April 2005, Issue 177

Simple USB Data Acquisition

DAQ VIA ARM

In this simple USB ARM DAQ application, the LPC2138 must read analog temperature sensor voltage at a timed interval. The data must be formatted and then sent to the MCB2130 board’s serial port. Therefore, you must use the LPC2138’s timer peripheral for the interval timer, its A/D peripheral for reading the analog temperature sensor voltage, and its UART peripheral for serial communication.

The LPC2138’s timer is a 32-bit timer/counter with a programmable 32-bit prescaler. It’s an extremely flexible timer given its capture channels, match registers, external outputs, and interrupt capabilities. I preloaded a timer match register for this application. The LPC2138 generates a timer interrupt when the timer counter matches this value. Its A/D converter is a 10-bit successive approximation A/D converter. A 10-bit reading of the analog temperature sensor provides more than enough resolution for the USB ARM DAQ example.

The LPC2138’s UART is your typical UART with data rate generation, but it also includes 16-byte receive and transmit FIFOs for added flexibility. Given that temperature data is sent out every few minutes, the data rate is set to 9,600 bps. Now that you’re familiar with the LPC2138’s peripherals, let’s move on to the embedded software.