Nuts & Volts - June 2007

56

June 2007

■ PHOTO 1. This is the WWWLSEEP-1 Programmer.

applied to SDA from the 1200, and the SDA also outputs data to the 1200 when requested. A clock signal generated by the 1200 is applied to synchronize I/O operation.

The complete programmer development project includes writing a Windows XP program to communicate with the programmer, and coding the 1200 firmware. Of course, in addition to creating the software serial port and communicating with a PC, the firmware must perform all read/write programming functions through SCL and SDA. The firmware is 854 bytes total, including the software serial port overhead.

Photo 1 shows the WWLSEEP-1 programmer. Notice that all the signals and power supply are provided through the DB- 9 connector; there is no separate power supply.

Also note that if the PC is not running the programmer software but running the Hyper Terminal program instead, it will show the character echo effect as in the first example.

Summing Up

We’ve seen two examples where a software serial port is useful. It can save you money and it can (nearly) do the same job as a hardware serial port. You’ll need some memory space, but the overhead is less than 100 bytes. For the 1K byte 1200, there is still a lot of program space left.

The timing requirement for the asynchronous communication is very forgiving at the 9600 bps speed. Examine to see what will happen if you change the DLY49us to 50 µs by adding an NOP (that’s equivalent to 1 µs) to the beginning of the routine. You’ll see it is still okay.

Of course, there are limitations. The 1 MHz frequency is too low if you want to get a higher baud rate. At higher speeds the half_bitime is reduced and it will be hard to keep sampling in the middle of each bit.

We may not be chip makers, but we can create a software serial port for any MCU when we need it. I have created a website containing the software serial port program source code for different MCUs — just go to www.geocities.com/ microappnotes. NV