AN2156 Freescale Semiconductor / Motorola, AN2156 Datasheet - Page 28

• Manufacturer Part Number: AN2156
• Description: Programming and Erasing FLASH and EEPROM Memories on the MC68HC908AS60A/AZ60A
• Manufacturer: Freescale Semiconductor / Motorola

Application Note
EEPROM Timebase Divider Initialization
28
NOTE:
NOTE:
The EEPROM is actually a FLASH cell surrounded by a logic state
machine. The state machine requires a constant timebase with a period
of 35 s to apply high voltage with the right timing during the erase and
program operations. If the constant timebase is not set properly, the
memory cells may be over-programmed and permanently damaged.
Motorola cannot guarantee EEPROM program and erase operations in
this case.
To generate this constant timebase, a clock divider value (EExDIV
value) has to be set in the EEPROM timebase divider registers
(EExDIVH and EExDIVL).
Follow these steps to initialize a timebase divider value:
The recommended frequency range of the reference clock is 250 kHz to
16 MHz.
Make sure that the EExDIV is calculated with the reference clock source
being used.
1. The reference clock source has to be selected. The EEDIVCLK bit
2. The EExDIV has to be calculated to derive a EExDIV value that
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in the CONFIG-2 register determines the clock source. (Refer to
Configuration Register 2 and EEPROM Timebase Divider
Control
– CGMXCLK (output of the crystal oscillator circuit)
– System bus clock
matches the user’s application. Use the formula:
EExDIV = INT [Reference Frequency(Hz) x (35 x10
– Example 1: When the reference clock is 4.9152 MHz, the
– Example 2: When the reference clock is 8 MHz, the EExDIV
EExDIV value is 172 (hex $AC).
value is 280 (hex $118).
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Registers). The two options for clock source are:
–6
) +0.5]
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AN2156