AN2021 datasheet(22/39 Pages) STMICROELECTRONICS

22 / 39 page

New features

AN2021

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2.2.1

Hardware impact

None.

2.2.2

Software impact

None if only CLKOUT is needed.

When the CLKOUT function is enabled by setting the CLKEN bit in the SYSCON register, by

default the CPU clock is output on P3.15.

To have access to the XCLKOUTDIV register, and thus to program the clock pre-scaling

factor, the XMISCEN bit in the XPERCON register and the XPEN bit in the SYSCON

2.3

New multiplexer for X-Interrupts

The limited number of XBus interrupt lines of the present ST10 architecture imposes some

constraints on the implementation of the new functionalities. In particular, the additional

XPeripherals XSSC, XASC, XI2C and XPWM need some resources to implement interrupt

and PEC transfer. For this reason, a complex but very flexible multiplexed structure for the

interrupt is proposed. In Figure 2, the principle is represented through a simple diagram,

which shows the basic structure replicated for each of the four X-interrupt vectors (XP0INT,

XP1INT, XP2INT and XP3INT).

It is based on a new 16-bit register XIRxSEL (x = 0,1,2,3), divided into 2 bytes:

●

Higher Byte (XIRxSEL[15:8]) Interrupt Enable bits

●

Lower Byte (XIRxSEL[7:0]) Interrupt Flag bits

Figure 2.

X-Interrupt basic structure

When different sources submit an interrupt request, the enable bits (Byte High of XIRxSEL

available vector. If more than one source is enabled to issue the request, the service routine

has to identify the real event to be serviced. This can easily be done by checking the flag

XIRxSEL[7:0] (x = 0, 1, 2, 3)

XIRxSEL[15:8] (x = 0, 1, 2, 3)

XPxIC.IR (x = 0, 1, 2, 3)

IT Source 7

IT Source 6

IT Source 5

IT Source 4

IT Source 3

IT Source 2

IT Source 1

IT Source 0

Enable[7:0]

Flag[7:0]

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