Serial Downloading

Notes: 1. tWLRH is valid for the Write Flash, Write EEPROM, Write Fuse Bits and Write Lock Bits commands.

2.tWLRH_CE is valid for the Chip Erase command.

Both the Flash and EEPROM memory arrays can be programmed using the serial SPI bus while RESET is pulled to GND. The serial interface consists of pins SCK, MOSI (input) and MISO (output). After RESET is set low, the Programming Enable instruction

needs to be executed first before program/erase operations can be executed. NOTE, in Table 128 on page 292, the pin mapping for SPI programming is listed. Not all parts use

the SPI pins dedicated for the internal SPI interface. Note that throughout the description about Serial downloading, MOSI and MISO are used to describe the serial data in and serial data out respectively. For ATmega128 these pins are mapped to PDI and PDO.

Serial Programming

Algorithm

2467B–09/01

ATmega128(L)

Figure 143. Serial Programming and Verify

+2.7 - 5.5V

VCC

PDI PE0

PDO PE1

SCK PB1

XTAL1

RESET

GND

Note: If the device is clocked by the internal oscillator, it is no need to connect a clock source to the XTAL1 pin.

When programming the EEPROM, an auto-erase cycle is built into the self-timed programming operation (in the serial mode ONLY) and there is no need to first execute the Chip Erase instruction. The Chip Erase operation turns the content of every memory location in both the Program and EEPROM arrays into $FF.

Depending on CKSEL Fuses, a valid clock must be present. The minimum low and high periods for the serial clock (SCK) input are defined as follows:

Low:> 2 CPU clock cycles for fck < 12 MHz, 3 CPU clock cycles for fck ≥ 12 MHz High:> 2 CPU clock cycles for fck < 12 MHz, 3 CPU clock cycles for fck ≥ 12 MHz

When writing serial data to the ATmega128, data is clocked on the rising edge of SCK.

When reading data from the ATmega128, data is clocked on the falling edge of SCK. See Figure 144, Figure 145 and Table 145 for timing details.

To program and verify the ATmega128 in the serial programming mode, the following sequence is recommended (See four byte instruction formats in Table 144):

1.Power-up sequence:

Apply power between VCC and GND while RESET and SCK are set to “0”. In some systems, the programmer can not guarantee that SCK is held low during power-up. In this case, RESET must be given a positive pulse of at least two CPU clock cycles duration after SCK has been set to “0”.

As an alternative to using the RESET signal, PEN can be held low during Poweron Reset while SCK is set to “0”. In this case, only the PEN value at Power-on Reset is important. If the programmer cannot guarantee that SCK is held low during power-up, the PEN method cannot be used. The device must be powered down in order to commence normal operation when using this method.

2.Wait for at least 20 ms and enable serial programming by sending the Programming Enable serial instruction to pin MOSI.

3.The serial programming instructions will not work if the communication is out of synchronization. When in sync. the second byte ($53), will echo back when issu-

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