EM357-RTR datasheet(33/241 Pages) SILABS

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Rev 1.3

5.2.1.5. Simulated EEPROM

Ember software reserves 8 kB of the main flash block as a simulated EEPROM storage area for stack and

customer tokens. The simulated EEPROM storage area implements a wear-leveling algorithm to extend the

number of simulated EEPROM write cycles beyond the physical limit of 20,000 write cycles for which each flash

cell is qualified.

5.2.2. RAM

5.2.2.1. RAM Overview

The EM35x has 12 kB of static RAM on-chip. The start of RAM is mapped to address 0x20000000. Although the

ARM® CortexTM-M3 allows bit band accesses to this address region, the standard MPU configuration does not

permit use of the bit-band feature.

The RAM is physically connected to the AHB System bus and is therefore accessible to both the ARM® CortexTM-

M3 microprocessor and the debugger. The RAM can be accessed for both instruction and data fetches as bytes,

half words, or words. The standard MPU configuration does not permit execution from the RAM, but for special

purposes the MPU may be disabled. To the bus, the RAM appears as 32-bit wide memory and in most situations

has zero wait state read or write access. In the higher CPU clock mode the RAM requires two wait states. This is

handled by hardware transparent to the user application with no configuration required.

5.2.2.2. Direct Memory Access (DMA) to RAM

Several of the peripherals are equipped with DMA controllers allowing them to transfer data into and out of RAM

autonomously. This applies to the radio (802.15.4-2003 MAC), general purpose ADC, and both serial controllers. In

the case of the serial controllers, the DMA is full duplex so that a read and a write to RAM may be requested at the

same time. Thus there are six DMA channels in total. See "8.7. DMA Channels" on page 101 and "10.1.4. DMA" on

page 174 for a description of how to configure the serial controllers and ADC for DMA operation. The DMA

channels do not use AHB system bus bandwidth as they access the RAM directly.

The EM35x integrates a DMA arbiter that ensures fair access to the microprocessor as well as the peripherals

through a fixed priority scheme appropriate to the memory bandwidth requirements of each master. The priority

scheme is as follows, with the top peripheral being the highest priority:

1. General Purpose ADC

2. Serial Controller 2 Receive

3. Serial Controller 2 Transmit

4. MAC

5. Serial Controller 1 Receive

6. Serial Controller 1 Transmit

5.2.2.3. RAM Memory Protection

The EM35x integrates two memory protection mechanisms. The first memory protection mechanism is through the

ARM® CortexTM-M3 Memory Protection Unit (MPU) described in “5.3. Memory Protection Unit”. The MPU may be

used to protect any area of memory. MPU configuration is normally handled by Ember software. The second

memory protection mechanism is through a fine granularity RAM protection module. This allows segmentation of

the RAM into 32-byte blocks where any block can be marked as write protected. An attempt to write to a protected

RAM block using a user mode write results in a bus error being signaled on the AHB System bus. A privileged

mode write is allowed at any time and reads are allowed in either mode. The main purpose of this fine granularity

RAM protection module is to notify the software of erroneous writes to system areas of memory. RAM protection is

configured using a group of registers that provide a bit map. Each bit in the map represents a 32-byte block of

RAM. When the bit is set the block is write-protected.

The fine granularity RAM memory protection mechanism is also available to the peripheral DMA controllers. A

in RAM, a management interrupt is generated. At the same time the faulting address and the identification of the

peripheral is captured for later debugging. Note that only peripherals capable of writing data to RAM, such as

received packet data or a received serial port character, can generate this interrupt.

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