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On-Board MCU

The Fusion HAT+ comes with an GD32E203C8T6 microcontroller. It is an ARM Cortex-M23 processor with a maximum clock frequency of 72MHz. The microcontroller has 64KB of Flash memory and 8KB of SRAM.

The onboard PWM and ADC are driven by the microcontroller. Communication between the Raspberry Pi and the microcontroller is established via the I2C interface. The I2C address used for communication is 0x17.

ADC

Register addresses is 1 byte, 0x10 to 0x19 are ADC channels 0 to 3. The ADC precision is 12 bit, and the value is 0 to 4095. See more details in fusion_hat.adc.ADC.

Address	Description
`0x10`	ADC channel 0 H
`0x11`	ADC channel 0 L
`0x12`	ADC channel 1 H
`0x13`	ADC channel 1 L
`0x14`	ADC channel 2 H
`0x15`	ADC channel 2 L
`0x16`	ADC channel 3 H
`0x17`	ADC channel 3 L
`0x18`	ADC 4 H (Battery Level)
`0x19`	ADC channel 4 L

PWM

Frequency and Period Settings

The PWM frequency is determined by the period value (ARR) and the prescaler (PSC). The principle is that the microcontroller’s internal clock runs at 72 MHz. Through prescaling, the clock is divided to obtain a frequency Fp, which is then further divided by the period count to get the PWM output frequency F. Therefore,

F = 72,000,000 / (PSC + 1) / (ARR + 1)

In general, by specifying the desired frequency and period, you can calculate the required prescaler. For example, to drive a servo motor, you need a 50 Hz frequency and a desired resolution of 12 bits (i.e., a period of 2¹² = 4096). The prescaler PSC can be calculated as:

PSC = 72,000,000 / F / (ARR + 1) - 1
= 72,000,000 / 50 / (4095 + 1) - 1
= 350.5625
≈ 351

Since PSC must be an integer, it is approximated to 351. Therefore, by setting PSC = 351 and ARR = 4096, you get an actual PWM frequency of:

72,000,000 / 352 / 4096 = 49.937 Hz ≈ 50 Hz.

By default, the PWM prescaler (PSC) and period (ARR) are set to 351 and 4095, respectively, resulting in a default frequency of approximately 50 Hz.

The pulse width corresponds to the duty cycle value within the period. For example, with the configuration above where the period (ARR) is 4096, setting the pulse width (CCR) to 2048 yields a 50% PWM output.

Duty Cycle = CCR / (ARR + 1)

Pulse width

To control the channel pulse width is rather simple, just write the value to the register.

But what is the value? If you want to set the PWM to 50% pulse width, you need to know exactly what the period is. Base on the above calculation, if you set the period to 4095, then set pulse value to 2048 is about 50% pulse width.

Address	Description	Address	Description
`0x60`	PWM0_CCR_H	`0x6C`	PWM6_CCR_H
`0x61`	PWM0_CCR_L	`0x6D`	PWM6_CCR_L
`0x62`	PWM1_CCR_H	`0x6E`	PWM7_CCR_H
`0x63`	PWM1_CCR_L	`0x6F`	PWM7_CCR_L
`0x64`	PWM2_CCR_H	`0x70`	PWM8_CCR_H
`0x65`	PWM2_CCR_L	`0x71`	PWM8_CCR_L
`0x66`	PWM3_CCR_H	`0x72`	PWM9_CCR_H
`0x67`	PWM3_CCR_L	`0x73`	PWM9_CCR_L
`0x68`	PWM4_CCR_H	`0x74`	PWM10_CCR_H
`0x69`	PWM4_CCR_L	`0x75`	PWM10_CCR_L
`0x6A`	PWM5_CCR_H	`0x76`	PWM11_CCR_H
`0x6B`	PWM5_CCR_L	`0x77`	PWM11_CCR_L

Prescaler

Registers from 0x40 are used to set the PWM prescaler (range: 0–65535).

Address	Description
`0x40`	PWM_TIM0_PSC_H
`0x41`	PWM_TIM0_PSC_L
`0x42`	PWM_TIM1_PSC_H
`0x43`	PWM_TIM1_PSC_L
`0x44`	PWM_TIM2_PSC_H
`0x45`	PWM_TIM2_PSC_L

Period

Registers from 0x50 are used to set the PWM period (range: 0–65535).

Address	Description
`0x50`	PWM_TIM0_ARR_H
`0x51`	PWM_TIM0_ARR_L
`0x52`	PWM_TIM1_ARR_H
`0x53`	PWM_TIM1_ARR_L
`0x54`	PWM_TIM2_ARR_H
`0x55`	PWM_TIM2_ARR_L

Button and LED Control

Item

Address

Description

USR Button (r/-)

0x24

Read the level state of the USR button:

0: Low level

1: High level

LED (r/w)

0x30

Read or control the LED:

0: off

1: on

2: toggle

Buzzer Switch Control

Item

Address

Description

Buzzer Switch (r/w)

0x31

Read or control the buzzer switch:

0: off

1: on

Servo Zeroing

When you press the button twice, all the PWM signals will be set to 1500us pulse, 20000us period. That is, the servo will be in the middle position. You should secure the servo arm to the servo in this state.
Press the button twice again, all the PWM signals will be set to 0 pulse.

Charging Status

Address

Value

0x25

0: Not Charging

1: Charging

Safe Shutdown

The Fusion HAT+ includes a built-in safe shutdown mechanism. It performs a software-based shutdown by reading the microcontroller’s register status via I2C.

Address

Value

0x26

0: No request

1: Low battery, shutdown requested

2: Shutdown requested by power button

To configure the Raspberry Pi shutdown signal, please refer to Configure Shutdown Behavior for detailed instructions.