RM0046 Boot Assist Module (BAM)
Doc ID 16912 Rev 5 831/936
Example 2020 MHz crystal
Consider case where using a 20 MHz crystal, user attempts to send 62.5 Kb/s
FlexCAN message.
– Time base, clocking at crystal frequency, would measure:
– 62.5 Kb/s = 320 clocks/bit => 29 * 320 = 9280 clocks
– To calculate PRESDIV = 9280/725 = 12r580 => PRESDIV = 12
– To calculate time quanta requirement:
– Time base result = 29 x (Presdiv+1) x (SYNCSEG + TSEG1 + TSEG2)
– 9280 = 29 x 13 x (1 + TSEG1 + TSEG2)
– 1 + TSEG1 + TSEG2 = 24.6~ 25 (need to round up - S/W must track remainder)
– From the lookup table, CANA_CR = 0x0C7F_200D.
– This give a baud rate of 61.538k Baud
– This equates to an error of: ~1.6%
This excludes cycle count error introduced due to software polling likely to be ~6 system
clocks.
33.6.2 Interrupt
No interrupts are generated by or are enabled by the BAM.
33.7 Censorship
Censorship can be enabled to protect the contents of the flash memory from being read or
modified. In order to achieve this, the censorship mechanism controls access to the:
● JTAG / Nexus debug interface
● Serial boot mode (which could otherwise be used to download and execute code to
query or modify the flash memory)
To re-gain access to the flash memory via JTAG or serial boot, a 64-bit password must be
correctly entered.
Caution:
When censorship has been enabled, the only way to regain access is with the password. If
this is forgotten or not correctly configured, then there is no way back into the device.
There are two 64-bit values stored in the shadow flash which control the censorship (see
Table 145 for a full description):
● Nonvolatile Private Censorship Password registers, NVPWD0 and NVPWD1
● Nonvolatile System Censorship Control registers, NVSCI0 and NVSCI1
Censorship password registers (NVPWD0 and NVPWD1)
The two private password registers combine to form a 64-bit password that should be
programmed to a value known only by you. After factory test these registers are
programmed as shown below:
● NVPWD0 = 0xFEED_FACE
● NVPWD1 = 0xCAFE_BEEF
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