Microcontrollers build in support for off-chip SDRAM and flash
By Robert Cravotta, Technical Editor -- EDN, October 26, 2006
ARM7-core-based microcontrollers, smaller and consuming less power than ARM-9-core devices, are workhorses for low-power embedded-system applications operating from batteries. However, to take advantage of built-in support for SDRAM, NAND flash, and CompactFlash off-chip memories, designers have had to use ARM9 or equivalent microcontrollers. Addressing the desire to use ARM7, Atmel's new AT91SAM7SE512, SAM7SE256, and SAM7SE32 microcontrollers bring built-in support for these off-chip memories to ARM7 applications. The EBI (external-bus-interface) controller supports 32-bit off-chip memory in TrueIDE mode, and it includes an integrated ECC (error-correction-code) coprocessor for NAND flash. The larger memory supports features such as multilanguage data that you load at runtime or more extensive data logging for applications such as portable heart monitoring.
Using systems with external memory entails additional security risks, so the SAM7SE devices employ techniques to protect the system from malicious actions. For example, the microcontrollers boot only from internal memory. Also, the devices employ a flash security bit that disables the fast-flash-programming interface and JTAG access to the on-chip flash memory. In addition, the memory-protection unit in the devices can limit code execution from on-chip flash to prevent executing code directly from external memory. The on-chip flash uses a dual-bank architecture to support a true read-while-write capability and to offer additional protections to the system from power loss or cable disconnects when using the in-application programming for firmware upgrades.
The SAM7SE supports a migration path for SAM7S users by maintaining software compatibility. The new devices expand the general-purpose-I/O count from 32 to 88 pins to accommodate the EBI or to enable use of the devices' on-chip peripherals without multiplexing the peripherals when the EBI is not in use. The EBI supports a static-memory interface with built-in support for multiple chip select to facilitate interfacing to FPGAs. The SAM7SE devices also include an 11-channel peripheral-DMA controller with three programmed-I/O controllers, to offload the data-movement processing from the CPU, including directly storing streaming data in the external memories.
On-chip communication interfaces include three USARTs, USB, TWI (two-wire interface), I2S, and SPI. The SAM7SE512 also includes four PWMs; an eight-channel, 10-bit ADC; three 16-bit timers; a watchdog timer; a real-time clock; a crystal oscillator; and supervisory features like those on 8-bit microcontrollers, including brownout detection and power-on- reset. The AT91SAM7SE512, including 512 kbytes of on-chip flash, is available now in 128-pin "green" QFP or BGA packages and sells for $8.43 and $9.38, respectively (10,000). The AT91SAM7SE256 and AT91SAM7SE32 with 256 or 32 kbytes of on-chip flash will be available in early 2007.
