Understanding Pinmux design in SHAKTI SoCs

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PinMux or Pin Multiplexing is an important concept in embedded systems. By this concept, we can use a single pin for various mutually exclusive functions. These few pins on the board serve multiple purposes and we have to let the microcontroller know what these are. In this article, we are going to see how to configure the pinmux registers for various peripherals.

How to configure PINMUX register?

The pinmux configuration register is a 32-bit register located at 0x40310. Two bits of the Pinmux register are allocated to each hardware pin so that a total of 4 (00, 01, 10, 11, also 11 is not used now) operations are possible. The default value of Pinmux Configure Register on reset is 0 (all pins are configured as GPIO).

Following are the pins and their corresponding peripheral mapping. We can enable each peripheral by entering the right values to the pinmux register.

Pin Default
[00]
Pinmux
[01]
Pinmux
[10]
IO [0]GPIO_0UART1_RX
IO [1]GPIO_1UART1_TX
IO [2]GPIO_2UART2_RX
IO [3]GPIO_3UART2_TXPWM_0
IO [4]GPIO_4
IO [5]GPIO_5PWM_1
IO [6]GPIO_6PWM_2
IO [7]GPIO_7
IO [8]GPIO_8
IO [9]GPIO_9PWM_3
IO [10]GPIO_10SPI2_CSPWM_4
IO [11]GPIO_11SPI2_MOSIPWM_5
IO [12]GPIO_12SPI2_MISO
IO [13]GPIO_13SPI2_CLK
JB [3]GPIO_14
JB [4]GPIO_15

Note: GPIO4, GPIO7, GPIO8, GPIO14, GPIO15 are dedicated pins i.e. the pins are not multiplexed.

Now, this is how the 32-bit pinmux register is designed.

Fig. Pinmux register configuration

Setting GPIO in Pinmux

To enable GPIO in the pinmux register, all we have to do is to enter 00 (binary) in the bit positions corresponding to the GPIO pins. Consider the following diagram.

Fig. enabling GPIOs in Pinmux register (Default)

Now we have set 0 in all the required bit positions.

The Pinmux register value for setting all GPIO pins is 0000 0000 0000 0000 0000 0000 0000 0000 i.e 0x0 (hex).


Setting SPI2 in PinMux

To enable SPI 2 in the pinmux register, we have to enter 01 (binary) in the corresponding bit positions. Consider the following register diagram,

Fig. enabling SPI2 in Pinmux register

The hex equivalent of 0000 0000 0001 0101 0100 0000 0000 0000 is 0x154000. To enable SPI 2, the pinmux register should contain 0x15400.


Setting UART in PinMux

To enable UART 1 and UART 2 in the Pinmux register, we have to enter 01 (binary) in the corresponding bit positions.

Fig. enabling UART 1&2 in Pinmux register

The hex equivalent of 0000 0000 0000 0000 0000 0000 0101 0101 is 0x55.

ConfigurationPinmux register value
UART 10x05
UART 20x50
UART 1 and UART 20x55
Fig. Pinmux value for UART

Setting PWM in pinmux register

To enable PWMs in the pinmux register, we have to enter 10 (binary) in the corresponding bit positions. Consider the following register diagram,

Fig. enabling PWMs in Pinmux register

Now we can enable the required PWM by enabling the corresponding bit positions. Consider the above figure, as we can see all the PWMs are enabled. The hex equivalent for that is 0000 0000 0000 0010 1010 1010 1000 0000 is 0x2AA80

ConfigurationPinmux register value
Only PWM 00x80
Only PWM 10x200
Only PWM 20x800
Only PWM 30x2000
Only PWM 40x8000
Only PWM 50x20000
All PWMs0x2AA80
Fig. Pinmux value for PWM

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