BCM2835 データシートの表示（PDF） - Broadcom Corporation

部品番号

コンポーネント説明

メーカー

BCM2835

BCM2835 ARM Peripherals

Broadcom Corporation 

Software accessing RAM directly must use physical addresses (based at 0x00000000).

Software accessing RAM using the DMA engines must use bus addresses (based at

0xC0000000).

1.3 Peripheral access precautions for correct memory ordering

The BCM2835 system uses an AMBA AXI-compatible interface structure. In order to keep

the system complexity low and data throughput high, the BCM2835 AXI system does not

always return read data in-order2. The GPU has special logic to cope with data arriving out-

of-order; however the ARM core does not contain such logic. Therefore some precautions

must be taken when using the ARM to access peripherals.

Accesses to the same peripheral will always arrive and return in-order. It is only when

switching from one peripheral to another that data can arrive out-of-order. The simplest way

to make sure that data is processed in-order is to place a memory barrier instruction at critical

positions in the code. You should place:

• A memory write barrier before the first write to a peripheral.

• A memory read barrier after the last read of a peripheral.

It is not required to put a memory barrier instruction after each read or write access. Only at

those places in the code where it is possible that a peripheral read or write may be followed

by a read or write of a different peripheral. This is normally at the entry and exit points of the

peripheral service code.

As interrupts can appear anywhere in the code so you should safeguard those. If an interrupt

routine reads from a peripheral the routine should start with a memory read barrier. If an

interrupt routine writes to a peripheral the routine should end with a memory write barrier.

2Normally a processor assumes that if it executes two read operations the data will arrive in order. So a read

from location X followed by a read from location Y should return the data of location X first, followed by the

data of location Y. Data arriving out of order can have disastrous consequences. For example:

a_status = *pointer_to_peripheral_a;

b_status = *pointer_to_peripheral_b;

Without precuations the values ending up in the variables a_status and b_status can be swapped around.

It is theoretical possible for writes to go ‘wrong’ but that is far more difficult to achieve. The AXI system

makes sure the data always arrives in-order at its intended destination. So:

*pointer_to_peripheral_a = value_a;

*pointer_to_peripheral_b = value_b;

will always give the expected result. The only time write data can arrive out-of-order is if two different

peripherals are connected to the same external equipment.

06 February 2012 Broadcom Europe Ltd. 406 Science Park Milton Road Cambridge CB4 0WW

© 2012 Broadcom Corporation. All rights reserved

Page 7

Share Link: