Reflet microcontroller

A micro-controller based on a Reflet CPU.

Part of the system are included as submodules, so you should run git submodule update --init --recursive after cloning this repository.

The CPU

The CPU is a Reflet CPU. More information and the toolchain can be found here: https://github.com/Arkaeriit/reflet.

Peripherals

All the Verilog files describing the peripherals and their submodules are in the peripheral folder.

All the peripherals are controlled and monitored with 8-bits registers to make them compatible with any Reflet CPU. Each register is only accessible with its exact address (even in 16-bit mode, editing the register at address 0xFF00 will not impact the register at address 0xFF01).

The peripherals are identified with a base address and are controlled by editing the register from the base address and a few bytes forwards. For example, with the 16-bit controller, the GPIO got 0xFF07 as its base address and is configured with the register at address 0xFF07 to 0xFF11.

To minimize the synthesis time and size of a reflet microcontroller, peripherals can be enabled or disabled with Verilog parameters.

Hardware info

This peripheral is always active. It is used to read the frequency of the system clock and which peripherals are enabled or not.

Offset with base address	Type	Register name	Effect
0	ro	clk_lsb	Contain the 8 LSB of the frequency of the main clock in MHz.
1	ro	clk_msb	Contain the 8 MSB of the frequency of the main clock in MHz.
2	ro	periph_1	Bits 0 to 2 contain info about the wordsize of the processor. 1 means that it is an 8-bit processor, 2 means a 16 bit, 3 32 bits, 4 64 bits, 5 128 bits, and 0 means any other wordsize. Bit 3 tells if the interrupt multiplexer is enabled. Bit 4 tells if the GPIO is enabled. Bit 5 tells if the timer is enabled. Bit 6 tells if the second timer is enabled. Finally, bit 7 tells if the UART is enabled.
3	ro	periph_2	Bit 0 tells if the PWM is enabled. Bit 1 tells if the seven-segment controller is enabled. Bit 2 tells if the power manager is enabled. Bit 3 tells if the synthesizer is enabled. Bit 4 tells if the extended io is enabled. Bits 5 to 7 are left to 0.

Interrupt multiplexer

The CPU got only 4 interrupt lines, but more than 4 peripherals can raise interrupts. This module lets the user control which peripherals can use interrupts and the priority of each interrupt.

Offset with base address	Type	Register name	Effect
0	r/w	int_en	Bit 0 control if the interrupt from the GPIO is enabled. Bit 1 control if the interrupt from the UART is enabled. Bit 2 control if the interrupt from the timer is enabled. Bit 3 control if the interrupt from timer 2 is enabled.
1	r/w	int_level	Bits 0 and 1 control the priority of the GPIO interrupt. Bits 2 and 3 control the priority of the UART interrupt. Bits 4 and 5 control the priority of the timer interrupt. Bits 6 and 7 control the priority of the timer 2 interrupt.
2	ro	reserved	Reserved for future used. Locked to 0 for now.
3	r/w	status	This register tells which interrupts are currently raised. Whenever a new interrupt appends, the related bit is raised to 1. To acknowledge the interrupt, its bit must be switched back to 0. Bit 0 tell the state of the GPIO interrupt. Bit 1 tells the state of the UART interrupt. Bit 2 tells the state of the timer interrupt. Bit 3 tells the state of the timer 2 interrupt.

GPIO

The most basic way to interact with the external world. It is made of a 16 bits parallel output and a 16 bits parallel input. The input can trigger an interrupt on the rising or falling edges of each input pin.

Offset with base address	Type	Register name	Effect
0	r/w	gpo_lsb	Control the 8 first pins of the parallel output.
1	r/w	gpo_msb	Control the 8 last pins of the parallel output.
2	ro	gpi_lsb	Contain the state of the 8 first pins of the parallel input.
3	ro	gpi_msb	Contain the state of the 8 last pins of the parallel input.
4	r/w	rise_int_lsb	Control which of the 8 first pins of the parallel input should raise an interrupt on a rising edge.
5	r/w	rise_int_msb	Control which of the 8 last pins of the parallel input should raise an interrupt on a rising edge.
6	r/w	fall_int_lsb	Control which of the 8 first pins of the parallel input should raise an interrupt on a falling edge.
7	r/w	fall_int_msb	Control which of the 8 last pins of the parallel input should raise an interrupt on a falling edge.

Timer

A 24 bits timer. This timer contains two prescalers (pre1 and pre2) and the main counter (mcnt). Each of these 3 values is on 8 bits. The timer raise an interrupt each (pre1 + 1) * (pre2 + 1) * (mcnt) cycles of the system clock.

Offset with base address	Type	Register name	Effect
0	r/w	pre1	Control the value of pre1.
1	r/w	pre2	Control the value of pre2.
2	r/w	mcnt	Control the value of mcnt.

Timer 2

A 23 bits timer. This timer behaves in a similar way to the main timer except that pre1 is on 7 bits instead of 8 and that we can select to use either the system clock or the output of the main timer as the input source. This makes it possible to chain timer and timer2 to have a resulting 47 bits timer.

Offset with base address	Type	Register name	Effect
0	r/w	pre1	Bit 0 should be set to 0 to use the system clock as an input source and set to 1 to use timer 1 as the input source. Bits 1 to 7 control the value of pre1.
1	r/w	pre2	Control the value of pre2.
2	r/w	mcnt	Control the value of mcnt.

UART

A UART module with a baud rate locked at 9600. Its behavior is very similar to the IO functions of the Reflet simulator. To send a message, write its value in the tx_data register and put a 0 in tx_cmd. To receive a message, either enable the interrupt in the interrupt multiplexer or wait for the value in the rx_cmd register to be set to 0. The data will then be readable in the rx_data register.

Offset with base address	Type	Register name	Effect
0	r/w	tx_cmd	Set to 0 to send a message.
1	r/w	tx_data	Write here the message to send.
2	r/w	rx_cmd	This register is automatically set to 0 when a byte is received.
3	r/w	rx_data	The byte received is readable here.

PWM

A PWM module with up to 8 bits of resolution. Its frequency is configurable but the higher the frequency, the smaller the resolution. Its period is controllable with the freq register and its duty cycle with the duty register. The resulting duty cycle will be duty/freq.

Offset with base address	Type	Register name	Effect
0	r/w	freq	Set the period of the PWM.
1	r/w	duty	Set the duty-cycle.

Seven segments display controller

Even if this could be done with GPIO and a timer, a controller for seven segments display could be useful. This module is made to control 4 digits displays with colons and dots. In the selection bits, a 1 indicates that the digit is active. In the segments bits, 0 indicates that the segments must be lit.

Offset with base address	Type	Register name	Effect
0	r/w	ctrl	Bit 0 is used to enable or disable the display. Bit 1 is used to enable the colon. Bits 4 to 7 are used to select which dots must be turned on. Bits 2 and 3 are unused.
1	r/w	num01	Bits 0 to 3 are used to set the value of the first number. Bits 4 to 7 are used to set the value of the second number.
2	r/w	num23	Bits 0 to 3 are used to set the value of the third number. Bits 4 to 7 are used to set the value of the fourth number.

Power manager

This module is meant to either reduce the speed of the processor or disable it until an interrupt happens. To reduce the speed, a PWM signal is sent to the enable pin of the CPU.

Offset with base address	Type	Register name	Effect
0	r/w	sleep	When bit 0 is set to 1, the processor is disabled until a detected interrupt is raised. When bit 1 is set to 1, the processor is slowed down by a factor controlled in the power register. Bits 2 and 3 are unused. Bit 4 to 7 are used to tell if interrupts 0 to 3 are watched. If bit 4 is set to 1, the module will watch for interrupt 0 and unlock the processor if interrupt 0 is raised. Bit 5 will enable interrupt 1 and so on.
1	r/w	power	This register will let you choose the duty cycle to send to the enable pin of the CPU. The duty cycle is (the content of this register + 1)/256.

Frequency synthesizer

The documentation will come when the module will be done

Extended IO

If you need more than the 16 inputs and 16 outputs of the GPIO module, you can use this one to have up to 256 additional inputs and 256 output. The IOs can be selected with an address on a register and controlled with the other one. The inputs can not raise interrupt. The actual number of IO is chosen with a parameter of the controller module.

Offset with base address	Type	Register name	Effect
0	r/w	address	Select which IO is selected.
1	r/w	control	Bit 0 contains the value in the selected input. Bit 1 contains the actual value of the selected output. To set the output, write the value you want in bit 2 and set bit 3 to 1. Bits 4 to 7 are unused.

Debug Helper

This module helps in debugging by printing over an UART line the content of the working register whenever a debug instruction is encountered. The data is sent at 9600 bauds in a little-endian fashion. When sending those data, the CPU is disabled to ensure that no debug instructions will be missed.

Controllers

The folder controller contains files used to make 8 bits or a 16 bits microcontroller with a Reflet CPU. As of now, for the 8-bit controller, you must copy the reflet_?bits_controller.v file and replace the data memory with a ROM with a valid Reflet program. For the 16-bit one, you can do so or choose to use the included bootloader instead.

8 bits controller

The 8 bits controller is very limited by its addressable space and thus, is only equipped with a limited amount of peripherals.

The memory map is the following:

Start	End	Content
0x00	0x7F	Instructions
0x80	0xEC	Data
0xED	0xFF	Peripherals
-----	----	-------
0xED	0xF0	Interrupt multiplexer
0xF1	0xF8	GPIO
0xF9	0xFB	Timer
0xFC	0xFF	UART

Top-level module

The top-level module is in the controller folder and is named reflet_8bit_controller. It can not be used as-is because there is no way to fill its instruction memory. To use it, you must copy the top-level module and replace the mem_inst module with a ROM.

The parameters for this module are the following:

Name	Description	Default value
clk_freq	The frequency of the main clock in Hertz.	1 000 000
debug_output	When set to 1, enable the debug helper module.	0
enable_interrupt_mux	If equal to 0, the interrupt multiplexer is disabled.	1
enable_gpio	If equal to 0, the GPIOs are disabled.	1
enable_timer	If equal to 0, the timer is disabled.	1
enable_uart	If equal to 0, the UART is disabled.	1
mem_resetable	If not set to 0, the memories are reset to 0 when the reset signal is pulled down. This can be incompatible with some FPGA's memory blocks.	0

The ports of this module are the following:

Name	Type	Description
clk	input	The main clock.
reset	input	Reset the controller when flipped to low.
debug	output	Turned on for a clock cycle when the CPU executes a debug instruction.
debug_tx	output	When debug_output is enabled, this line prints the content of the working register on debug instructions.
quit	output	Is turned on from the moment the CPU executes a quit instruction and stops itself.
gpi	16-bit input	The input of the GPIO.
gpo	16-bit output	The output of the GPIO.
rx	input	The rx line of the UART.
tx	output	The tx line of the UART.

16 bits controller

The 16 bits controller is equipped with all the peripherals and its bigger addressable space makes it way easier to program for.

The memory map is the following:

Start	End	Content
0x0000	0x7CFF	Instructions
0x7D00	0x7FFF	Bootloader
0x8000	0xFEFF	Data
0xFF00	0xFFFF	Peripherals
------	------	-------
0xFF00	0xFF03	Hardware info
0xFF04	0xFF07	Interrupt multiplexer
0xFF08	0xFF0F	GPIO
0xFF10	0xFF12	Timer
0xFF13	0xFF15	Timer 2
0xFF16	0xFF19	UART
0xFF1A	0xFF1B	PWM
0xFF1C	0xFF1E	Seven segments controller
0xFF1F	0xFF20	Power manager
0xFF21	0xFF21	Frequency synthesizer
0xFF22	0xFF23	Extended IO

Top-level module

The top-level module is in the controller folder and is named reflet_16bit_controller. Unlike its 8-bit counterpart, it can be used on its own as it is equipped with a bootloader that can be used to load a program in the data memory. Of course, if it is needed to only use a single program with an implementation, the mem_inst module can be replaced with a ROM.

The parameters for this module are the following:

Name	Description	Default value
clk_freq	The frequency of the main clock in Hertz.	1 000 000
debug_output	When set to 1, enable the debug helper module.	0
enable_interrupt_mux	If equal to 0, the interrupt multiplexer is disabled.	1
enable_gpio	If equal to 0, the GPIOs are disabled.	1
enable_timer	If equal to 0, the timer is disabled.	1
enable_timer2	If equal to 0, the timer2 is disabled.	1
enable_uart	If equal to 0, the UART is disabled.	1
enable_pwm	If equal to 0, the PWM is disabled.	1
enable_segments	If equal to 0, the seven segments display controller is disabled.	1
enable_power_manager	If equal to 0, the power manager is disabled.	1
enable_synth	If equal to 0, the frequency synthesizer is disabled.	1
enable_ext_io	If equal to 0, the extended IO is disabled.	1
data_size	Size in bytes of the data memory.	100
inst_size	Size in bytes of the instruction memory.	128
mem_resetable	If not set to 0, the memories are reset to 0 when the reset signal is pulled down. This can be incompatible with some FPGA's memory blocks.	0
ext_io_size	The size of the extended io ports.	128
swift_bootloader	Use a 50 ms bootloader timeout instead of 4 s.	0

The ports of this module are the following:

Name	Type	Description
clk	input	The main clock.
reset	input	Reset the controller when flipped to low.
reset_limited	input	Reset the CPU, peripherals, and data memory but preserves the instruction memory.
debug	output	Turned on for a clock cycle when the CPU executes a debug instruction.
debug_tx	output	When debug_output is enabled, this line prints the content of the working register on debug instructions.
quit	output	Is turned on from the moment the CPU executes a quit instruction and stops itself.
gpi	16-bit input	The input of the GPIO.
gpo	16-bit output	The output of the GPIO.
rx	input	The rx line of the UART.
tx	output	The tx line of the UART.
pwm	output	Output of the PWM.
segments	7-bit output	Seven-segment control, an output value of 0 means that the segment is on.
seg_select	4-bit output	Seven segments digit selection, an output value of 1 means that the digit is selected.
seg_colon	output	Control of the colon indicator of the seven-segment display, a value of 0 means that the colon is turned on.
seg_dot	output	Control of the decimal dot indicator of the seven-segment display, a value of 0 means that the dot is turned on.
synth_out	output	Output of the frequency synthesizer.
ext_io_in	selectable size input	Input of the extended IO module.
ext_io_out	selectable size output	Output of the extended IO module.

Using the bootloader

The bootloader is very easy to use. After the processor started, transmit the program through the UART connection and wait 4 seconds for the program to start. If 4 seconds is too long, you can use the parameter swift_bootloader to lower it to 50 ms.