UM10147_2
© NXP B.V. 2008. All rights reserved.
User manual
Rev. 02 — 28 April 2008
26 of 134
NXP Semiconductors
UM10147
P89LPC952/954 User manual
2.8 CPU Clock (CCLK) modification: DIVM register
The OSCCLK frequency can be divided down, by an integer, up to 510 times by
configuring a dividing register, DIVM, to provide CCLK. This produces the CCLK
frequency using the following formula:
CCLK frequency = f
osc
/ (2N)
Where: f
osc
is the frequency of OSCCLK, N is the value of DIVM.
Since N ranges from 0 to 255, the CCLK frequency can be in the range of f
osc
to f
osc
/510.
(for N = 0, CCLK = f
osc
).
This feature makes it possible to temporarily run the CPU at a lower rate, reducing power
consumption. By dividing the clock, the CPU can retain the ability to respond to events
other than those that can cause interrupts (i.e. events that allow exiting the Idle mode) by
executing its normal program at a lower rate. This can often result in lower power
consumption than in Idle mode. This can allow bypassing the oscillator start-up time in
cases where Power-down mode would otherwise be used. The value of DIVM may be
changed by the program at any time without interrupting code execution.
2.9 Low power select
The P89LPC952/954 is designed to run at 18 MHz (CCLK) maximum. However, if CCLK
is 8 MHz or slower, the CLKLP SFR bit (AUXR1.7) can be set to a logic 1 to lower the
power consumption further. On any reset, CLKLP is logic 0 allowing highest performance.
This bit can then be set in software if CCLK is running at 8 MHz or slower.
3.
A/D converter
3.1 General description
The P89LPC952/954 has a 10-bit, 8-channel multiplexed successive approximation
analog-to-digital converter module. A block diagram of the A/D converter is shown in
. The A/D consists of an 8-input multiplexer which feeds a sample-and-hold circuit
providing an input signal to one of two comparator inputs. The control logic in combination
with the SAR drives a digital-to-analog converter which provides the other input to the
comparator. The output of the comparator is fed to the SAR.
3.2 A/D features
•
10-bit, 8-channel multiplexed input, successive approximation A/D converter.
•
Eight result register pairs.
•
Six operating modes
–
Fixed channel, single conversion mode
–
Fixed channel, continuous conversion mode
–
Auto scan, single conversion mode
–
Auto scan, continuous conversion mode
–
Dual channel, continuous conversion mode
–
Single step mode