Source: https://mokxi.com/learn/a4988-pinout Learn # A4988 pinout: every pin, the microstep table and the current limit Written by the Mokxi team Start building free Or see every lesson. - 455 parts on the bench - 43 boards running now - 1.00x real time, on every board Press Run, then click a mode switch to change the step resolution MS1, MS2 and MS3 are on 5 V, so the A4988 is at 1/16 step and 3200 pulses turn the NEMA 17 once. The A4988 carrier is a small board with 16 pins in two rows of eight, a chip under a stick-on heat sink and a trimmer for the current. The left row is logic: EN, MS1, MS2, MS3, RST, SLP, STEP and DIR. The right row is power: VMOT, GND, 2B, 2A, 1A, 1B, VDD and GND. That is the order Pololu’s carrier has them in, and the order Mokxi’s part uses. The circuit above is an Uno sending 3200 pulses to an A4988 and a NEMA 17, then 3200 back. The three switches are MS1, MS2 and MS3, all on 5 V to start, which is 1/16 step: the board shows "1/16" and the shaft turns once each way. Click a switch while it runs and the board shows the new resolution. ## The 16 pins The two GND pins are one ground plane on the board, so either will do, and Mokxi treats them the same way. Pololu asks for an electrolytic capacitor of at least 47 µF across VMOT and GND close to the board, because the motor’s wiring throws voltage spikes the chip is not rated for; Mokxi does not model those spikes, so the circuit runs without one, and a bench board should not. Pololu also warns that plugging or unplugging the motor while the driver is powered can destroy it. A4988 carrier pins, top to bottom, and what each does when nothing is wired to it Pin | Row | What it does | Left unconnected EN | Logic | Enable, active low. HIGH lets the outputs go; the step count carries on. | Pulled low by 100 k: outputs on MS1 | Logic | Microstep select | Pulled low by 100 k MS2 | Logic | Microstep select | Pulled low by 50 k MS3 | Logic | Microstep select | Pulled low by 50 k RST | Logic | Reset, active low. Holds the outputs off and the position at home. | Floats, which is a reset: link it to SLP SLP | Logic | Sleep, active low. STEP is ignored for 1 ms after it goes high. | Pulled up by 10 k: awake STEP | Logic | One step on each rising edge | Floats: drive it DIR | Logic | Which way each step goes | Floats: drive it VMOT | Power | Motor supply, 8 to 35 V | Outputs off GND | Power | Motor ground, one plane with the other GND | Use either GND 2B | Power | Coil 2 | Coil 2 open 2A | Power | Coil 2 | Coil 2 open 1A | Power | Coil 1 | Coil 1 open 1B | Power | Coil 1 | Coil 1 open VDD | Power | Logic supply, 3 to 5.5 V; sets the input thresholds | Chip off GND | Power | Logic ground | Use either GND ## MS1, MS2 and MS3: the microstep table Each mode pin is pulled low inside the chip, so a board with nothing on them runs in full step. Every row of the table below was set on Mokxi’s A4988 and read back off the board. The combinations the datasheet leaves out (MS3 high without both of the others) are read as full step: in the circuit above, MS1 switched to ground with the other two still high shows "full". In full step the chip holds each coil at 70.7 percent of the limit, the four points where both coils carry the same current, and a driver switched into full step from a microstep goes to the nearest of them. A4988 step resolution, and pulses for one turn of a 200-step motor MS1 | MS2 | MS3 | Resolution | Steps a turn LOW | LOW | LOW | Full step | 200 HIGH | LOW | LOW | Half step | 400 LOW | HIGH | LOW | Quarter step | 800 HIGH | HIGH | LOW | Eighth step | 1600 HIGH | HIGH | HIGH | Sixteenth step | 3200 ## Vref and the current limit formula The current limit is I = VREF / (8 × RS), where RS is the current sense resistor beside the chip and VREF is the voltage on the trimmer, measured to GND. Pololu’s carriers made since 2017 use 0.068 ohm resistors and earlier ones 0.05 ohm; Mokxi’s default is 0.1 ohm, which many clones use. Read the code on yours: R100 is 0.1 ohm, R068 is 0.068 and R050 is 0.05. To set a limit, work backward: VREF = 8 × RS × I. For the 1.7 A rating of Mokxi’s NEMA 17 on a 0.1 ohm board that is 1.36 V, and most people set a little under the rating. Mokxi gives the coils the currents the formula says: with VREF at 0.8 V on a 0.1 ohm board each coil carried 0.707 A at rest, one half-step put the whole 1.000 A in one coil and none in the other, and at 0.4 V each coil carried 0.354 A. Click the A4988 in the circuit and change vref and rsense: the board prints the limit it works out. The A4988 current limit for each common sense resistor Sense resistor | VREF for a 1 A limit | Limit at VREF 0.8 V 0.05 ohm | 0.40 V | 2.00 A 0.068 ohm | 0.54 V | 1.47 A 0.1 ohm | 0.80 V | 1.00 A ## Wiring an A4988 to an Arduino Pins 3 and 4 are the ones the circuit above uses; any two digital pins do. The sketch for this wiring is on the A4988 and NEMA 17 tutorial, and AccelStepper takes the same pins as AccelStepper stepper(AccelStepper::DRIVER, 3, 4). An A4988 on an Arduino Uno A4988 pin | To | Why STEP | Pin 3 | One step per rising edge DIR | Pin 4 | Which way RST | SLP, on the board | Or the driver never wakes MS1, MS2, MS3 | 5 V, GND or nothing | The resolution, from the table VDD | 5 V | Logic supply GND (logic) | GND | Shared with the Uno VMOT | The motor supply’s +, 12 V here | 8 to 35 V GND (motor) | The motor supply’s − | 1A, 1B | One coil | A pair, found with a meter 2A, 2B | The other coil | ## Logic levels, and wiring it to an ESP32 VDD is the logic supply, 3 to 5.5 V, and it also sets what counts as high: the inputs switch at 0.3 and 0.7 of VDD. That matters on a 3.3 V board. With VDD on 5 V a high has to reach 3.5 V, which an ESP32’s pins never do, and on an ESP32-C3 the shaft did not move. With VDD on the ESP32’s own 3.3 V pin it turned. In Mokxi a VDD of 2.9 V left the chip off and 3.0 V ran it. The DRV8825 has the same footprint with no VDD pin, and reads 2.2 V as high whatever its supply, so it needs no change for a 3.3 V board; the DRV8825 guide has its own table and formula. ## What Mokxi’s A4988 leaves out The chopper as a circuit, with its decay modes and its whine at low speed; coil inductance; the 2 A rating, heat and thermal shutdown; and the 1 microsecond minimum STEP pulse, which no Arduino sketch gets near. The pull resistors are Pololu’s layout, and clones vary. ## Questions What do MS1, MS2 and MS3 do on an A4988? They set the step resolution: all low is full step, MS1 alone is half, MS2 alone a quarter, MS1 and MS2 an eighth, and all three high a sixteenth. They are pulled low, so unconnected pins mean full step. What is the A4988 Vref formula? I = VREF / (8 × RS). On a 0.1 ohm board that is 1.25 A for every volt of VREF, so 0.8 V is a 1 A limit; on Pololu’s 0.068 ohm boards it is 1.47 A at 0.8 V. Why does the A4988 need RST connected to SLP? RST floats on the carrier and a floating reset holds the driver off. SLP is pulled up on the board, so linking the two keeps RST high and the driver running. Can an A4988 run from 3.3 V logic? Yes, if VDD is on 3.3 V too. The inputs switch at 0.7 of VDD, so with VDD on 5 V a 3.3 V pin never reads as high. What voltage does the A4988’s VMOT need? From 8 to 35 V. In Mokxi the outputs stay off at 7.9 V and come on at 8 V. Related ## Keep going Arduino A4988 and NEMA 17 Stepper: Wiring and Code Arduino DRV8825: Wiring, Code and Microstepping Stepper Motor Vibrating but Not Turning: Fixes AccelStepper Arduino Library: Examples and Homing Stepper Motor Wiring: 4, 5 and 6 Wire Motors Arduino Uno Pinout: ATmega328P Pins, PWM, I2C, SPI How the A4988 and DRV8825 are modeled The NEMA 17 stepper motor The stepper motor simulator Sources ## Where the facts on this page come from - Pololu A4988 carrier: the pins, the sense resistors since 2017, the current limit and the capacitor - Allegro A4988 datasheet: the microstep table, the input thresholds and the pull-downs ## Build this for real Open the editor, change a value and watch the number move with it. Nothing to install, and no account needed. Start building free Open the editor