UART4 Unavailable on STM32F750N8-DK?

[DRobe.4]

I want to use only the Tx pin on UART4, it is on PA0/CN5-1 of the Arduino connector. I setup

// Initialize UART4 for Debug Terminal 250 kbaud using 50 MHz PCLK1 /////////////////////////////////

   PeriphClkInitStruct.PeriphClockSelection = RCC_PERIPHCLK_TIM

                                                                  |RCC_PERIPHCLK_USART1

                                                                  |RCC_PERIPHCLK_USART6

                                                                  |RCC_PERIPHCLK_UART4;               // enable clocks to USART1/USART6/UART4/TIM

 PeriphClkInitStruct.Usart1ClockSelection = RCC_USART1CLKSOURCE_PCLK2;      // USART1 runs on PCLK2 = 100 MHz

 PeriphClkInitStruct.Usart6ClockSelection = RCC_USART6CLKSOURCE_PCLK2;      // USART6 runs on PCLK2 = 100 MHz

 PeriphClkInitStruct.Usart6ClockSelection = RCC_UART4CLKSOURCE_PCLK1;      // USART4 runs on PCLK1 = 50 MHz

 PeriphClkInitStruct.TIMPresSelection    = RCC_TIMPRES_ACTIVATED;

   RCC->APB1ENR |= RCC_APB1ENR_UART4EN;

      __HAL_RCC_GPIOA_CLK_ENABLE();

      GPIO_InitStruct.Alternate = GPIO_AF8_UART4;

      GPIO_InitStruct.Mode       = GPIO_MODE_OUTPUT_PP;            // PA0=Tx

      GPIO_InitStruct.Pin        = GPIO_PIN_0;

      GPIO_InitStruct.Pull       = GPIO_NOPULL;

      GPIO_InitStruct.Speed      = GPIO_SPEED_MEDIUM;

      HAL_GPIO_Init(GPIOA, &GPIO_InitStruct);

   UART4->CR1   = 0x00;                                       // reset

   UART4->BRR  = 400;                                          // 100,000,000/2/250000=200 (running on PCLK1 = 50 MHz)

      UART4->CR1  = 0x1008;                                                   // Enable Tx and 9-bit length using bit12=1

      UART4->CR1 |= USART_CR1_UE;                                          // Enable UART4

When I observed CN5-1 on a scope, no sign of activity.

To debug further, I went to STM32CubeMX and tried to enable UART4 and it was unavailable even though I ensured that no ADC/SDMMC/ETH were enabled. I am using the LCD but it should not conflict. What am I doing wrong?

[Tesla DeLorean]

GPIO_InitStruct.Mode       = GPIO_MODE_AF_PP;  

[DRobe.4]

Thanks, but that had no effect. Maybe the answer involves the peculiar CubeMX unavailability of UART4... other USARTs work. BTW, just in case I wasn't clear, my Keil IDE does not object to the code. Also, the code that I pasted above is only a small part of the whole code, so my next step is to delete everything unrelated to UART4... I was hoping that there was a really simple solution.

[DRobe.4]

The stripped down version works, so there was some conflict, I guess.