July 2005, Issue 180

Solar-Powered Water Pump Controller
Cypress PSoC High Integration Challenge 2004 Contest Winner

ISRs

There are three main interrupt sources in the pump controller. The most active is the CY8C27443’s 8-bit sigma-delta ADC ISR, which samples at a frequency of 10,416 Hz. The ADC samples are intercepted and processed by the background thread. The internal temperature-sampling ISR uses a relatively slow 166-kHz clock to minimize the CPU’s loading. It’s triggered once every second to infer the ambient temperature of the sealed pump inverter cabinet.

The second most active is the inverter three-phase waveform generator ISR, which operates at the switching frequency of 5,859 Hz. It generates a composite waveform of preprogrammed gain and frequency by triggering at each terminal count of PWM-A corresponding to the start of each switching period. The generator steps through a 360° look-up table containing a normalized composite sine wave at a variable phase velocity that’s proportional to the desired output line frequency:

where N represents the entries in the look-up table (256). FS is the switching frequency (5,859 Hz). FL is the line frequency (25 to 65 Hz).

All three PWM generators use a phase position variable by accessing the look-up table at offsets of 0°, 120°, and 240°. The sine wave values are stored in a normalized form in the look-up table with the 0 through 127 representing 0 through 100%.

The sine wave values for the three phases are instantaneously multiplied by the inverter’s output using the single-cycle MAC unit in the CY8C27443. The resultant values are then renormalized by dividing the result by 128 and then loaded into the PWM duty cycle registers. The MAC unit’s numerical range enables the output gain to have a range from 0 to 127, which corresponds to 0 to 100% of rated voltage output.