324 Real Time Motor Controller :: Quicker, easier and cheaper to make your own chip!

Introduction

The Real Time Motor Controller (RTMC) is designed to control a stepper motor such as the SEQ_28BYJ_48. The step_table is programmed with the coil positions for the motor, and each step advances the table_idx to the next coil position. The size of the step is programmable via step_size. The step_size may be positive or negative. The example motor supports step_size in [-2, -1, 1, 2]. The table_last set the limit for the table_idx before resetting to 0. It would be set to 7 for the example motor if abs(step_size) == 1 or 6 if abs(step_size) == 2.

The step control increments the table_idx + step_size. The run control continously increments table_idx, with a pause of step_delay cycles between increments.

SPI Peripheral Protocol

Control of the RTMC is accomplished via its SPI peripheral interface and simple read/write protocol.

SPI must be byte-oriented and send most significant bit first.

SPI mode must be CPOL=0, CPHA=0.

1-byte OPCODE: NOP=0, RD=1, WR=2.

1-byte ADDRESS: Select the register

2-byte DATA: Transfer in two bytes, MSB first.

1-byte RESULT: BUSY=0, ACK=1, ACK_DATA=2

The SPI peripheral should operate at up to 1/2 the core clock frequency. My target is 50 MHz core and 25 MHz SCK.

Write Operation

Tx: WR, ADDR, DATA0, DATA1

Rx: Loop reading 1-byte RESULT until ACK seen.

Read Operation

Tx: RD, ADDR

Rx: Loop reading 1-byte RESULT until ACK_DATA seen.

Rx: DATA0, DATA1

Memory Map

16-bit Register	Offset	Access	Description
id	0x00	R	ID: {version, idcode}
gpio	0x01	RW	GPIO: {mc_oe[7:0], gpo[3:0], gpi[3:0]}
step_ctrl	0x02	RW	Step Control: {run, step, reserved[4:0], table_last[3:0], step_size[4:0]}
step_stat	0x03	R	Step Status: {reserved[7"0], state[3:0], table_idx[3:0]}
step_delay0	0x04	RW	Step Delay: Most significant 16 bits, unsigned.
step_delay1	0x05	RW	Step Delay: Least significant 16 bits, unsigned.
step_count0	0x06	R, WC	Step Count: Most significant 16 bits, signed. WC = write-to-clear
step_count1	0x07	R, WC	Step Count: Least significant 16 bits, signed.
delay_count0	0x08	R, WC	Delay Count: Most significant 16 bits, unsigned.
delay_count1	0x09	R, WC	Delay Count: Least significant 16 bits, unsigned.
step_table[0]	0x10	RW	Motor State 0: 8-bits mapped to uio[7:0].
...			...
step_table[15]	0x1F	RW	Motor State 15: 8-bits mapped to uio[7:0].

Pinout

Inputs

ui[0]: General Purpose Input gpi[0]

ui[1]: General Purpose Input gpi[1]

ui[2]: General Purpose Input gpi[2]

ui[3]: General Purpose Input gpi[3]

ui[4]: SPI0.cs

ui[5]: SPI0.sck

ui[6]: SPI0.tx

ui[7]: Connected to uo[6]

Outputs

uo[0]: General Purpose Output gpo[0]

uo[1]: General Purpose Output gpo[1]

uo[2]: General Purpose Output gpo[2]

uo[3]: General Purpose Output gpo[3]

uo[4]: Connected to ^uio_in

uo[5]: Connected to ui[7]

uo[6]: Connected to ena

uo[7]: SPI0.rx

Bidirectional pins

uio[0]: Motor Control mc[0]

uio[1]: Motor Control mc[1]

uio[2]: Motor Control mc[2]

uio[3]: Motor Control mc[3]

uio[4]: Motor Control mc[4]

uio[5]: Motor Control mc[5]

uio[6]: Motor Control mc[6]

uio[7]: Motor Control mc[7]

How to test

Connect up the external hardware, program the registers, and write a 1 to the run bit. MicroPython code in the works, but not until the chips come back.

External hardware

Assuming use of the TT Demo board.

Utilize the RP2040 SPI0 in controller mode to communicate with the RTMC.

Connect uio[3:0] to a SEQ_28BYJ_48 motor + ULN2003 Driver.

uio[7:4] could optionally be connected to a second motor driven in tandem.

324 Real Time Motor Controller