I have an Adafruit Feather nRF52840 which I'm trying to use to transmit data over a UART serial connection to a Raspberry Pi. The Raspberry Pi is running a Python test script (details below) which configures its serial port per the MIDI specification which says:
"The hardware MIDI interface operates at 31.25 (+/- 1%) Kbaud, asynchronous, with a start bit, 8 data bits (D0 to D7), and a stop bit. This makes a total of 10 bits for a period of 320 microseconds per serial byte. The start bit is a logical 0 (current on) and the stop bit is a logical 1 (current off)".
My Zephyr code configures the nRF52840 in the same way. I then send 8 bytes with known values. The Raspberry Pi script prints what it receives to the console.
The Pi is printing 5 times more bytes than are being transmitted and the byte values bear no resemblance to those that were transmitted. I believe this is indicative of a mismatch between the UART configs on the transmitting nRF52840 and the Pi.
Here's the Raspberry Pi receiver script:
import serial
ser = serial.Serial('/dev/ttyAMA0', baudrate=31250)
while True:
data = ord(ser.read(1))
print("Decimal: "+str(data)+"\t\tBinary: "+format(data,'008b')+"\t\tHex: 0x"+format(data,'02x'))
Note that I validated the Python script by connecting a commercial MIDI keyboard directly to the Pi and pressing a key (note=F3). The results were as expected and are presented here:
MIDI Keyboard sending NOTE ON then NOTE OFF to the Receiver (key played, F3) Decimal: 144 Binary: 10010000 Hex: 0x90 Decimal: 65 Binary: 01000001 Hex: 0x41 Decimal: 52 Binary: 00110100 Hex: 0x34 Decimal: 128 Binary: 10000000 Hex: 0x80 Decimal: 65 Binary: 01000001 Hex: 0x41 Decimal: 59 Binary: 00111011 Hex: 0x3b NOTE ON is 1001nnnn 0kkkkkkk 0vvvvvvv where n is the channel, k is the note number and v is the velocity value. NOTE OFF is 1000nnnn 0kkkkkkk 0vvvvvvv where n is the channel, k is the note number and v is the velocity value. So the data received: 10010000 01000001 00110100 - NOTE ON (channel 1), note=F3, velocity=52 10000000 01000001 00111011 - NOTE OFF (channel 1), note=F3, velocity=59 Results are as expected. Python script validated.
Here's my Zephyr code:
// C code
#include <zephyr/kernel.h>
#include <zephyr/drivers/uart.h>
#include <math.h>
/* Get UART1 device */
static const struct device *uart = DEVICE_DT_GET(DT_NODELABEL(uart1));
// 1200, 2400, 4800, 9600, 19200, 31250, 57600, 115200
#define BAUDRATE 31250
int main(void)
{
printk("uart_tx_text V1.3 - baudrate=%d\n",BAUDRATE);
if (!device_is_ready(uart)) {
printk("ERROR: uart1 is not ready\n");
return -1;
}
printk("UART1 is ready\n");
/* UART configuration */
const struct uart_config uart_cfg = {
.baudrate = BAUDRATE,
.parity = UART_CFG_PARITY_NONE,
.stop_bits = UART_CFG_STOP_BITS_1,
.data_bits = UART_CFG_DATA_BITS_8,
.flow_ctrl = UART_CFG_FLOW_CTRL_NONE
};
int ret = uart_configure(uart, &uart_cfg);
if (ret < 0) {
printk("ERROR configuring UART: %d",ret);
return ret;
}
printk("UART1 has been configured - start receiver now\n");
k_msleep(10000);
int p=0;
for (p=1; p<9;p++) {
uint8_t val=(pow(2,p)-1);
uart_poll_out(uart, val);
printk("TX Decimal: %d Hex: %02x\n",val, val);
}
return 0;
}
// DTS overlay
/*
* Copyright (c) 2025
* SPDX-License-Identifier: Apache-2.0
*/
&uart1 {
compatible = "nordic,nrf-uarte";
status = "okay";
current-speed = <31250>;
pinctrl-0 = <&uart1_default>;
pinctrl-1 = <&uart1_sleep>;
pinctrl-names = "default", "sleep";
};
&pinctrl {
uart1_default: uart1_default {
group1 {
psels = <NRF_PSEL(UART_TX, 1, 10)>;
};
group2 {
psels = <NRF_PSEL(UART_RX, 1, 9)>;
bias-pull-up;
};
};
uart1_sleep: uart1_sleep {
group1 {
psels = <NRF_PSEL(UART_TX, 1, 10)>,
<NRF_PSEL(UART_RX, 1, 9)>;
low-power-enable;
};
};
};
// prj.conf
CONFIG_GPIO=y
CONFIG_SERIAL=y
CONFIG_UART_USE_RUNTIME_CONFIGURE=y
CONFIG_LOG=y
CONFIG_USE_SEGGER_RTT=y
CONFIG_SEGGER_RTT_PRINTF_BUFFER_SIZE=512
And here are my test results:
// output from the Adafruit Feather nRF52840 00> *** Booting nRF Connect SDK v3.1.1-e2a97fe2578a *** 00> *** Using Zephyr OS v4.1.99-ff8f0c579eeb *** 00> uart_tx_text V1.3 - baudrate=31250 00> UART1 is ready 00> UART1 has been configured - start receiver now 00> TX Decimal: 1 Hex: 01 00> TX Decimal: 3 Hex: 03 00> TX Decimal: 7 Hex: 07 00> TX Decimal: 15 Hex: 0f 00> TX Decimal: 31 Hex: 1f 00> TX Decimal: 63 Hex: 3f 00> TX Decimal: 127 Hex: 7f 00> TX Decimal: 255 Hex: ff // Output from the Raspberry Pi receiver script Receiver: Decimal: 129 Binary: 10000001 Hex: 0x81 Decimal: 78 Binary: 01001110 Hex: 0x4e Decimal: 198 Binary: 11000110 Hex: 0xc6 Decimal: 39 Binary: 00100111 Hex: 0x27 Decimal: 237 Binary: 11101101 Hex: 0xed Decimal: 136 Binary: 10001000 Hex: 0x88 Decimal: 139 Binary: 10001011 Hex: 0x8b Decimal: 200 Binary: 11001000 Hex: 0xc8 Decimal: 220 Binary: 11011100 Hex: 0xdc Decimal: 107 Binary: 01101011 Hex: 0x6b Decimal: 239 Binary: 11101111 Hex: 0xef Decimal: 11 Binary: 00001011 Hex: 0x0b Decimal: 224 Binary: 11100000 Hex: 0xe0 Decimal: 220 Binary: 11011100 Hex: 0xdc Decimal: 107 Binary: 01101011 Hex: 0x6b Decimal: 239 Binary: 11101111 Hex: 0xef Decimal: 11 Binary: 00001011 Hex: 0x0b Decimal: 224 Binary: 11100000 Hex: 0xe0 Decimal: 196 Binary: 11000100 Hex: 0xc4 Decimal: 107 Binary: 01101011 Hex: 0x6b Decimal: 207 Binary: 11001111 Hex: 0xcf Decimal: 11 Binary: 00001011 Hex: 0x0b Decimal: 113 Binary: 01110001 Hex: 0x71 Decimal: 198 Binary: 11000110 Hex: 0xc6 Decimal: 107 Binary: 01101011 Hex: 0x6b Decimal: 207 Binary: 11001111 Hex: 0xcf Decimal: 11 Binary: 00001011 Hex: 0x0b Decimal: 224 Binary: 11100000 Hex: 0xe0 Decimal: 198 Binary: 11000110 Hex: 0xc6 Decimal: 59 Binary: 00111011 Hex: 0x3b Decimal: 207 Binary: 11001111 Hex: 0xcf Decimal: 3 Binary: 00000011 Hex: 0x03 Decimal: 232 Binary: 11101000 Hex: 0xe8 Decimal: 198 Binary: 11000110 Hex: 0xc6 Decimal: 59 Binary: 00111011 Hex: 0x3b Decimal: 207 Binary: 11001111 Hex: 0xcf Decimal: 139 Binary: 10001011 Hex: 0x8b Decimal: 232 Binary: 11101000 Hex: 0xe8 Decimal: 228 Binary: 11100100 Hex: 0xe4 Decimal: 255 Binary: 11111111 Hex: 0xff
My UART config params look correct in my Zephyr C code and I am not getting an error when I call uart_configure. And yet the evidence would suggest that the configuration is not being set as requested. The transmitting nRF52840 and receiving Pi are presumably not using the same parameters which is why the Pi reports nonsense results.
Note that this Pi has been used for MIDI applications many times before and is therefore a reliable, known quantity. My validation test also verifies that the problem is not at the receiver end of the communication.
Could someone please help with this? Is this a Zephyr bug? Or is it my code?
Thanks in anticipation
