Introduction
This discussion deals with the PICAXE-08M as an accessory to a Basic Stamp or similar to control the speed of a motor or the intensity of a lamp using the background PWM capability of the PICAXE-08M. The interface is relatively simple, consisting of two leads, STOP and PULSE from the Stamp or similar to the PICAXE-08M.
In the following, the PICAXE code is offered which the user may easily use to program a PICAXE-08M. You can do this yourself, or I will supply them for $5.00 each.
Note that multiple slaves may be accommodated on a single Basic Stamp, each slave requiring a STOP and PULSE output from the BS2.
(Note that although a Parallax Basic Stamp is considerd in this discussion, the master processor might well be a PICAXE-18, -18A or 18X, particularly in applications where multiple PWM channels are required).
Background
The PWM commands associated with the Basic Stamp is not what one would think of as PWM, but rather a series of logic ones and zeros over a cycle of 256 slots such that the output is high an average is num / 256. For example;
1 0 0 0 0 0 0 0 ... ' 1/256 1 1 0 0 1 1 0 0 ... ' 64/256 1 0 1 0 1 0 1 0 ... ' 128/256Thus, the period changes, depending on the duty cycle and I have found the transitions so fast that I was uncertain whether the signal was propagating through an optocoupler. True PWM is high for num / 256 ths of the time, followed by low for the balance of the time. For example;
1 0 0 0 0 0 0 0 ... ' 1/256 1 1 1 1 ... 0 0 ... ' 64/256, 64 logic one states followed by 192 logic zero states. 1 1 1 1 ... 0 0 ... ' 128/256, 128 logic one states followed by 128 logic zero states.
In addition, the PWM command associated with the Stamp requires the full resources of the processor. That is, the PWM output only occurs while executing the PWM command. There is little to no time for the Stamp to do anything else and at $49.00, the Stamp is relatively expensive to dedicate to simply performing the PWM task.
Using the PCAXE-08M
The PICAXE-08M provides background true PWM capability on Pin 2 (term 5).
PWMOut 2, 255, PWMDuty4The period is 255 + 1 clock ticks or 256 us (3.9 kHz) for 4.0 MHz operation. When operated at 8 MHz, the period is 256 * 0,5 us or 128 us (7.6 kHz).
In the following program, the PWMDuty is initialized to zero. When the STOP input (Pin3) is at a logic one, the PWM output (Pin2) is low (motor or lamp is off). When the STOP input is brought low, the PWMOut command is executed using the current value of PWMDuty.
Normally, the PULSE input on Pin4 (terminal 3) is at a logic one. When brought low, the PICAXE is interrupted and the number of positive transitions of the Pulse input are counted over a 100 ms window. At the conclusion of the 100 ms window, the PICAXE waits for the PULSE input to again go high. The number of counts received during this window then becomes the new PWMDuty.
' PWMSlave.Bas (PICAXE-08M)
'
' Basic Stamp PICAXE-08M
'
'
' P7 (term 12) --------------- STOP ----- Pin3 (Term 4)
'
' P6 (term 11) --------------- PULSE ---- Pin4 (Term 3)
'
' Pin2 / PWM (Term 5) ------- To TIP41, TIP122, IRLZ44
' ULN2803, L293DNE or similar
' 10K to +5 VDC pull up resistors on STOP and PULSE inputs
'
' Stamp to PWM PICAXE-08M
'
' copyright, P H Anderson, Baltimore, MD, Nov 8, '05
Symbol PWMDuty = B0
Symbol Counts = B1
Symbol PWMDuty4 = W1 ' uses B2 & B3
'''''''''''''''''''
Poke $8f, $71 ' set OSCON register to 8 MHz
Dirs = %00000100 ' Pin2 is output
PWMDuty = 0 ' initialize to zero
Main:
SetInt %00000000, %00010000 ' interrupt on low on Pin4
If Pin3 = 1 Then StopPWM
' otherwise
PWMDuty4 = 4 * PWMDuty
PWMOut 2, 255, PWMDuty4
Main2:
Goto Main
StopPWM:
PWMOut 2, 255, 0
Goto Main2
Interrupt:
Count 4, 200, Counts ' 100 ms window when running at 8 MHz
PWMDuty = Counts
SetInt %00000000, %00010000
WaitForOne:
Pause 10
If Pin4=0 then WaitForOne
' wait for Pin4 to go back to logic 1
Return
Basic Stamp 2
The following program illustrates a test program which causes the PWM output of the PICAXE-08M to completely off, to 128/256 duty to 254/256 duty.
' Program PWM_STAMP.BS2
'
' Illustrates a Basic Stamp interface with a PICAXE-08M programmed with
' PWM firmware discussed above.
'
' The program begins with a high state on Pulse (P6). The motor is held off
' for nominally two seconds with a high on the Stop (P7).
'
' The Duty cycle is then set to 128 by bringing Pulse (P6) low and then sending 128
' pulses within a 100 ms window. This is followed by a delay to assure the 100 ms
' window has elapsed prior to returning the Pulse output to its normal high state.
'
' As the Stop output is low, the PWM Duty at term 5 of the PICAXE is now 128 / 256.
' One can view this using and LED (with series 330 Ohm resistor) from the PICAXE PWM
' output to ground. This is followed by a two second delay.
'
' The PWM Duty is then set to 254 / 256 causing a test LED to appear near full
' brilliance.
'
' This sequence of LED off, followed by 128/256 duty followed by 254/256 duty
' is continually repeated.
'
' Basic Stamp PICAXE-08M
'
'
' P7 (term 12) --------------- STOP ----- Pin3 (Term 4)
'
' P6 (term 11) --------------- PULSE ---- Pin4 (Term 3)
'
' Pin2 / PWM (Term 5) ------- To TIP41, TIP122, IRLZ44
' ULN2803, L293DNE or similar
'
' 10K to +5 VDC pull up resistors on STOP and PULSE inputs
'
'
' copyright, Peter H Anderson, Baltimore, MD, Nov, '05
N Var Byte
High 6 ' Pulse output is high when idle
Top:
High 7 ' Stop Output
Pause 2000
Low 6 ' low on Pulse to interrupt the PICAXE-08M
For N = 1 to 128
High 6 ' then send the number of pulses within 100 ms
Low 6
Next
Pause 100 ' be sure the 100 ms window has elapsed
High 6 ' done writing the PWMDuty to the PICAXE-08M
Low 7 ' low on Stop causes PWMOut at the specified duty
Pause 2000 ' pause to admire
Low 6 ' now adjust PWM Duty while motor is turning.
For N = 1 to 254
High 6 ' set to Duty of 254
Low 6
Next
Pause 100 ' be sure the 100 ms window has elapsed
High 6 ' done writing to the PICAXE
Pause 2000 ' pause to admire
Goto Top ' repeat