PIC16C621A-20/SO - Allegro MicroSystems, Inc.

Data Sheet

26113A*

8-BIT

MICROCONTROLLER

The Microchip Technology PIC16C621A-20/P and PIC16C621A-

20/SO 8-bit microcontrollers can make designing with step motors easy,

inexpensive, and productive. A reference design technique is integral to

the implementation of a system that includes the power circuitry, a low-

cost, 8-bit, preprogrammed (with proprietary source code) microcontrol-

ler and the other components needed to complete the control hardware.

These programmed PIC16C621As (provided only as part of a chip set

with the A3955 or A3957 or as part of a demonstration board) eliminate

the need for software development, expedite the product creation, and

hasten the time to market.

The reference design can be utilized directly or integrated into a

larger printed wiring board. A further benefit is the compactness of the

circuit layout. Power-driver output ratings presently available with these

devices are 50 V and ±1.5 A (with the A3955 or A3957). A similar

device for 46 V and either 1.5 A (with the SLA7042M) or 3 A (with the

SLA7044M) is planned. The reference design supports stepping formats

that include full-step, half-step, quarter-step, eighth-step, and sixteenth-

step (microstepping) increments for a two-phase stepping motor.

Microchip’s PIC16C621A-20/P is furnished in an 18-pin dual in-line

plastic package for through-hole applications. The PIC16C621A-20/SO

is furnished in a 20-lead wide-body, shrink-pitch, small-outline plastic

package (SSOP) with gull-wing leads for minimum area, surface-mount

applications.

FEATURES

■Full-, Half-, Quarter-, Eighth-, or Sixteenth-Step Increments

■DC to 20 MHz Clock Input

■Power-On Reset

■Brown-Out Reset

■High-Speed CMOS Technology

■Low Power, <20 mA @ 5 V, 20 MHz (Typically 9 mA)

PIC16C621A

ABSOLUTE MAXIMUM RATINGS

Supply Voltage, VDD .......................... 7.0 V

Input Voltage Range,

VI........................ -0.3 V to VDD + 0.6 V

RESET Voltage, VRESET ...................... 14 V

Input Clamp Current, IIK ............... ±20 mA

Output Clamp Current, IOK ............ ±20 mA

Operating Temperature Range,

TA.................................... 0°C to +70°C

Storage Temperature Range,

TS.............................. -55°C to +150°C

Caution: These CMOS devices have input

static protection (Class 3) but are still

susceptible to damage if exposed to extremely

high static electrical charges.

910

. PP-071A

MODE SELECT

DIRECTION

CONTROL IN

OSC/CLOCK IN

OSC/CLOCK OUT

SUPPLYV

PFD

PHASE

PFD

PHASE

MODE SELECT

STEP IN

RESET

GROUND

MONITOR OUT

SHIFT CLOCK

SERIAL DATA OUT

STROBE OUT

PIC16C621A-20/P

(DIP)

These devices are manufactured by Microchip Technology,

Inc. for Allegro MicroSystems, Inc. They are only intended

for use as application-development tools in combination with

the A3955 or A3957 microstepping motor drivers. Quanti-

ties are limited. A software license for production quantities

is available.

PIC16C621A

8-BIT

MICROCONTROLLER

PIC16C621A-20/SO

(SSOP)

0.65 mm (0.026”) pitch

RECOMMENDED OPERATING CONDITIONS

over operating temperature range

Logic Supply Voltage Range, VDD ............... 4.5 V to 5.5 V

High-Level Input Voltage, VIH ............................ ≥ 0.85VDD

Low-level input voltage, VIL ................................. ≤0.15VDD

. PP-071-1

PFD

PHASE

PFD

PHASE

MONITOR OUT

SHIFT CLOCK

SERIAL DATA OUT

STROBE OUT

MODE SELECT

DIRECTION

CONTROL IN

OSC/CLOCK IN

OSC/CLOCK OUT

SUPPLYV

GROUND

MODE SELECT

STEP IN

RESET

SUPPLYV

GROUND

156

To ease and simplify the design effort, the user only

provides the following signals: (a) direction, (b) stepping

clock (8x the full-step frequency), (c) mode logic (three

inputs determine the operation for full, half, quarter, or

eighth stepping), (d) reset input (initiates a ‘detent’

position), and (e) recirculation control (this allows estab-

lishing the percent of fast- vs slow-decay in the phase

winding). The microcontroller program providess auto-

matic recirculation control. This eliminates the need for

evaluating the impact of stepping rate vs the sinusoidal

current profile.

Although recirculation control can provide slight

improvements (i.e., lower current ripple, reduced motor

heating [a few degrees], and diminish audible noise levels

[minimal differences]), this entails an evaluation of the

motor (and step frequencies) to determine the proper ratio

of fast- and slow-decay. The benefits of tuning the

recirculation ratios are small, and the time and effort

required can be considerable. Hence, the uninitiated user

should opt for the automatic recirculation control, and

avoid the essentially unnecessary activity.

FUNCTIONAL DESCRIPTION

MICROCONTROLLER OPERATION

Although ‘hardware’ control of the microstepping ICs

is feasible, without a specific (ASIC), monolithic IC

controller the prime solution becomes a ‘software’ option.

From the user’s perspective, a ‘preprogrammed’ micro-

controller appears little, or no, different than a ‘dedicated’

controller and sequencer IC expressly created for mi-

crostepping applications of the power-driver ICs. Further,

the flexibility of a software-based drive is certainly a basic

benefit (high-volume production of 8-bit microcontrollers

transposes to low-cost circuitry).

As an indicator of the logic signals needed to control

the power ICs, Table 1 lists the required A3955 inputs to

the 3-bit DAC for eighth-step operation (the similar

A3957 uses a 4-bit DAC for sixteenth-step operation).

These I/O signals are serial data from the microcontroller,

then converted to a parallel mode by a 74HC595 as the

‘interface’ between the microcontroller and the two

microstepping power ICs.

PIC16C621A

8-BIT

MICROCONTROLLER

The versatility offered by software control allows the

operating modes listed in Table 1. This table itemizes the

various logic inputs that determine direction, stepping

format, reset, 1/8th vs 1/16th sub-steps, etc. Note that

during power up, shift clock (SCLK) is sampled for a pull-

up or pull-down resistor to establish the fractional step

limit. A pull up sets up a 1/8th-step format (for the

A3955) and pull down sets up 1/16th-operation (for the

A3957).

Table 2 lists the microcontroller terminal descriptions

and provides the essence of the circuit operation (a

schematic illustrating a typical stepper design follows). A

brief description of the microcontroller I/O should clarify

the connections of the various elements of the drive

electronics.

FUNCTIONAL DESCRIPTION (cont’d)

Table 1 — Controller/sequencer IC operational logic

Binary inputs Operating mode Comments

DIR MS2 MS1 MS0 (Command executed on L → H of CLK) (Applicable power ICs)

0000 CW, Full step (single-phase) A3955/57

0001 CW, Half step (constant torque) A3955/57

0010 CW, 1/4 step (constant torque) A3955/57

0011 CW, 1/8th step (constant torque) A3955/57

0100 CW, 1/16th step (constant torque) A3957 only

0101 Disable A3955/57 holding torque At present position

0110 Enable A3955/57 holding torque From present position

0111 Reset PIC16C621A sequencer IC A3955/57

1000 CCW, full step (single-phase) A3955/57

1001 CCW, half step (constant torque) A3955/57

1010 CCW, 1/4 step (constant torque) A3955/57

1011 CCW, 1/8th step (constant torque) A3955/57

1100 CCW, 1/16th step (constant torque) A3957 only

1101 Disable A3955/57 holding torque At present position

1110 Enable A3955/57 holding torque From present position

1111 Reset PIC16C621A sequencer IC A3955/57

PIC16C621A

8-BIT

MICROCONTROLLER

Table 2 — Microcontroller terminal descriptions

DIP SSOP Function Description Comments, connections, etc.

1 1 Input Mode select 1 Static and/or dynamic control of stepping mode

2 2 Input Mode select 2 Static and/or dynamic control of stepping mode

3 3 Input Step in Governs full-step rate (÷8 for A3955; ÷16 for A3957)

4 4 Input Reset Resets DIR., MS2, MS1, and MS0 to 0000 (detent)

5 5, 6 Power Ground Logic power return

6 7 Output Monitor out Signals full-step rotor alignment (active low)

7 8 I/O Shift clock Pull up for A3955; pull down for A3957

8 9 Output Serial data out Shifts 8-bit serial data to 74HC595 serial input

9 10 Output Strobe out Latches 8-bit data into 74HC595 (latch clock in)

10 11 Output Phase B Controls current direction in phase B

11 12 Output PFD B Phase B recirculation control

12 13 Output Phase A Controls current direction in phase A

13 14 Output PFD A Phase A recirculation control

14 15, 16 Power Supply (VDD) Recommended range: 4.5 V to 5.5 V

15 17 Output Osc/clock out Crystal oscilator connection

16 18 Input Osc/clock in Crystal oscillator connection/external clock input

17 19 Input Direction control Determines direction of step motor rotation

18 20 Input Mode select 0 Static and/or dynamic control of stepping mode

FUNCTIONAL DESCRIPTION (cont’d)

Mode-select inputs

These three inputs (MS2, MS1, and MS0) determine

the stepping format, disable/enable motor power, and reset

the controller/sequencer. In conjunction with the direction

input, the mode inputs control the sixteen operating states

listed. Deactivating stepper power in any position except

‘detent’ (i.e., a single phase activated) results in the motor

rotor advancing, or retracting, from its intermediate

position and alignment with a natural (i.e., minimum-

reluctance flux field) orientation. The absolute position

may be affected by inertia, load, fractional position,

ringing, etc. and cannot be determined without feedback.

Phase currents must be maintained to immobilize the

rotor/load in any intermediate position.

Step (clock) input

The sequencer stepping-clock frequency is a multiple

of the actual stepping rate. The A3955 requires a step-

ping-clock input frequency equal to eight times the

intended stepping rate for the motor; the A3957 requires a

stepping clock rate that is sixteen times the actual stepping

rate of the motor. However, neither design necessitates

that the step frequency be varied should the operating

mode(s) be switched during operation. Shifting from one

stepping format does not mandate a simultaneous (and

equivalent) change in the clock frequency.

Using a 20 MHz crystal (maximum limit for the

PIC16C621A) allows a 50 kHz stepping clock for the

A3955, and this equates to 6 250 full steps per second (50

kHz/8). For the A3957 this 50 kHz stepping clock trans-

PIC16C621A

8-BIT

MICROCONTROLLER

lates to 3 125 full steps per second (50 kHz/16). These

frequencies represent the attainable limits with the

PIC16C621A.

Although not a necessity, a stepping clock with a 50%

duty cycle represents the simplest technique for providing

an appropriate (≤50 kHz) stepping clock rate. The step

clock varies depending upon the start, acceleration,

slewing, deceleration, and stop trajectories mandated by

the motion system control and ‘point-to-point’ timing

objectives.

Reset input

The preprogrammed microcontroller incorporates two

‘software’ reset states that are serially loaded with MS2,

MS1, and MS0 all high. However, the direct hardware

reset is actuated with a logic 0 (active low) on this input

An input low level defaults to the 0000 binary state and

sets the rotor to its natural (or detent) position with one-

phase energized.

Monitor output

An output low signal indicates a rotor alignment

corresponding to a single phase on position. Any changes

in the operating mode (microstepping to full-step, etc.)

should coincide with the interval that the monitor output is

in the low state. This alleviates noise problems, excessive

ringing, etc. that may result from changing the stepping

modes on-the-fly. Nonlinear (such as S-curve) accelera-

tion profiles can exploit this signal to achieve very

smooth, quiet stepper operation.

Shift clock output

This I/O terminal serves a dual purpose. On power

up, the microcontroller samples this terminal as an input.

Connecting a pull-up resistor results in 1/8th-step format;

while a pull-down resistor configures the controller for its

1/16th-step mode (A3957 only). This provides versatility,

simplicity, and cost-effectiveness for most users.

Operating in its output mode, this I/O constitutes the

shift clock signal for the 74HC595. Data is transferred

from the microcontroller serial-data output to the serial-

data input of the 74HC595. This 8-bit serial format is

converted into parallel signals controlling the 3-bit (or 4-

bit) DAC input lines to the two microstepping power ICs.

Serial data from the serial data output is valid on the low-

to-high clock transitions and eight clock pulses shift serial

control signals into the 74HC595. A basic timing diagram

(showing serial data, shift clock, and strobe) is depicted.

Signal timing is controlled by the preprogrammed micro-

controller; data entered into the 74HC595 shift register is

then latched by the low-to-high transition of the strobe

input.

SCLK

Dwg. WP-040A

SDO D7 D6 D5 D4 D3 D2 D1 D0

Serial data, shift clock, and strobe

Serial data output

The binary signal instructions that control each of the

microstepping power ICs is shown in table 1. The first

3-bits (or 4-bits) control the digital-to-analog conversion

in one power IC, while the next 3 (or 4-bits) ratio the

second power driver current. The microcontroller moni-

tors all the various static inputs (i.e., Direction, Mode

Selects, Reset), and by exploiting the Stepping Clock for

its input frequency, transfers the 8-bit data commands to

the power driver ICs via the serial-to-parallel interface IC.

The microcontroller utilizes look-up tables to provide

overall control of direction, stepping format, and recircu-

lation mode (PFD). The microcontroller reads inputs and

then outputs time-based signals to control both microstep-

ping ICs.

Strobe output

After the 8-bit serial data has been loaded into the

shift register, a low-to-high transition on the strobe output

transfers the serial data from the shift register into the

eight ‘D’ flip-flops that compose the parallel-data outputs.

This ‘latched’ data controls microstepping current ratios

for both power ICs, and is ‘updated’ after eight step

clocks.

FUNCTIONAL DESCRIPTION (cont’d)

PIC16C621A

8-BIT

MICROCONTROLLER

Direction control outputs

Two of the microcontroller outputs are devoted to

direction control. The direction signals follow the se-

quencing to provide the 1/8th-step ‘sinusoidal’ wave-

forms. Direction outputs are parallel, and connected

directly to the power IC inputs (A3955 in the schematic).

Obviously, both direction signals must coincide with the

8-bit serial-to-parallel conversion signals that control the

phase current ratios (D2, D1, and D0).

PFD control outputs

Another pair of microcontroller outputs constitute

parallel, direct control of the recirculation paths during

each PWM current cycle. The percentage of fast-decay

(or four-quadrant) to slow-decay (or two-quadrant)

recirculation determines how effectively the output current

tracks a sinusoidal waveform. Mixed-mode operation was

utilized for those portions of each microstepping cycle in

which the current is decreasing. The application of

mixed-mode decay allows the PWM current to properly

decay during each fixed-off-time interval.

The typical application shown includes a pair of

49.9 kΩ variables and allows setting the ratios of fast- and

slow-decay to match the individual characteristics of the

step motor, step frequency, supply voltage, etc. As the

stepping frequency is increased, waveform anomalies

become more pronounced, and the benefit of mixed mode

(or fast decay) also becomes very noticeable.

A comprehensive discussion of mixed-mode operation

is included in Allegro Technical Paper STP 97-5.

Oscillator input/clock input

This device connection serves as one terminal for a

parallel-cut crystal (oscillator 2 is the other). In the

typical application, two 20 pF capacitors are recommend-

ed, and a series resistor from oscillator out may be re-

quired if AT-cut crystals are utilized.

Also, this terminal is used if an external clock (from a

system computer, etc.) is connected here in lieu of a

crystal or ceramic resonator. If so, the oscillator out

connection is left open. The 20 MHz frequency limit

applies to using any clock source.

Oscillator output/clock output

This is the other device connection used for the

internal crystal oscillator (or ceramic resonator) that

supplies the microcontroller with an oscillator/time base

(≤20 MHz).

Direction input

This input controls the sequencing of the winding

currents. Reversing direction may necessitate deceleration

(perhaps full stop) to avoid problems associated with the

load inertia causing the motor to overshoot the proposed

stop/reverse position.

A more extensive discussion of this microcontoller

and microstepping is included in Allegro Technical Paper

STP 99-11.

CONTROL OF PHASE CURRENTS

The power-driver ICs regulate the winding currents,

and the user establishes the design value via a stable

voltage reference and a current-sensing resistor for each

winding (R8 and R19).

FUNCTIONAL DESCRIPTION (cont’d)

PIC16C621A

8-BIT

MICROCONTROLLER

Low-cost microstepping drive circuitry

This “typical” application is available as a product evaluation kit (APEK3955SLB-02)

VCC

REF

PFD

PHASE

GND

VBB

OUTA

OUTB

SENSE

GND

A3955

STEP

MS2

MS1

MS0

DIR

MON

SCLK

SDO

VCC

SCLR

RCK

SER

SCK

GND

Q'H

OSC IN

OSC OUT

PFDA

PHASEA

VDD

GND

PFDB

PHASEB

CLK

MS2

MS1

MS0

DIR

MON

A3955

A3957

PIC16C621A

74HC595

VCC

REF

PFD

PHASE

GND

VBB

OUTA

OUTB

SENSE

GND

A3955

D3B A3957

D3A A3957

SDO

+5 V

10 kΩ50 kΩ

33 kΩ

22 pF

10 kΩ (5)

10 kΩ

4.99 kΩ

10 kΩ

470 pF

39 kΩ

0.56 Ω

100 kΩ

0.1 µF

470 pF

39 kΩ

20 MHz

+5 V

10 kΩ

50 kΩ

33 kΩ

47 µF

LOAD

SUPPLY

LOAD

SUPPLY

Dwg. EP-068A

STEP MOTOR

PHASE A

STEP MOTOR

PHASE B

47 µF

!N4934 (4)

PIC16C621A

8-BIT

MICROCONTROLLER

The products described here are manufactured by Microchip

Technology, Inc. for Allegro MicroSystem, Inc.

Microchip Technology, Inc. reserves the right to make, from time to

time, such departures from the detail specifications as may be

required to permit improvements in the performance, reliability, or

manufacturability of their products. Therefore, the user is cautioned

to verify that the information in this publication is current before

placing any order.

These products are not authorized for use as critical components in

life-support appliances, devices, or systems without express written

approval.

The information included herein is believed to be accurate and

reliable. However, Microchip Technology, Inc. and Allegro

MicroSystem, Inc. assume no responsibility for its use; nor for any

infringements of patents or other rights of third parties which may

result from its use.