NUC505-YO2G datasheet(79/130 Pages) NUVOTON

79 / 130 page

NUC505

July. 26, 2018

Page 79 of 130

Rev.1.08

USBH

SDH

8 (Highest)

Table 6.2-3 AHB Bus Priority Order in Fixed Priority Mode If two or more master modules request to

access AHB bus at the same time, the higher priority request will get the permission to access AHB

bus.

Priority Sequence

(PRISEL = 0)

AHB Bus Priority

1 (Lowest)

Cortex-M4 I

Cortex-M4 D

Cortex-M4 System

SPIM

USBD

USBH

SDH

8 (Highest)

Table 6.2-3 AHB Bus Priority Order in Fixed Priority Mode

The SPI flash controller normally has the lowest priority (except CPU interface) under the fixed priority

mode. The NUC505 provides a mechanism to raise the priority of CPU request to the highest. If the

CPUHPRI bit (bit-4 of SYS_AHBCTL control register) is set to 1, the PRISTS bit (bit-5 of

SYS_AHBCTL control register) will be automatically set to 1 while an unmasked external IRQ occurs.

Under this circumstance, the ARM core will become the highest priority to access AHB bus.

The programmer can recover

the original priority order by directly writing “1” to clear the PRISTS bit.

For example, this can be done that at the end of an interrupt service routine. Note that PRISTS only

can be automatically set to 1 by an external interrupt when CPUHPRI = 1. It will not take effect for a

programmer to directly write 1 to PRISTS

to raise ARM core’s AHB priority.

Round Robin Priority Mode

6.2.7.2

Round-robin priority mode is selected if PRISEL = 1. The AHB bus arbiter uses a round robin

arbitration scheme for every master module to gain the bus ownership in turn. That is the requestor

having the highest priority becomes the lowest-priority requestor after it has been granted access.

Rotate rule Example

6.2.7.3

In the default sequence of AHB Master Bus, the priority is I

2S>SDH > USBH > USBD >SPIM >M4(S) >

M4(D) > M4(I).

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