MC9S12XD256MAL Datasheet, PDF (1165/1348 Pages) Freescale Semiconductor, Inc – Freescale’s Scalable Controller Area Network

Chapter 28 256 Kbyte Flash Module (S12XFTX256K2V1)
28.4.1.1 Writing the FCLKDIV Register
Prior to issuing any Flash command after a reset, the user is required to write the FCLKDIV register to
divide the oscillator clock down to within the 150 kHz to 200 kHz range. Since the program and erase
timings are also a function of the bus clock, the FCLKDIV determination must take this information into
account.
If we define:
• FCLK as the clock of the Flash timing control block
• Tbus as the period of the bus clock
• INT(x) as taking the integer part of x (e.g. INT(4.323) = 4)
then FCLKDIV register bits PRDIV8 and FDIV[5:0] are to be set as described in Figure 28-24.
For example, if the oscillator clock frequency is 950kHz and the bus clock frequency is 10MHz,
FCLKDIV bits FDIV[5:0] should be set to 0x04 (000100) and bit PRDIV8 set to 0. The resulting FCLK
frequency is then 190kHz. As a result, the Flash program and erase algorithm timings are increased over
the optimum target by:
(200 – 190) ⁄ 200 × 100 = 5%
If the oscillator clock frequency is 16MHz and the bus clock frequency is 40MHz, FCLKDIV bits
FDIV[5:0] should be set to 0x0A (001010) and bit PRDIV8 set to 1. The resulting FCLK frequency is then
182kHz. In this case, the Flash program and erase algorithm timings are increased over the optimum target
by:
(200 – 182) ⁄ 200 × 100 = 9%
CAUTION
Program and erase command execution time will increase proportionally
with the period of FCLK. Because of the impact of clock synchronization
on the accuracy of the functional timings, programming or erasing the Flash
memory cannot be performed if the bus clock runs at less than 1 MHz.
Programming or erasing the Flash memory with FCLK < 150 kHz should
be avoided. Setting FCLKDIV to a value such that FCLK < 150 kHz can
destroy the Flash memory due to overstress. Setting FCLKDIV to a value
such that (1/FCLK+Tbus) < 5µs can result in incomplete programming or
erasure of the Flash memory cells.
If the FCLKDIV register is written, the FDIVLD bit is set automatically. If the FDIVLD bit is 0, the
FCLKDIV register has not been written since the last reset. If the FCLKDIV register has not been written
to, the Flash command loaded during a command write sequence will not execute and the ACCERR flag
in the FSTAT register will set.
Freescale Semiconductor
MC9S12XDP512 Data Sheet, Rev. 2.21
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