STS1TX datasheet(54/70 Pages) STMICROELECTRONICS

54 / 70 page

MCU interface

STS1TX

54/70

DocID025109 Rev 1

In STANDBY state, no oscillator is available as the clock source. In order to allow the MCU

to better handle this event, and avoid a potential dead state situation, a dedicated procedure

is forecast when the STS1TX enters STANDBY state. A few extra clock cycles can be

provided to the MCU before actually stopping the clock (an interrupt is generated to notify

the MCU of this event).

The number of extra cycles can be programmed through the MCU_CK_CONF configuration

or to switch on any auxiliary clock generator. The maximum transition time from READY to

STANDBY is then:

Equation 9

where fclk is the digital clock frequency (typically 26 MHz).

The transition to SLEEP state causes the MCU clock source to change from XO to RCO.

Similarly, when the STS1TX exits SLEEP to any active state, the source is the XO. Both

these transitions are implemented in order to be glitch-free. This is guaranteed by

synchronizing both transitions, switching on the rising or falling edge of the RCO clock.

The clock provided to the MCU depends on the current state:

Table 33. MCU clock vs. state

State

Source oscillator

MCU clock

SHUTDOWN

N/A

STANDBY

N/A

Tail

SLEEP

RC Osc

RC/1 or RC/128

READY

TUNING

XTAL

XTAL/N

TREADY STANDBY

fclk

--------

512

1192

⁄

-----------------

98304

fclk

----------------

⋅

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