed. In order to ensure atomic operation interrupts should be disabled before writing to SPMCSR.

The CPU is halted during the Page Erase operation.

19.4.3Page Load

To write an instruction word:

•Set up the address in the Z-pointer

•Set up the data in R1:R0

•Write “00000001” to SPMCSR

•Execute an SPM instruction within four clock cycles after writing SPMCSR

The content of PCWORD in the Z-register is used to address the data in the temporary buffer. The temporary buffer will auto-erase after a Page Write operation, or by writing the RWWSRE bit in SPMCSR. It is also erased after a system reset.

Note that it is not possible to write more than one time to each address without erasing the temporary buffer.

If the EEPROM is written in the middle of an SPM Page Load operation, all data loaded will be lost.

19.4.4Page Write

To execute Page Write:

•Set up the address in the Z-pointer

•Write “00000101” to SPMCSR

•Execute an SPM instruction within four clock cycles after writing SPMCSR

The data in R1 and R0 is ignored. The page address must be written to PCPAGE. Other bits in the Z-pointer must be written to zero during this operation.

The CPU is halted during the Page Write operation.

175

8246B–AVR–09/11