Freescale Semiconductor 56F8122 - Manual (Page 95)

Flash Access Blocking Mechanisms
56F8322 Technical Data, Rev. 10.0
Freescale Semiconductor 95
Preliminary
EXAMPLE 1: If the system clock is the 8MHz crystal frequency because the PLL has not been set up,
the input clock will be below 12.8MHz, so PRDIV8=FM_CLKDIV[6]=0. Using the following equation
yields a DIV value of 19 for a clock of 200kHz, and a DIV value of 20 for a clock of 190kHz. This
translates into an FM_CLKDIV[6:0] value of $13 or $14, respectively.
EXAMPLE 2: In this example, the system clock has been set up with a value of 32MHz, making the FM
input clock 16MHz. Because that is greater than 12.8MHz, PRDIV8=FM_CLKDIV[6]=1.Using the
following equation yields a DIV value of 9 for a clock of 200kHz, and a DIV value of 10 for a clock of
181kHz. This translates to an FM_CLKDIV[6:0] value of $49 or $4A, respectively.
Once the LOCKOUT_RECOVERY instruction has been shifted into the instruction register, the clock
divider value must be shifted into the corresponding 7-bit data register. After the data register has been
updated, the user must transition the TAP controller into the RUN-TEST/IDLE state for the lockout
sequence to commence. The controller must remain in this state until the erase sequence has completed.
For details, see the JTAG Section in the 56F8300 Peripheral User Manual.
Note: Once the lockout recovery sequence has completed, the user must reset both the JTAG TAP controller
(by asserting TRST) and the device (by asserting external chip reset) to return to normal unsecured
operation.
7.2.4 Product Analysis
The recommended method of unsecuring a programmed device for product analysis of field failures is via
the backdoor key access. The customer would need to supply Technical Support with the backdoor key
and the protocol to access the backdoor routine in the Flash. Additionally, the KEYEN bit that allows
backdoor key access must be set.
An alternative method for performing analysis on a secured microcontroller would be to mass-erase and
reprogram the Flash with the original code, but modify the security bytes.
To insure that a customer does not inadvertently lock himself out of the device during programming, it is
recommended that he program the backdoor access key first, his application code second and the security
bytes within the FM configuration field last.
SYS_CLK
(2)
) (
< <
(DIV + 1)
150[kHz] 200[kHz]
SYS_CLK
(2)(8)
) (
< <
(DIV + 1)
150[kHz] 200[kHz]