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Micro-ISP
IV User Manual V2.03
Series
ATmega103/603 - ISP Notes
1. RESET circuit
This should be an 'Active LOW'
RESET circuit. To prevent
PROG_ACTIVE
inevitable EEPROM corruption,
an electronic brownout circuit is
recommended such as a 'RESET
1
2
3
Supervisor' device available from
Dallas Semiconductor, Maxim,
Seiko etc.
2. PEN
The programmer does not
support the 'PEN' method of
entering RESET. The PEN pin
should therefore be tied high.
3. MOSI/MISO connections
The Atmega microcontrollers differ from other AVR/89S device in that the ISP programming
is performed via the UART pins 'PD1/RXD' and 'PD0/TXD' pins instead of MOSI and MISO.
This connection system can lead to driver contention problems with the 'PROG-MOSI' line
and the output driver of an RS-232 Transceiver chip. Basically, if the 'RS-232-RXD' line is
asserted, the programmer may not be able to assert the 'PROG_MOSI' pin. Possible
workarounds include placing a resistor i.e. R5 in the path from the RS-232 transceiver to the
RXD pin of the microcontroller or using a jumper i.e. J1 which the user must manually move
before attempting to program the device.
4. Protection resistors
It is good EMC design practice to place some limiting resistance on each processor I/O line to
protect against damage from transients. i.e. R1, R3, R4, R5. A very low value of e.g. 10
ohms is recommended. Further protection can be afforded by using ferrites instead of
resistors. The ATmega devices are very sensitive to the voltage slew rate, so use of any
component which slews the edges of the programming signals may cause unreliable
programming.
5. SCK Noise filtering
If noise spikes on the SCK pin are found to be a problem, a resistor i.e. R6 may be inserted
between the SCK pin and ground.
6. Active LED
An LED may be placed on the user target system to indicate when the programmer is active.
The value of R7 should be chosen to suit the target supply voltage.
Further sources of information:
AN100 - ISP Design Notes for the Atmel ATmega Microcontroller
Family
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