2
0943E–AVR–08/08
AVR910
Figure 1. Six-wire Connection Between Programmer and Target System
To assure proper communication on the three SPI lines, it is necessary to connect ground on the
programmer to ground on the target (GND).
To enter and stay in Serial Programming mode, the AVR microcontroller reset line has to be kept
active (low). Also, to perform a Chip Erase, the Reset has to be pulsed to end the Chip Erase
cycle. To ease the programming task, it is preferred to let the programmer take control of the tar-
get microcontroller reset line to automate this process using a fourth control line (Reset).
To allow programming of targets running at any allowed voltage (2.7 - 6.0 V), the programmer
can draw power from the target system (V
CC
). This eliminate the need for a separate power sup-
ply for the programmer. Alternatively, the target system can be supplied from the programmer at
programming time, eliminating the need to power the target system through its regular power
connector for the duration of the programming cycle.
Figure 2 shows the connector used by this In-System Programmer to connect to the target sys-
tem. The standard connector supplied is a 2 x 3 pin header contact, with pin spacing of 100 mils.
Figure 2. Recommended In-System Programming Interface Connector Layout (Top View)
Hardware
Design
Considerations
To allow In-System Programming of the AVR microcontroller, the In-System Programmer must
be able to override the pin functionality during programming. This section describes the details
of each pin used for the programming operation.
GND
The In-System Programmer and target system need to operate with the same reference voltage.
This is done by connecting ground of the target to ground of the programmer. No special consid-
erations apply to this pin.
RESET
The target AVR microcontroller will enter Serial Programming mode only when its reset line is
active (low). When erasing the chip, the reset line has to be toggled to end the erase cycle. To
simplify this operation, it is recommended that the target reset can be controlled by the In-Sys-
tem Programmer.
Immediately after Reset has gone active, the In-System Programmer will start to communicate
on the three dedicated SPI wires SCK, MISO, and MOSI. To avoid driver contention, a series
resistor should be placed on each of the three dedicated lines if there is a possibility that exter-
nal circuitry could be driving these lines. The connection is shown in Figure 3. The value of the
resistors should be chosen depending on the circuitry connected to the SPI bus. Note that the
PC 9-PIN
SERIAL PORT
IN-SYSTEM
PROGRAMMER
TARGET AVR MCU
AT90S
XXXX
VCC
RESET
MISO
MOSI
SCK
VCC
RES
MISO
MOSI
SCK
GND
TXD
RXD
GND
TXD
RXD
GND
1
3
5
4
6
2
VCC
MOSI
GND
MISO
SCK
RESET
