Multichannel signal generators are used in many fields of science. For example, they are used to provide signals for ultrasonic phased arrays and usually depend on costly FPGAs or DSPs. We present a design based on the synchronization of multiple low-cost RP2040 microcontrollers to generate digital square wave signals (PWM) and a granularity of 4 ns. It can be scaled up for high output counts. The CPU clocks are synchronized, and PIO modules are used to generate synchronized interrupts across the system. This ensures a stable phase relation of all output signals with a remaining jitter of 0.17 ns. We demonstrate the generation of chirp signals by utilizing the DMA for hardware-level data transfer. This signal generator can utilized to control ultrasonic phased arrays and generate arbitrary test patterns for digital circuits.