3

The raspberry pi 4 has at least two uarts (as shown here), I know where I can connect to one of them. Pins 8, 10 and 12 have a uart as an option.

enter image description here Source:

This site describes accessing uart1, where is it? Do I have access to it?

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4

The preliminary Pi4 datasheet shows there are 6 UARTs.

UART GPIO
0    14/15
1    14/15
2    0/1
3    4/5
4    8/9
5    12/13

Given that UART 0/1 are both connected to the same GPIO you can only use one of them at any one time. So potentially there are 5 accessible UARTs at any one time.

You can enable each of the UARTs on the Pi4B by making appropriate entries in /boot/config.txt.

See /boot/overlays/README for details.

At the simplest add the following entry to enable UARTx.

dtoverlay=uartx

1

As Joan has stated only 1 of UART0/1 is accessible (UART0 /dev/ttyAMA0 is connected to Bluetooth on later model Pi).
There are additional UART on the Pi4.

I haven't actually tried these.
I doubt UART2 would be usable as GPIO0/1 are used for HATs (and shown as reserved).
I suspect that the SDA0/SCL0 connected to these pins are used for other internal functions.

I have extracted the GPIO Alternate Functions from the data sheet, as I have been unable to find any existing list.

This shows the mode and function of each.

   ALT0       ALT1       ALT2       ALT3       ALT4       ALT5      
 0 SDA0       SA5        PCLK       SPI3_CE0_N TXD2       SDA6      
 1 SCL0       SA4        DE         SPI3_MISO  RXD2       SCL6      
 2 SDA1       SA3        LCD_VSYNC  SPI3_MOSI  CTS2       SDA3      
 3 SCL1       SA2        LCD_HSYNC  SPI3_SCLK  RTS2       SCL3      
 4 GPCLK0     SA1        DPI_D0     SPI4_CE0_N TXD3       SDA3      
 5 GPCLK1     SAO        DPI_D1     SPI4_MISO  RXD3       SCL3      
 6 GPCLK2     SOE_N      DPI_D2     SPI4_MOSI  CTS3       SDA4      
 7 SPI0_CE1_N SWE_N      DPI_D3     SPI4_SCLK  RTS3       SCL4      
 8 SPI0_CE0_N SDO        DPI_D4     _          TXD4       SDA4      
 9 SPI0_MISO  SD1        DPI_D5     _          RXD4       SCL4      
10 SPI0_MOSI  SD2        DPI_D6     _          CTS4       SDA5      
11 SPI0_SCLK  SD3        DPI_D7     _          RTS4       SCL5      
12 PWM0       SD4        DPI_D8     SPI5_CE0_N TXD5       SDA5      
13 PWM1       SD5        DPI_D9     SPI5_MISO  RXD5       SCL5      
14 TXD0       SD6        DPI_D10    SPI5_MOSI  CTS5       TXD1      
15 RXD0       SD7        DPI_D11    SPI5_SCLK  RTS5       RXD1      
16 FL0        SD8        DPI_D12    CTS0       SPI1_CE2_N CTS1      
17 FL1        SD9        DPI_D13    RTS0       SPI1_CE1_N RTS1      
18 PCM_CLK    SD10       DPI_D14    SPI6_CE0_N SPI1_CE0_N PWM0      
19 PCM_FS     SD11       DPI_D15    SPI6_MISO  SPI1_MISO  PWM1      
20 PCM_DIN    SD12       DPI_D16    SPI6_MOSI  SPIl_MOSI  GPCLK0    
21 PCM_DOUT   SD13       DPI_D17    SPI6_SCLK  SPI1_SCLK  GPCLK1    
22 SD0_CLK    SD14       DPI_D18    SD1_CLK    ARM_TRST   SDA6      
23 SD0_XMD    SD15       DPI_D19    SD1_CMD    ARM_RTCK   SCL6      
24 SD0_DATO   SD16       DPI_D20    SD1_DAT0   ARM_TDO    SPI3_CE1_N
25 SD0_DAT1   SD17       DPI_D21    SD1_DAT1   ARM_TCK    SPI4_CE1_N
26 SD0_DAT2   TE0        DPI_D22    SD1_DAT2   ARM_TDI    SPI5_CE1_N
27 SD0_DAT3   TE1        DPI_D23    SD1_DAT3   ARM_TMS    SPI6_CE1_N

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