Advanced Robotics with the Toddler (Paralax, student guide, v1.3, 2004)

Appendix B: Toddler PCB Schematic · Page 209

Appendix B: Toddler PCB Schematic

Appendix C: Blue Bell Designs Co-Processor Chip · Page 211

Appendix C: Blue Bell Designs Co-Processor

INTRODUCING BLUE BELL DESIGN

Parallax customers make many accessories for our robot and some of them are for sale. This appendix is an example of a co-processor from Blue Bell Design (www.bluebelldesign.com). At press time for this text the Blue Bell Design product is not available through Parallax, but may be in the future..

Chapters 8 and 9 discussed Real Time Programming and some of the difficulties you can get into when trying to keep up with some task that requires servo refreshing at a relatively high rate. The Toddler challenge is keeping up with refreshing both the servos every 20 ms.

How does it help?

A Co-Processor is an additional processor added to the main processor, which, in Toddler’s case, is the BASIC Stamp. Various manufacturers put different features in their Co-Processors. In Chapter #9, this text discussed the benefits of a ramping-type servo controller. That type of controller would perform the Do_Movement subroutine with little help from the main processor. The Blue Bell Design Co-Processor has ramping servo controllers for up to 8 servos, plus five analog to digital converter inputs (10-bit), and extra inputs and outputs. Best of all, it is even available with a PC board to easily attach to your Toddler.

Where do the interface pins come from?

Obviously, if the BASIC Stamp is to talk to this Co-Processor, it will require some I/O pins from the BASIC Stamp. In this case it’s four. If you were counting the earlier programs, you’ll see that the program in Chapter #8 had only a few spare pins left. Chapter #9 had even fewer left. Evaluating the pin usage carefully you can see the display LEDs can go. This isn’t an altogether great solution but would work in a pinch. The pin-out of the PC board also requires moving some functions to different I/O ports but that is even less of an issue thanks to the flexibility of the new PIN type of variable in PBASIC 2.5. It turns out, however that the unused servo controller outputs from the CoProcessor can also be used as regular digital outputs. Since we only have two servos on Toddler, we can use servo channels 4->7 for the indicator LEDs and still have channels 2 and 3 as spare servo controllers. Maybe for a tilt and pan camera mount? Now we are

Page 212 · Advanced Robotics with the Toddler

actually ahead on the pins as we used 4 Toddler I/O to get 6 Co-Processor I/O. The two Toddler freed up pins can again be used for a Memsic 2125 Accelerometer.

How do I drive the Servo Controller?

The four interface lines to the Co-Processor are reset, timer complete, serial data to the Co-Processor and serial data from it. You’ll need to do a co-processor reset in the initialization routine. Just pull the reset line low for a few milliseconds to do that. To tell a servo controller to go to a certain position, you send two bytes. The first byte tells the co-processor that you want a certain channel to move to a new place. The second byte tells where you want it to go. Remember we used a WORD variable for PULSOUT to drive the servos before. Here we only need a byte because, rather than being general case like PULSOUT, the servo controller byte defines how many steps of 4 us are added to a base value of 994 us. In other words, a 0 sent to the servo controller gives 0.994 ms. Send a 255 and get 2.014 ms. That is almost exactly the 1 to 2 ms range specified for most servos. If you need wider pulses for special applications, there is a “coarse” mode that spreads the range somewhat. Sending a total of four bytes then tells the controller where both the servos need to be and it will continue to refresh them every 20 ms with no further help from the toddler main processor.

As we discussed in Chapter #9, you really don’t want the servo to move to the final position at full speed. That is why we had TiltStep and StrideStep in the Do_Movement subroutine. To get the servo controllers to ramp to position, just send two bytes per servo channel. The first tells the controller to store the ramp value and enable ramping. The second value contains the ramp rate for that channel. Unlike TiltStep and StrideStep, the servo controller adds only a quarter of the value sent for each 20 milliseconds cycle (it keeps track of the fractions internally). This allows for up to a 20 second full range transition. The maximum ramped speed is 31 counts, which takes 0.66 seconds for the transition. Setting the ramp value back to zero disables ramping for that channel. Ramp rates stay in effect until rewritten or the co-processor is reset. They are also not dependent on the loop speed in the BS2 as they are incremented once per 20 ms not once each time through the control loop. Since we have only been using a single rate for tilt and another one for stride, we can write the ramp values at initialization time and not worry about it anymore. In the following code we used TiltStep*4 to keep the steps sizes like the previous programs.

How do I know when the servo is finished moving?

One benefit of calculating in the BASIC Stamp is that you constantly know where the servo is at any given time. The disadvantage is, of course, that it takes a lot of power from the processor! Since the servo controller in the co-processor is now doing this for you, you’ll have to figure out how long it will take to move the servo to the new position yourself. This is easily done by finding the difference from where you are to where you are going and dividing by the step rates. Now you have almost nothing to do but wait until the servo controller is finished with the move. If you use a PAUSE instruction, the processor stops while it is waiting. That isn’t much help! You could also read the current servo position out of the servo controller but there is a better way. Here is where a unique feature of the Blue Bell Design Co-Processor comes into play. There are 8 timer channels that keep time in increments of 20 ms. You can set a timer value and periodically check to see if any are finished by looking at the single Timer Expired pin. That pin goes high to signify a timer just completed. It is reset by reading a timer status byte that tells which timer(s) finished. Here we’ll use timer 0.

SEROUT To_Co_Proc, Baud, [(128),DelayTime] ' write Timer 0 with delay value

To test the timers and reset the flag:

Now let’s see the whole code.

Page 214 · Advanced Robotics with the Toddler

'-----[ Title ]--------------------------------------------------------------

'Toddler CoP Program A.1 FSM Walker.bs2

'This program has real time servo programming and sensor integration.

'Toddler now wanders in a straight line until it sees or hits something.

'by H. W. Lewis (www.bluebelldesign.com)

'{$STAMP BS2}

'{$PBASIC 2.5}

'-----[ I/O Definitions ]----------------------------------------------------

'each time through - NOT REALLY NEEDED

'CoProc V1.1 pin 21 pulled up = 2400 Baud

'CoProc V1.1 pin 21 grounded = 9600 Baud

'Co-Proc servo channels 4,5,6 and 7 are used

'for LED drive - gives back lots to I/O pins

'to Stamp

'servo 4 = left bumper, 5 = rt bumper, 6 = Left vision, 7 = Right vision

'For Bumper LEDs, low = on. For vision LEDs, High = on.

'This sounds weird but it allows easy wiring on the PCB.

'uses servo channel 6 output for LED drive

'executing vision sequence for left side

'uses servo channel 7 output for LED drive

'executing vision sequence for right side

'uses servo channel 4 output for LED drive

'executing bumper sequence for left side

'uses servo channel 5 output for LED drive

Page 216 · Advanced Robotics with the Toddler

Appendix C: Blue Bell Designs Co-Processor Chip · Page 217

'-----[ Basic Movement Tables ]----------------------------------------------

'These tables contain Basic Movements consisting of bytes containing

'above Basic Movement Codes to describe movement sequences.

'An xx indicates the end of a list.

'Note that PivotLeft and PivotRight aren't necessarily entered at the start

'of their tables

'by keeping MxOffset value from Forward.

'by keeping MxOffset value from Forward.

'bumper hit assumes SL when hit happened.

'-----[ Initialize ]---------------------------------------------------------

SEROUT To_Co_Proc, Baud, [TiltRamp,(TiltStep*4)]

'write rate to servo 0

SEROUT To_Co_Proc, Baud, [StrideRamp,(StrideStep*4)]

'write rate to servo 1

Page 218 · Advanced Robotics with the Toddler

SEROUT To_Co_Proc, Baud, [LeftBumpLEDOff]

' turn off LED

SEROUT To_Co_Proc, Baud, [RightBumpLEDOff]

' turn off LED

ResetFeet:

CurrentTilt = CenterTilt

CurrentStride = CenterStride

SEROUT To_Co_Proc, Baud, [TiltServo,CurrentTilt]

'Set Servo 0 = Tilt

SEROUT To_Co_Proc, Baud, [StrideServo,CurrentStride]

'Set Servo 1 = Stride

'DEBUG "resetCC", CR, lf

'DEBUG "Forward = ", hex Forward, cr

'DEBUG "PivotLeft = ", hex PivotLeft, cr, "PivotRight = ", hex PivotRight,

cr

'DEBUG "RBumper = ", hex RBumper, cr, "LBumper = ", hex LBumper, cr

'newMove = BumpL

'newMove = BumpR

'-----[ Main Code ]----------------------------------------------------------