MC9S08JM16 Datasheet, PDF (55/386 Pages) Freescale Semiconductor, Inc – Microcontrollers

Chapter 4 Memory
4.7.1 Flash Clock Divider Register (FCDIV)
Bit 7 of this register is a read-only status flag. Bits 6 through 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.
R
W
Reset
7
DIVLD
0
6
PRDIV8
5
DIV5
4
DIV4
3
DIV3
2
DIV2
0
0
0
0
0
= Unimplemented or Reserved
Figure 4-5. Flash Clock Divider Register (FCDIV)
1
DIV1
0
0
DIV0
0
Table 4-6. FCDIV Register Field Descriptions
Field
7
DIVLD
6
PRDIV8
5:0
DIV[5:0]
Description
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.
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.
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 DIV5:DIV0 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, Equation 4-2, and Table 4-6.
if PRDIV8 = 0 – f_FCLK = f_Bus ÷ ([DIV5:DIV0] + 1)
Eqn. 4-1
if PRDIV8 = 1 – f_FCLK = f_Bus ÷ (8 × ([DIV5:DIV0] + 1))
Eqn. 4-2
Table 4-7 shows the appropriate values for PRDIV8 and DIV5:DIV0 for selected bus frequencies.
MC9S08JM16 Series Data Sheet, Rev. 2
Freescale Semiconductor
55