SPIRIT1 datasheet(71/94 Pages) STMICROELECTRONICS

71 / 94 page

SPIRIT1

MCU interface

Doc ID 022758 Rev 3

71/94

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 forecasted when the SPIRIT1 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 12

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 SPIRIT1 exits SLEEP to any active state, the source is the XO. Both

Table 35.

MCU_CK_CONF configuration register

MCU_CK_CONF[4:0]

Clock source

Division ratio

XO_RATIO

RCO_RATIO

Don’t care

RCO

1/128

0Don’t care

2/3

1/2

1/3

1/4

1/6

1/8

71/12

81/16

91/24

1/36

1/48

1/64

1/96

1/128

1/192

READY STANDBY

clk

--------

512

192

⁄

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

98304

clk

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

⋅

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