Page 195 - Embedded Microprocessor Systems Real World Design

firmware writes anything to the ROM space. This technique is useful when all the
available 1/0 decodes are used up and trace capability needs to be added after the
hardware is finalized. The one drawback to this method occurs in systems that have
flash or other writable memory, where a careless write sequence actually could
change the data.
Like the write to an 1/0 port, this method can use different addresses for dif-
ferent data types. Note that if the Motorola-type bus is used, a DTACKmust be gen-
erated for the debug write cycle.

Read from ROM

Some microcontrollers, such as the 8051, lack the capability of writing to the ROM
space. However, they do have instructions that can read from ROM to do things
like table lookups. Figure 6.3 shows a circuit that can be used to generate trace data
by this method. A low-going strobe is generated when a read is performed from
any PROM address above 8000 h. When using this circuit, the trace data is captured
from the low-order address lines, A0 through A7, and the data lines are ignored.
The circuit shown in Figure 6.3 assumes that the ROM is located in the lower
32K. If the ROM is larger than 32K, a wider NAND gate can be used to further
decode the address bits, generating a strobe only for the upper 1K or less. The
following code shows how this would be implemented on an 8051 family
microcontroller:

MOV DPTR,#8000 h ; starting address-upper 32 k
MOV A,#CONSTANT ; constant is the trace value
MOV A,@A+DFTR ; output the value.

This method can use different addresses to iden@ different types of trace data,
but both addresses must be in the upper 8 bits of the address field, since the lower

ROM Cs
OUTPUT S m
OT
A15 I

-
) AOA7 * TRACEDATA

Figure 6.3
Generating Trace Data by Read from ROM.

176 Embedded Microprocessor Systems