UM10413
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© NXP B.V. 2011. All rights reserved.
User manual
Rev. 1 — 16 December 2011
213 of 268
NXP Semiconductors
UM10413
MPT612 User manual
•
If V
DDC
is present, the RTC and the 32 kHz oscillator are powered by V
DDC
during
normal operation
and
during Deep power-down mode. If the SRAM block is selected
to remain active during Deep power-down mode, it remains powered by V
DDC
.
•
If V
DDC
is removed from the power supply pin, the power supply for the RTC and
32 kHz oscillator switches to the battery power supply on pin V
DD(RTC)
. If the SRAM
block is selected to remain active during Deep power-down mode, the SRAM power
supply switches to V
DD(RTC)
as well - if Deep power-down mode and SRAM active
have been selected in register PWRCTRL; see
.
Remark:
Simply removing power from pin V
DDC
does not cause the SRAM to be powered
from the battery supply. In order to keep SRAM powered, Deep power-down mode must
be entered:
1. Set bit 0 in register PWRCTRL to logic 0.
2. Set bit 1 in register PWRCTRL to logic 1 to allow the SRAM to remain powered.
24.8 Reference clock divider (prescaler)
The reference clock divider (as of now referred to as the prescaler) allows generation of a
32.768 kHz reference clock from any peripheral clock frequency greater than or equal to
65.536 kHz (2
32.768 kHz). This permits the RTC to run always at the proper rate
regardless of the peripheral clock rate. Basically, the prescaler divides the peripheral clock
(PCLK) by a value which contains both an integer portion and a fractional portion. The
result is not a continuous output at a constant frequency, some clock periods will be one
PCLK longer than others. However, the overall result can always be 32,768 counts per
second.
The reference clock divider consists of a 13-bit integer counter and a 15-bit fractional
counter. The reasons for these counter sizes are as follows:
1. For frequencies that are expected to be supported by the MPT612, a 13-bit integer
counter is required. This can be calculated as 160 MHz divided by 32,768 minus
1 = 4881 with a remainder of 26,624. Thirteen bits are required to hold the value
4881, but actually supports frequencies up to 268.4 MHz (32,768
8192).
2. The remainder value could be as large as 32,767, which requires 15 bits.
24.8.1 Prescaler integer register (PREINT - 0xE002 4080)
This is the integer portion of the prescale value, calculated as:
PREINT = int (PCLK / 32768)
1. The value of PREINT must be greater than or
equal to 1.
Table 206. Reference clock divider registers
Name
Size
Description
Access
Address
PREINT
13
prescale value, integer portion
R/W
0xE002 4080
PREFRAC 15
prescale value, fractional portion
R/W
0xE002 4084