Cannot write to serial - c++

Question

I am using the ESP32 DevKitC-v1 (clone) with FreeRTOS and attempting to write bytes to the serial port.
My code (below) causes the following exception:
Guru Meditation Error: Core 0 panic'ed (LoadProhibited). Exception was unhandled.
Core 0 register dump:
PC : 0x400ea8fe PS : 0x00060730 A0 : 0x800eb825 A1 : 0x3ffdfdf0
A2 : 0x00000001 A3 : 0x00000055 A4 : 0x00000001 A5 : 0x00000000
A6 : 0x00000002 A7 : 0xff000000 A8 : 0x00000000 A9 : 0x60010000
A10 : 0x00000055 A11 : 0x00000001 A12 : 0x3ffdfd54 A13 : 0x3ffd1068
A14 : 0x00000000 A15 : 0x00000000 SAR : 0x00000004 EXCCAUSE: 0x0000001c
EXCVADDR: 0x00000055 LBEG : 0x400014fd LEND : 0x4000150d LCOUNT : 0xffffffff
Backtrace: 0x400ea8fe:0x3ffdfdf0 0x400eb822:0x3ffdfe10 0x400ebf0d:0x3ffdfe50 0x400e1511:0x3ffdfe80
Rebooting...
ets Jun 8 2016 00:22:57
rst:0xc (SW_CPU_RESET),boot:0x13 (SPI_FAST_FLASH_BOOT)
configsip: 0, SPIWP:0xee
clk_drv:0x00,q_drv:0x00,d_drv:0x00,cs0_drv:0x00,hd_drv:0x00,wp_drv:0x00
mode:DIO, clock div:2
load:0x3fff0018,len:4
load:0x3fff001c,len:6372
load:0x40078000,len:11276
load:0x40080000,len:6084
entry 0x4008032c
I have been able to find limited information regarding FreeRTOS and UART operations, I apologise in advance however if I have missed something obvious. I have been trying to self resolve the issue for about 3 days now.
My code:
#define ECHO_TEST_TXD (GPIO_NUM_17)
#define ECHO_TEST_RXD (GPIO_NUM_16)
#define ECHO_TEST_RTS (UART_PIN_NO_CHANGE)
#define ECHO_TEST_CTS (UART_PIN_NO_CHANGE)
static void prvSerialRelayMQTTCommand()
{
const TickType_t xFiveSeconds = pdMS_TO_TICKS( 5000UL );
/* Configure parameters of an UART driver,
* communication pins and install the driver */
uart_config_t uart_config = {
.baud_rate = 9600,
.data_bits = UART_DATA_8_BITS,
.parity = UART_PARITY_DISABLE,
.stop_bits = UART_STOP_BITS_1,
.flow_ctrl = UART_HW_FLOWCTRL_DISABLE
};
uart_param_config(UART_NUM_2, &uart_config);
uart_set_pin(UART_NUM_2, ECHO_TEST_TXD, ECHO_TEST_RXD, ECHO_TEST_RTS, ECHO_TEST_CTS);
uart_driver_install(UART_NUM_2, BUF_SIZE * 2, 0, 0, NULL, 0);
while (1) {
uart_write_bytes(UART_NUM_2, 0b00000000, 1);
vTaskDelay(xFiveSeconds);
uart_write_bytes(UART_NUM_2, 0b00000001, 1);
vTaskDelay(xFiveSeconds);
uart_write_bytes(UART_NUM_2, 0b00000011, 1);
vTaskDelay(xFiveSeconds);
uart_write_bytes(UART_NUM_2, 0b00000111, 1);
vTaskDelay(xFiveSeconds);
uart_write_bytes(UART_NUM_2, 0b00001111, 1);
vTaskDelay(xFiveSeconds);
}
}

Answer 1

Invalid pointer arguments
The second argument to uart_write_bytes() should be a const char * (see reference). You are passing it an invalid pointer.
Change
uart_write_bytes(UART_NUM_2, 0b00000000, 1);
to
char c = '\0';
uart_write_bytes(UART_NUM_2, &c, 1);
and similarly for the other calls.
However, this isn't necessarily your only problem.
Stack size for FreeRTOS task
A LoadProhibited error occurs when the ESP32 attempts to read or write an invalid memory address. The relevant address is stored in the EXCVADDR register - which in this case we can see is 0x00000055. This means that your calls to uart_write_bytes() (even though invalid) are not the cause of the reset - because you did not pass the address 0x00000055 in any of the calls.
I suspect your problem is that you are not allocating a large enough stack for your FreeRTOS task. If you create a task with a stack size of configMINIMAL_STACK_SIZE, for example, that might not be enough for using the UART. A stack overflow would also result in a LoadProhibited error.
The stack size is the third parameter passed to xTaskCreate(). Try verifying that it is at least 1024, and increase as needed.