Nuts and Volts - July 2015

July 2015 13

SHARPENING YOUR TOOLS OF CREATIVITY

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connected to anything, so pin B. 7 can be used for any purpose you want without creating an electrical problem; third, don’t forget to install the short bare jumper that connects the RxD pin on the PL2303HX to pin C.0 on the 20M2.

The software for our first experiment this month (hserin20M2. bas) is available for downloading at the article link. Now would be a good time to download it, along with the other programs we will be using this month. If you read through the program listing for the hserin20M2. bas program, you will see that — aside from the fact we’re using a different processor — there are two minor differences from the hserinFromPC. bas program that we used in the previous column.

First, the pin assignments for the

RxPin and the TxPin have changed.

On the 20M2 processor, the hserin pin is pin B. 6, and the hserout pin is pin C.0. We aren’t using the hserout command in our first experiment, but we will in our second experiment.

Using pin C.0 for the serout command will enable us to use the same hardware setup for the next experiment, as well.

The second difference has to do with the “undocumented A.0 pin” on the 20M2 processor. We’ve already discussed this issue (in the February

2012 Primer), so I’ll just briefly re-state it at this point. The PICAXE compiler recognizes A.0 as an undocumented name for the Serial Out pin on the

20M2 processor, and accepts a few undocumented “pseudo” commands, including high A.0, low A.0, toggle A.0, and serout A.0. Unfortunately, it’s not possible to define a name for the undocumented A.0 pin: including a symbol LED = A.0 statement in a program produces a syntax error.

However, whenever we want to blink the LED on the 20M2 Serial Out pin, we can just use a combination of high A.0 and low A.0 commands (with suitable pauses), or even a toggle A.0 command. (As we’ll soon see, the same approach also works with the 20X2 processor.)

With the above information in mind, let’s replicate Experiment

4 from the last Primer. This time, however, we’re using a 20M2 processor. You may want to refer back to the previous Primer installment for the details of setting up the experiment and your terminal program.

When you’re ready to proceed, use Figures 1 and 2 to assemble your breadboard circuit, and download the hserin20M2.bas program to the 20M2. The results you obtain by typing one or two characters into the terminal program should be the same as last time.

Experiment 2: Adding hserout, and Speeding Things Up

In our previous hardware serial experiments with the 08M2, we weren’t able to use the hserout command. As I explained then, on both the 08M2 and 14M2, the hserout command is only available on the processor’s Serial Out pin (C.0 on the 08M2, and B.0 on the

14M2). As a result, if we were to use the hserout command, every time we downloaded a new program to the processor we would also be sending “garbage” characters out to the PC. In order to avoid that problem, we used the serout command with pin C. 2 on the 08M2.

Fortunately, on both the 20M2 and 20X2 processors, the hserout pin does not conflict with the Serial Out pin. On both 20-pin processors, the hserout pin is pin C.0 and the hserin pin is pin B. 6, which means that we’re now free to experiment with the hserout command and speed things up a bit.

Figure 3 presents the valid serial baud rates for hardware serial I/O, along with the baud rates that are supported by the Cool Term program. (If you’re using a different terminal program, you will need to check your documentation for the valid baud rates.)

For each baud rate, the entries in the middle column of Figure 3 show the required value for the baud_setup parameter of the hsersetup command. The “X” in each entry must be replaced by the appropriate value (in MHz) for the clock speed at which the PICAXE processor is running.