Nuts and Volts - September 2015

comments refer to the corresponding numbers along the right edge of the program listing:

[1] We’re using the same hsersetup command we used previously. The mode parameter is configured as follows: background receive to scratchpad, true serial input and output, hserin and hserout pins enabled.

[ 2] We’ve already discussed the setintflags command that we’ll be using this month, so there’s no need to elaborate further.

[ 3] If you have used the setint command in any of your projects, you know that the first statement in the interrupt subroutine is usually setint off. The purpose of that statement is to make sure the interrupt subroutine itself will not be interrupted while it’s being executed. If that were to happen, the program would behave erratically.

In the current program, it’s highly unlikely that additional background serial data would be received while the current data is being processed, but including the setintflags off statement protects the program from that remote possibility. Of course, if a second microprocessor were transmitting the data, you would need to make sure to include a brief pause between data transmissions.

[ 4] At this point, it’s important to remember that the interrupt is triggered as soon as the first data byte is received in the background. Even at a rate of 115,200 baud, a string of data that’s possibly as long as 128 characters will still be in the process of being received at the beginning of the interrupt. Therefore, a program delay is needed at this point to make sure all the data has arrived before we begin to process it.

In order to determine how long the delay should be, we need to do a little approximate arithmetic.

• Each incoming byte contains one bit: 1 start + 8 data + 1 stop • Therefore: 128 bytes = 1280 bits • Form a proportion: 115,200 bits / 1 sec = 1280 bits / x sec • Therefore: 115,200 x = 1280 1

• So, x = 0.011 seconds, or 11 mS (I decided to increase the delay to 15 mS, just to be safe.) If you want to run the program using a different baud rate, the necessary delay will need to be recalculated. To save you the trouble, Figure 3 gives the necessary delay for selected alternate baud rates.

[ 5] When the program first begins to run, hserinptr is automatically initialized to 0 (as are all PICAXE variables).

Therefore, when the interrupt occurs, the first received byte is stored at location 0 in the scratchpad, and hserinptr is automatically incremented to 1 in preparation for the next byte.

This process continues for every byte that’s received in the background.

As a result, when all the data has been received, hserinptr points to the location in the scratchpad that is one greater than that of the last received character. Therefore, at this point, we decrement hserinptr by 1, so that it points to the location of the last received byte.

[ 6] Here, we loop through the scratchpad locations from location 0 to the location of the last received byte using direct addressing to get each character, and then echo it back to the terminal program.

[ 7] Before returning from the interrupt subroutine, we need to reset hserinptr to 0, so that storage of the next serial input data will again begin at location 0 in the scratchpad.

[ 8] We also need to reset the hserinflag to 0 in preparation for the next background serial transmission.

[ 9] The very last thing we need to do before returning from the interrupt subroutine is to re-enable the interrupt so that the next background serial transmission will again trigger it.

At this point, we’re ready to test the Hser20X2 IntDirect.bas program, but before we do that, I want to correct a mistake I made in the hserial20X2.bas program we used in the previous Primer. In that program, I included a note stating that pin A.0 is “undocumented” on the 20X2 (as it is on the 20M2). However, that’s not the case. If you refer to the 20X2 pinout, you will see that pin A.0 is, in fact, documented. In the present program, I corrected that error.

Before running the Hser20X2IntDirect.bas program, make sure your terminal software is set up as described previously, and that your breadboard circuit is set up with a 20X2 processor in place (see Figure 2).

When you’re ready, download the program to your breadboard setup and test several different input strings. An exact duplicate of each string should be echoed back to the terminal. If not, you will need to check your breadboard setup for wiring errors.

Experiment 2: Hserin with an Interrupt and Indirect Addressing

This experiment essentially replicates Experiment 1, except this time we’re using indirect addressing to echo the data back to the terminal. In Experiment 1, the