MCP3461RT-ESLASHST datasheet(55/118 Pages) MICROCHIP

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DS20006404C-page 55

MCP3461/2/4R

5.11

ADC Start-up Timer

The device includes an intelligent start-up timer circuit

for the ADC, which ensures that the ADC is properly

biased and that internal nodes are properly settled

before each conversion. This timer ensures the proper

conditions for the ADC to convert with its full accuracy

for each conversion.

The ADC can operate in three different modes: ADC

Shutdown, Standby and Conversion, as described in

Section 5.9 “ADC Operating Modes”. The ADC

start-up timer manages the time for the transitions

between each mode. These transitions can be instan-

taneous or can take a maximum of 256 DMCLK

periods, depending on the type of transition, and the

current status of the ADC and of the internal start-up

timer.

The timer will always try to reduce the transition time

from one state to another, but will also allow enough

time for the internal circuitry to settle to the proper

internal operating points.

The transitions from Standby or Conversion mode to

ADC Shutdown mode are always immediate. They

reset the internal start-up timer to 256 DMCLK periods

(TADC_SETUP).

The transitions from ADC Shutdown to Standby or

Conversion mode start the internal start-up timer that

decrements from 256 to 0. The timer only decrements

after a small delay of two MCLK periods in case of a

transition caused by an SPI command. This small delay

is necessary to overcome any possible synchronization

issue between the two asynchronous clocks: MCLK

and SCK. The timer will immediately decrement

(without the synchronization delay) if the transitions are

generated by the internal state machine (for example,

when the transitions are generated by the Scan

sequence). Once the timer reaches 0 (when the user

has clocked 256 DMCLK periods), the device reaches

its internal proper operating points and will either stay

in Standby mode (if ADC_MODE[1:0] = 10) or start the

Conversion mode (if ADC_MODE[1:0] = 11).

The transition from Standby to Conversion mode and

vice versa is immediate once the timer has reached 0 (if

ADC_MODE[1:0] = 11). If the transition from Standby to

Conversion mode occurs, and if the timer has not yet

reached 0, the timer will continue to decrement to 0

before effectively starting the conversion. The timer

cannot decrement faster than 256 DMCLK periods when

the ADC transitions from ADC Shutdown mode to Con-

version mode (from ADC Shutdown mode, the ADC is

allowed 256 DMCLK periods to power-up and settle to

its desired operating point before starting conversions).

The start-up time has been sized at 256 DMCLK clock

periods for the part to be able to settle in all conditions

and with all possible clock frequencies as specified.

Table 5-12 summarizes the behavior of the internal

start-up timer as a function of the ADC_MODE[1:0] bits

setting.

Rewriting ADC_MODE[1:0] bits without changing the

bit settings does not modify the internal timer and

cannot shorten the start-up delay necessary to start

accurate conversions. A synchronization delay of two

MCLK

periods

occurs

after

each

rewrite

ADC_MODE[1:0] = 1x.

In Scan mode, when CONV_MODE[1:0] = 11

(Continuous mode), the ADC may be placed in ADC

Shutdown mode and restarted in between each Scan

cycle depending on the TIMER[23:0] bits setting (see

Section 5.15.5 “Delay Between

Scan Cycles

(TIMER[23:0])”). If the TIMER register is programmed

with a decimal code greater than TADC_SETUP = 256,

the internal timer will automatically place the part in

ADC Shutdown mode at the end of the cycle and will

start to transition to the next cycle 256 DMCLK periods

before the end of the TIMER delay.

This lowers the power consumed during the TIMER

delay as much as possible. If the value of the TIMER

delay is less than 256 DMCLK periods, the part will not

enter ADC Shutdown mode and stay in Standby during

the TIMER delay (in this case, the power consumed is

equivalent to the Conversion mode power consumption).

In order to catch the start of the conversion in case of

complex sequences of transitions, it can be useful to

enable the EN_STP bit so that the part will generate a

pulse on the IRQ pin to indicate a conversion start.

Figure 5-11 shows different cases of transitions

between modes and shows the internal state of the

start-up timer for each step.

TABLE 5-12:

ADC START-UP TIMER BEHAVIOR AS A FUNCTION OF ADC_MODE[1:0] SETTINGS

ADC_MODE[1:0]

ADC State

ADC Start-up Timer Behavior

Conversion

The ADC start-up timer decrements to 0. The conversion

starts when it reaches 0.

Standby

The ADC start-up timer decrements to 0. The ADC is ready to

convert when it reaches 0.

ADC Shutdown

ADC start-up timer is reset to TADC_SETUP = 256.

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