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MCUs offer access to high-capacity external storage

Posted: 02 Nov 2006 ?? ?Print Version ?Bookmark and Share

Keywords:Atmel? SAM7? ARM7? AT91SAM7SE? MCU?

Atmel's SAM7SE MCUs

Atmel Corp. has added three new MCUs to its SAM7 family of ARM7-based USB MCUs. Available in flash memory densities of 32-, 256- and 512Kbytes, the SAM7SE MCUs are said to be the only ARM7-based MCUs to include an external bus interface (EBI) that provides access to massive amounts of external NAND Flash, SDRAM, CompactFlash, SRAM and ROM storage. Atmel said the ability to efficiently store and retrieve over a gigabyte of data makes SAM7SE MCUs suitable for data logging applications such as ambulatory medical monitoring.

SDRAM, NAND interface
The EBI on AT91SAM7SE MCUs supports 32bit NAND flash, SDRAM and CompactFlash memories in TrueIDE mode. It includes a NAND flash controller with a complementary error-correcting code controller that compensates for bits that are lost by NAND flash memory over time. A built-in memory controller offers up to eight configurable chip selects and supports a wide range of 16bit and 32bit static devices including SRAM, ROM, flash, memory mapped LCD and FPGA.

Data logging applications have usually been implemented with costly, power-hungry ARM9-based MCUs because they were the only MCUs available that supported SDRAM and NAND flash memories. Conventional ARM7 MCUs require intensive bit manipulation through the general purpose I/O in these applications, which significantly reduces the available processing power. With the advent of Atmel's SAM7E family, designers can now use these smaller-footprint, ARM7-based MCUs to implement battery-powered, portable versions of these applications.

A major problem when external memory or logic is used is that it makes the on-chip flash vulnerable to external, potentially unauthorized, access. SAM7E MCUs have several features that secure the code: 1) the MCU cannot boot from external memory; 2) a flash security bit disables the Fast Flash Programming Interface (FFPI) and JTAG access to the on-chip flash memory; and 3) a memory protection unit (MPU) can be configured to enforce code execution from on-chip flash, thereby preventing execution from an outside source.

Dual-bank flash memory on the 512KByte SAM7SE allows true read-while-write capability so systems can be reprogrammed while continuing to operate. The dual-bank memory also protects the system from power failures or other errors that can cause an unrecoverable system failure.

10Mbps on-chip bandwidth
Modern communications protocols have maximum data rates that conventional ARM7 MCUs cannot support. For example full-speed USB operates at 12Mbps and high-speed SPI operates at 25Mbps. At only 4Mbps, conventional ARM7s become so bogged down they cannot effectively process data. Like Atmel's other SAM7 MCUs, the SAM7SE-series has an 11-channel peripheral DMA controller (PDC) that increases on-chip bandwidth to 10Mbps while maintaining 96 percent of the CPU's cycles for application processing. The PDC can store the streaming content directly in the external NAND Flash or SDRAM without CPU intervention.

The SAM7SE series includes the same supervisory features found on 8bit MCUs including a brown-out detector, power-on- reset, real-time clock, crystal oscillator, watchdog timer and three 16bit timers. Communication interfaces include three USARTs, USB, TWI, I?S (SSC) and SPI. The device also has four pulse width modulators (PWM), an 8-channel, 10bit ADC and 88 I/O pins.

Traditional MCUs multiplex the peripherals, frequently creating a bottleneck when there are multiple peripheral accesses. Three individual PIO controllers avoid this problem. When the EBI is not in use, all peripherals can be accessed simultaneously.

The AT91SAM7SE512 is available in 128-pin Green QFP or BGA packages and priced at $8.43 and $9.38, respectively, in quantities of 10,000. The AT91SAM7SE256 and AT91SAM7SE32 will be available in early 2007.

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