Datasheet

Chapter 4 Memory
MC9S08SH8 MCU Series Data Sheet, Rev. 3
54 Freescale Semiconductor
control registers (FOPT, FPROT) at reset. There is also an 8-byte comparison key in FLASH memory.
Refer to Table 4-3 and Table 4-4 for the absolute address assignments for all FLASH registers. This section
refers to registers and control bits only by their names. A Freescale Semiconductor-provided equate or
header file normally is used to translate these names into the appropriate absolute addresses.
4.7.1 FLASH Clock Divider Register (FCDIV)
Bit 7 of this register is a read-only flag. Bits 6:0 may be read at any time but can be written only one time.
Before any erase or programming operations are possible, write to this register to set the frequency of the
clock for the nonvolatile memory system within acceptable limits.
if PRDIV8 = 0 — f
FCLK
= f
Bus
÷ (DIV + 1) Eqn. 4-1
if PRDIV8 = 1 — f
FCLK
= f
Bus
÷ (8 × (DIV + 1)) Eqn. 4-2
Table 4-7 shows the appropriate values for PRDIV8 and DIV for selected bus frequencies.
76543210
R DIVLD
PRDIV8 DIV
W
Reset 00000000
= Unimplemented or Reserved
Figure 4-5. FLASH Clock Divider Register (FCDIV)
Table 4-6. FCDIV Register Field Descriptions
Field Description
7
DIVLD
Divisor Loaded Status Flag — When set, this read-only status flag indicates that the FCDIV register has been
written since reset. Reset clears this bit and the first write to this register causes this bit to become set regardless
of the data written.
0 FCDIV has not been written since reset; erase and program operations disabled for FLASH.
1 FCDIV has been written since reset; erase and program operations enabled for FLASH.
6
PRDIV8
Prescale (Divide) FLASH Clock by 8
0 Clock input to the FLASH clock divider is the bus rate clock.
1 Clock input to the FLASH clock divider is the bus rate clock divided by 8.
5:0
DIV
Divisor for FLASH Clock Divider — The FLASH clock divider divides the bus rate clock (or the bus rate clock
divided by 8 if PRDIV8 = 1) by the value in the 6-bit DIV field plus one. The resulting frequency of the internal
FLASH clock must fall within the range of 200 kHz to 150 kHz for proper FLASH operations. Program/Erase
timing pulses are one cycle of this internal FLASH clock which corresponds to a range of 5 μs to 6.7 μs. The
automated programming logic uses an integer number of these pulses to complete an erase or program
operation. See Equation 4-1 and Equation 4-2.