This project is an experiment to generate a 60kHz square wave with good frequency accuracy using the UART on an ESP32-C3 development board. This technique sets the UART's baud rate to double the desired frequency, disables parity, flow control, and enables one stop bit. Sending an endless series of 0x55 generates a square wave where each byte represents 10 of the bits:
1. START bit is 0
2. 1 (LSb)
3. 0
4. 1
5. 0
6. 1
7. 0
8. 1
9. 0 (MSb)
10. STOP bit is 1
- This requires 12000 bytes/second (counting the START and STOP bits, 10 bits are transmitted per byte) which is easy when using interrupts to feed DMA.
- The bits are sent in LSB-first order, so we have to send bytes of 0x55.
- Generate a square wave at frequency 1/2 the baud rate.
- Baud rate generator has pretty good granularity of settable frequency.
- Need baud rate of 120000 to generate 60kHz square wave this way.
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Set UART BRG with divider=666 dividerFract=11 (11/16 = 0.6875)
- ESP32-C3-devkitM-1 has 160MHz CPU clock with
APB_CLK=80MHz. - This is the maximum input frequency to the UART module.
- Yields baud rate of 80.0e6/(666 + 11/16) = 119996.250117 or ~4Hz error.
- ESP32-C3-devkitM-1 has 160MHz CPU clock with
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This code could use explicit DMA, but it's hardly worth it. The Espresso SDK driver probably does DMA for its buffers anyway. So all that's needed is to keep sending data when the FIFO goes empty.