Rainbow Electronics AT89C5131 - Manual (Page 42)

42
AT89C5131
4136A–USB–03/03
In-System Programming
(ISP) The In-System Programming (ISP) is performed without removing the microcontroller
from the system. The ISP facility consists of a series of internal hardware resources
coupled with internal firmware to facilitate remote programming of the AT89C5131
through the serial port.
The Atmel ISP feature has made in-circuit programming in an embedded application
possible with a minimum of additional expenses in components and circuit board area.
The ISP function through UART uses four pins: TxD, RxD, V
SS
, V
CC
. Only a small con-
nector needs to be available to interface the application to an external circuit in order to
use this feature.
Using In-System
Programming The ISP feature allows a wide range of baud rates in the user application. It is also
adaptable to a wide range of oscillator frequencies. This is accomplished by measuring
the bit-time of a single bit in a received character. This information is then used to pro-
gram the baud rate in terms of timer counts based on the oscillator frequency. The ISP
feature requires that an initial character (an uppercase U) be sent to the AT89C5131 to
establish the baud rate. The ISP firmware provides auto-echo of received characters.
Once baud rate initialization has been performed, the ISP firmware will only accept Intel
Hex-type records. Intel Hex records consist of ASCII characters used to represent hexa-
decimal values and are summarized below:
:NNAAAARRDD..DDCC
AT89C5131 will accept up to 16 (10h) data bytes. The “AAAA” string represents the
address of the first byte in the record. If there are zero bytes in the record, this field is
often set to ‘‘0000’’. The “RR” string indicates the record type. A record type of “00” is a
data record. A record type of “01” indicates the end-of-file mark. In this application, addi-
tional record types will be added to indicate either commands or data for the ISP feature.
The “DD” string represents the data bytes. The maximum number of data bytes in a
record is limited to 16 (decimal). The “CC” string represents the checksum byte. ISP
commands are summarized in Table 40.
As a record is received by the AT89C5131, the information in the record is stored inter-
nally and a checksum calculation is performed and compared to ‘‘CC’’.
The operation indicated by the record type is not performed until the entire record has
been received. Should an error occur in the checksum, the AT89C5131 will send an “X”
out the serial port indicating a checksum error. If the checksum calculation is found to
match the checksum in the record, then the command will be executed. In most cases,
successful reception of the record will be indicated by transmitting a “.” character out the
serial port (displaying the contents of the internal program memory is an exception). In
the case of a Data Record (record type ‘‘00’’), an additional check is made. A “.” charac-
ter will NOT be sent unless the record checksum matched the calculated checksum and
all of the bytes in the record were successfully programmed. For a data record, an “X”
indicates that the checksum failed to match, and an “R” character indicates that one of
the bytes did not properly program.
Atmel_ISP, a software utility to implement ISP programming with a PC, is available from
Atmel. Please visit our web site www.atmel.com.