The Microcontroller Idea Book - Jan Axelson's Lakeview Research

• BASIC-52’s ON EX1 instruction will also respond to external interrupt 1, but the
response time will be much slower. ON EX1 has priority, however, and Listing 13-6
will not execute if an ON EX1 statement has executed in BASIC-52.
• When BASIC-52 jumps to an assembly-language interrupt routine, it automatically
pushes psw on the stack. But popping psw on returning from the interrupt routine is not
automatic, so the interrupt routine must include an instruction to do so.
• Unlike other subroutines, which end with ret, interrupt routines must end with reti.
To test Listing 13-6, upload it to RAM at 4013h, and execute these two lines of BASIC-52
code, to ensure that the interrupt is enabled:
IE=IE.OR.84h
TCON=TCON.OR.04h
Now, each time pin 13 goes low, pin 1 should toggle, as it did with Listing 13-1.
Adding Custom Commands and Instructions
Calling Assembly-language Routines
Another feature of BASIC-52 is the ability to add up to 16 custom keywords representing
commands or instructions that you define with assembly-language routines. Listing 13-7 is
an example program for doing so.
To add custom keywords, you must have code memory from 2000h to 2071h, because
BASIC-52 looks for special information at several addresses in this area.
On bootup, BASIC-52 examines the data at address 2048h. If bit 5 is set, BASIC-52 assumes
that you have added custom keywords, and it looks for additional information in a token
table and vector table.
At 2078h, BASIC-52 expects to find the starting address of a token table, which lists your
new keywords. At 2070h, it expects the starting address of a vector table, which lists the
assembly-language routines that the new keywords execute.
You may upload both tables to any free area of combined code/data memory. If you’re using
an external EPROM programmer, you can use code-only memory in the 8052-BASIC
system.
In the token table, each new keyword is assigned a number, in sequence from 10h to 1Fh.
The name of the keyword is surrounded by quotation marks, and a 0 indicates the end of the
token. The final keyword in the list ends in 0FFh, to signify the end of the list of tokens.
Listing 13-7 adds three keywords, which toggle, set, and clear bit 0 of Port 1.
The Microcontroller Idea Book 233