June 2000, Issue 107

Low-Cost Software
Bell-202 Modem

APPLICATION DEVELOPMENT

Adding functions to an SX28AC-based design is as simple as going to a web site and downloading the desired virtual peripherals. New software development and programming are equally painless.

The chip is programmed over its oscillator pins (26, 27) using the SX-Key development system (a 0.5² × 1.5² module that connects at one end to a PC by a standard DE-9 connector and at the other end to the system board by a four-pin header interface connector). The SX-Key software, which includes an editor, programmer, and debugger, can be installed on a PC and run under Windows.

With the SX-Key software loaded and the PC connected to the circuit through the hardware module, software development begins immediately. The first screen is a device setup window.

It lets you set parameters such as the number of pins and on-chip memory size of the SX-series MCU you’re using, the clock source (an external frequency generator in this design, although a range of internal oscillator frequencies can also be selected), and additional options like stack extensions and an on-chip watchdog timer. A configuration window lets you select the desired communication port and specify the erase and program times for the flash program memory.

With the chip parameters established, an Edit window lets you write the program. All timing-dependent operations must be run within an interrupt service routine, so the code needs to be written using an interrupt indicator.

An on-chip hardware timer produces a real-time clock/counter (RTCC) signal that generates interrupts. The RTCC can be set to establish how many clock cycles occur before an interrupt must be serviced.

The general program flow is:

(load RTCC countdown)
run application code
RTCC times out, issues interrupt
	interrupt service routine
		run peripheral routine
	load RTCC countdown
	RETI (return from interrupt)
run application code

Listing 1 shows the initialization part of the program for this modem design.

After resetting the SX chip, the I/O ports and the default values for some software variables must be initialized. The final line of code in the reset entry code typically loads the Option register with the proper value to enable RTCC clock rate and interrupts.

In this design, the contents of the RTCC are incremented each clock cycle. If the RTCC overflows, an interrupt request follows. The RTCC overflow interrupt is essential to the virtual-peripheral concept, as it provides a jitter-free time base that is used as the time slice for the peripherals. Much like an RTOS, this master clock can then increment other clocks specific to each task.

The completed program and the virtual peripherals are loaded into the MCU’s on-chip program memory through the SX-Key module. A Debug window (see Photo 1) displays the result of programming the chip and provides interactive debugging options.

The left column shows the contents of registers 00–0F of the selected RAM bank in hex and binary numbers. The contents of registers 10–1F of the remaining RAM banks are displayed on the right side.

In the middle of the Debug window, the contents of the M and W registers are shown as well as interrupt and skip flags. Beneath that is listed the address, opcode, and assembler mnemonic of the selected part of the program memory.

Buttons for emulation functions are arranged across the window bottom:

• step—executes one instruction
• walk—executes multiple instructions, one at a time
• run—executes instructions at full speed
• stop—halts execution of a walk or run operation
• reset—resets the MCU
• exit—closes the Debug window

To debug the program, just reset the contents of any of the registers. All the changed registers are marked with red during debugging so you don’t lose the track of what you’re doing.

When you’re satisfied that you’ve corrected the bugs, reprogram the chip. Because you’re working with flash memory, you can go through as many iterations as you want to get it right.

AT LAST, A CHOICE

For about $7, I was able to design a compact fully functional embedded modem that meets the needs of all kinds of applications. It was possible because designers have a choice not only of modems, but also of every imaginable type of embedded system.

Stephen Holland is a senior applications engineer at Scenix Semiconductor. Before joining Scenix in early 1998, he worked for over six years in the electronics and computer industries in Canada and Hong Kong. You may reach him at stephen.holland@scenix.com.