Sonntag, 12. August 2012

NXP LPC11U14 and the LPCXpresso Board

The NXP LPC11xx series microcontrollers include an ARM Cortex-M0 CPU with SRAM, Flash and numerous peripherals. The low-cost LPCXpresso development board offers a debug adapter (JTAG, more precisely SWD) via USB. This is used with the Eclipse-based LPCXpresso IDE. Fortunately NXP offers it for Linux users too, even including the drivers for the debug connection.

After rapt contemplation I concluded, that most, if not all, of my ideas for nice projects require the connection to a PC. I prefer USB, so the LPC11Uxx microcontrollers including a USB 1.1 (12 MBit) device core come in handy.

NXP really tries to establish a community around their microcontrollers with forums and near-open-source software. The ARM Cortex-M0 MCUs are positioned as 8-bit and 16-bit MCU replacement. Therefore they have quite small packages (QFN33, LQFP48). The LPC1102 is sold in a WLCSP-16 package of which I've posted microscope images some time ago.

The low-cost LPCXpresso boards come without pin headers. I've added sockets, because its easier to connect wires to them.

One feature of these boards is that they can be used as debug adapter for custom boards. The connections from the debug adapter part to the device part of the board are layed out to holes for a 8x2 pin head with 100mil pitch. When delivered, the connection is made with small solder dots which can be removed using desoldering braid. I've added the pin head and put 8 jumpers on it to re-connect the on-board microcontroller.

One massive drawback of these microcontrollers is their extremley slow Flash memory. This gets even worse because it is (nearly) kept secret. The datasheet as well as the web page praise the fast operating frequency of 50 MHz. You have to dig deeply into the user manual to find a third-level section called "Flash memory access" describing a register called "Flash configuration register". Only the specific description of the two bits "Flash memory access time" reveals that wait-states (this word is avoided too) are required at higher frequencies. The maximum flash frequency is 20 MHz, so at the praised 50 MHz CPU frequency only every third cycle is an active one, effectively dividing the performance by three! Only the timers and other peripherals can utilize the higher clock frequency.

While ranting, nearly all ARM microcontrollers have slow flash and nearly all companies try to obscure that fact. Some microcontrollers try to speed up things using some kind of cache, others put loads of (fast) SRAM for code storage. It is still disappointing to be mucked around.

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