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Saving Programs

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Saving Programs

In Chapter 3’s experiments, the BASIC-52 programs that you wrote were stored in RAM. This is fine for temporary use, but every time you power down, your program disappears and you have to start over.

This chapter shows you two ways to save BASIC-52 programs more permanently: by adding nonvolatile memory to the BASIC-52 system, and by downloading your programs to your host system’s disk. The nonvolatile memory may be battery-backed RAM, EEPROM, or EPROM. You can also use this memory for storing assembly-language programs or data that you want to save when you power down or reset. Disk storage is a convenient way to save programs if you want to edit them off-line, upload them to a different BASIC-52 system, or just save back-up copies.

Nonvolatile Memory Options

One of BASIC-52’s handiest features is its programming commands that store programs in nonvolatile (NV) memory: EPROM, EEPROM, or battery-backed RAM. The commands assume that the NV memory is addressed beginning at 8000h in external data memory.

With the addition of NV memory, you have two areas that may contain BASIC-52 programs: the NV memory, addressed beginning at 8000h, and the RAM, addressed beginning at 0. To distinguish between the two areas, you can call the memory beginning at 8000h the EPROM

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Figure 4-1. Pinouts for Dallas Semiconductor’s 8K NVRAM and SmartSocket.

space (even though it may contain NVRAM, EEPROM, or EPROM), and call the memory beginning at 0, up to 7FFFh or the top of RAM, the RAM space.

BASIC-52’s programming commands are designed to meet the requirements for EPROMs, using either of two programming algorithms, or procedures. You can use the same commands to store programs in NVRAM or EEPROM. Like EPROMs, these devices provide nonvolatile storage—in other words, their contents don’t disappear when power is removed. Plus, they have two advantages over EPROMs: they don’t need any special programming voltages, and they don’t need ultraviolet exposure to erase. This makes them much more convenient to use.

For these reasons, the first circuit we’ll look at offers a choice of NVRAM or EEPROM for nonvolatile storage. Later, we’ll add circuits that allow you to program EPROMs, for those who want this option.

NVRAM

Dallas Semiconductor offers NVRAM chips that you can use for nonvolatile storage. These work exactly like static RAM, except that they contain a lithium cell and backup circuits that retain the RAM’s contents when the main power supply is removed. The backup is

Saving Programs

guaranteed for at least ten years. Dallas also makes a product called the SmartSocket, which consists of an IC socket with an embedded lithium cell and backup circuits. To create a NVRAM, you plug your own static RAM chip into the SmartSocket.

Eight kilobytes is a convenient size that will store many short BASIC-52 programs, or fewer longer ones. For an 8K NVRAM, you can use a DS1225 NVRAM, or a DS1213B or DS1213C SmartSocket with a 6264 or similar static RAM. Figure 4-1 shows the pinouts.

The 1213B and 1213C SmartSockets differ only in that the 1213B will also accept a 24-pin 2K SRAM, with pins 1, 2, 27, and 28 unused, and the 1213C will also accept a 32K SRAM, which has address inputs at pins 1 and 26.

The DS1225 offers a choice of two write-protect voltages. On the -AB version, write protection is guaranteed when the power supply is less than 4.5V, and write operations are allowed when the power supply is greater than 4.75V. The SmartSockets use these same voltages. On the -AD and -Y versions of the DS1225, write protection is guaranteed when the supply is less than 4.25V, and write operations are allowed when the supply is greater than 4.5V. Either type should work in a BASIC-52 system with a regulated +5V supply. Access times of 250 nanoseconds or less are fine for the NVRAM.

Don’t be confused by the fact that Dallas describes its devices by the number of bits they store, rather than the number of bytes. For example, they call the 8-kilobyte DS1225 a 64K device.

You can order NVRAMs directly from Dallas Semiconductor (no minimum order), and from other vendors.

EEPROM

The other option for program storage is EEPROM. A typical EEPROM is guaranteed for 10,000 to 100,000 write cycles, compared to infinite write cycles for NVRAM. Access times for reading an EEPROM are similar to those for static RAM, but writing to EEPROM takes much longer. Most require 2 to 10 milliseconds after a write operation before you can access the chip again. In spite of the drawbacks, I’ve included EEPROM as an option because an 8K EEPROM may cost less than a comparable NVRAM.

A typical part number for an 8K EEPROM is 2864 or 28C64. Figure 4-2 shows the pinout for a 28(C)64 EEPROM. Notice that its pinout, too, is very similar to that of a 6264 static RAM.

EEPROMs have two common ways of indicating that they are busy performing a write operation and are unable to be accessed. In one type, when the EPROM is busy, the data pins hold the last-written data, but with one or more bits inverted. BASIC-52’s programming

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Figure 4-2. Pinout for 8K EEPROM.

commands verify each byte after programming it, so the inverted data automatically keeps BASIC-52 from programming another byte until the EEPROM is ready to receive it.

Other EEPROMs have a busy output, usually at pin 1, which goes low when the EEPROM is busy. For this type, you can tie the busy output to pin 12 of U1. BASIC-52’s programming commands wait for a high logic level at this pin after programming each byte. Note that this means that pin 12 of the 8052-BASIC must be high (or not connected) during programming of any device. However, using the BUSY output is optional, since programming won’t continue until the programmed byte verifies.

Whether you choose EEPROM or NVRAM, be sure to ask for a data sheet for the device you buy, so you can verify its pinout, capacity, and timing characteristics.

Adding NVRAM or EEPROM

Figure 4-3 shows the added circuits for the NVRAM or EEPROM at U8. Because the circuits are an addition to Figure 3-1’s circuits, the parts continue the same numbering sequence, beginning with U8. AND gate U3C is the third gate of Figure 3-1’s U3. Table 4-1 is a parts list of the components needed to add Figure 4-3’s circuits to Figure 3-1.

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time, there is a small chance that the right combination of outputs will cause a write operation to occur at U8.

Since this could destroy the information stored in the chip, we need a way to prevent U8 from being written to for a brief time after power-up. OR gate U9A prevents accesses to U8 until RESET goes low. The delay caused by the charging of R1 through C1 (in Figure 3-1) ensures that the reset algorithm has enough time to bring the port pins high.

U8’s Chip Select (pin 20) goes low only when both of these are true: RESET is low, and the 8052-BASIC is reading or writing to an address from 8000h to 9FFFh.

Output-enable (pin 22) connects to RDANY, to allow U8 to be accessed as data or program memory. This enables U8 to store assembly-language routines as well as BASIC-52 programs.

For writing to U8, AND gate U3C allows a choice of two control signals. WRITE is the conventional signal for writing to data memory. In addition, BASIC-52 uses a special PGM PULSE signal to store BASIC-52 programs in NV memory beginning at 8000h. Either of these signals will bring WE on U8 low.

Saving Programs

Jumper J4 is optional. It enables you to write-protect U8 by jumpering WE to +5V. You might want to do this if you have critical programs or data stored in U8, and you want to be sure that you don’t overwrite them accidentally.

Wiring Tips

When you add the circuits for NV memory, use sockets for U8 and U9. If you previously tied unused pins 9 and 10 of U3 to ground or +5V, be sure to remove these connections before you wire the ones shown in Figure 4-3. Since pins 4, 5, 9, 10, 12, and 13 of U9 are unused CMOS inputs, you should wire these to +5V or ground. You may instead use a 74LS32 for U9. If you do so, leave the unused inputs open.

Using the Programming Commands

When Figure 4-3’s circuit is added, you’re ready to power up and try the programming commands. Begin by entering any simple BASIC-52 program, such as one of the examples in Chapter 3.

Setting MTOP

If you have a 32K RAM at U7, you have an additional step to perform before you store a program in U8. On bootup, BASIC-52 tests contiguous memory and sets MTOP to the highest value it finds below E000h. But BASIC-52’s programming commands won’t work unless MTOP is below 8000h. To enable program storage, type the following command:

MTOP=7FFFh

This ensures that BASIC-52 won’t try to store RAM programs, variables, or strings in the area that you’ve reserved for permanent program storage (although it doesn’t prevent you from writing to the area with BASIC-52’s XBY operator). If U7 is an 8K device, MTOP is 1FFFh, well below 8000h, so you don’t have to worry about changing it.

Saving a Program

To copy the current program from U7 to U8, type

FPROG

The screen will display the number 1, indicating that this is the first BASIC-52 program to be stored in the device, and after a short delay, the READY prompt should return.

PROG is an alternate command that uses a slower programming algorithm, and should also work.

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If BASIC-52 is unable to program the chip, you’ll see this:

ERROR: PROGRAMMING

If you get this error message, double-check your wiring. When the programming command executes, pins 20, 22, and 27 should toggle, along with the address and data lines.

Running a Stored Program

When you have a program saved, you can run it from the NV memory. BASIC-52’s RAM and ROM commands switch from RAM mode, where BASIC-52 runs the program stored in RAM (U7), to ROM mode, where it looks in U8 for programs to run. When you’ve programmed successfully, run your program by typing

ROM

RUN

or

RROM

You can store multiple programs, space permitting, and run each by specifying its number. For example, to run the second program stored, type

RROM2

To return to editing programs in RAM, type

RAM

Another useful command is XFER. In ROM mode, type

XFER

to copy the current program from ROM into RAM, where you can edit it, and then use FPROG to store the revised version in U8 if you wish.

Adding Bootup Options

The commands FPROG1-FPROG6 enable you to store additional information besides programs. FPROG1 saves the current baud rate and causes BASIC-52 to boot immediately to the READY prompt, without waiting to receive a SPACE character. FPROG2 saves the current baud rate and also tells BASIC-52 to automatically run the first program in NV

Saving Programs

memory on bootup. This is what allows you to disconnect the system from its host and run it as a stand-alone system.

You can also permanently store a value for MTOP in U8. If you have a 32K RAM at U7, storing MTOP will ensure that you can use FPROG, and that your stored programs will be preserved when you reboot or power down.

If U7 is 32K, type

MTOP=7FFFH

FPROG3

Now, when your system boots up, MTOP will automatically be set to 7FFFh. FPROG3 also saves the baud rate and boots to the READY prompt without requiring you to press the space bar.

If you want to save MTOP and also run a program on bootup, use FPROG4, which combines the features of FPROG2 and FPROG3. FPROG5 is another useful command. It prevents BASIC-52 from clearing external data memory on bootup. FPROG6 enables you to add your own assembly-language reset routine.

If you use FPROG2-FPROG6, BASIC-52 will no longer auto-detect your host’s baud rate. You must use the baud rate and crystal value that were in use when you executed the FPROG command.

Erasing NV Memory

Eventually, your NVRAM or EEPROM will fill with programs, or you may just want to erase what you’ve stored and start fresh. Listing 4-1 is a program that erases U8 by writing 0FFh to all locations.

To use the program, enter the listing and type RUN. The READY prompt will return when erasing is complete. Line 30 verifies each erasure, and is required only for EEPROM,

Listing 4-1. Erases NVRAM or EEPROM.

10 FOR I=8000H TO 9FFFH

20 XBY(I)=0FFH

30 IF XBY(I)<>0FFH THEN GOTO 30

40 NEXT I

50 END