LPC2921 datasheet(35/81 Pages) PHILIPS

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LPC2921_2923_2925_0

Preliminary data sheet

Rev. 00.01 — 24 October 2008

35 of 81

NXP Semiconductors

LPC2921/2923/2925

ARM9 microcontroller with CAN and LIN

6.14.4.1

Functional description

The ADC block diagram, Figure 6, shows the basic architecture of each ADC. The ADC

functionality is divided into two major parts; one part running on the MSCSS Subsystem

clock, the other on the ADC clock. This split into two clock domains affects the behavior

from a system-level perspective. The actual analog-to-digital conversions take place in the

ADC clock domain, but system control takes place in the system clock domain.

A mechanism is provided to modify configuration of the ADC and control the moment at

which the updated configuration is transferred to the ADC domain.

The ADC clock is limited to 4.5 MHz maximum frequency and should always be lower

than or equal to the system clock frequency. To meet this constraint or to select the

desired lower sampling frequency, the clock generation unit provides a programmable

fractional system-clock divider dedicated to the ADC clock. Conversion rate is determined

by the ADC clock frequency divided by the number of resolution bits plus one. Accessing

ADC registers requires an enabled ADC clock, which is controllable via the clock

generation unit, see Section 6.15.2.

Each ADC has four start inputs. Note that start 0 and start 2 are captured in the system

clock domain while start 1 and start 3 are captured in the ADC domain. The start inputs

are connected at MSCSS level, see Section 6.14 for details.

6.14.4.2

Pin description

The two ADC modules in the MSCSS have the pins described below. The ADCx input

pins are combined with other functions on the port pins of the LPC2921/2923/2925. The

VREFN and VREFP pins are common for both ADCs. Table 20 shows the ADC pins.

Fig 6.

ADC block diagram

002aae251

ADC

start 2

ADC

start 0

APB clock

(BASE_MSCSS_CLK)

ADC clock

(up to 4.5 MHz)

(BASE_ADC_CLK)

APB system bus

IRQ scan

IRQ compare

ADC2 IN[7:0]

ADC

start 1

ADC

start 3

sync_out

ADC DOMAIN

SYSTEM DOMAIN

ADC

CONTROL

ADC

REGISTERS

3.3 V

ADC2

3.3 V

ADC1

ANALOG

MUX

conversion data

update

configuration data

IRQ

ADC1 IN[7:0]

ANALOG

MUX

3.3 V IN

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