Unformatted text preview: atively, the target can have power
supplied through the In-System Programming connector
for the duration of the programming cycle Figure 3. Connecting ISP Programming Cable to Target SPI Bus
To avoid problems, the In-System Programmer should be able to keep the entire Target System
Reset for the duration of the programming cycle. The target system should never attempt to
drive the three SPI lines while Reset is active.
SCK When programming the AVR in Serial mode, the In-System Programmer supplies clock information on the SCK pin. This pin is always driven by the programmer, and the target system should
never attempt to drive this wire when target reset is active. Immediately after the Reset goes
active, this pin will be driven to zero by the programmer. During this first phase of the programming cycle, keeping the SCK Line free from pulses is critical, as pulses will cause the target AVR
to loose synchronization with the programmer. When in synchronization, the second byte ($53),
will echo back when issuing the third byte of the programming enable instruction. If the $53 did
not echo back, give Reset a positive pulse, and issue a new Programming Enable command.
Note that all four bytes of the of the Programming Enable command must be sent before starting
a new transmission.
The target AVR microcontroller will always set up its SCK pin to be an input with no pull up
whenever Reset is active. See also the description of the Reset wire. 3
0943E–AVR–08/08 The minimum low and high periods for the Serial Clock (SCK) input are defined in the programming section of the datasheet. For the AT90S1200 they are defined as follows:
Low: >1 XTAL1 clock cycle
High: >4 XTAL1 clock cycles
MOSI When programming the AVR in Serial mode, the In-System Programmer supplies data to the
target on the MOSI pin. This pin is always driven by the programmer, and the target system
should never attempt to drive this wire when target reset is active.
The target AVR microcontroller will always set up its MOSI pin to be an input with no pull up
whenever Reset is active. See also the description of the Reset wire. MISO When Reset is applied to the target AVR microcontroller, the MISO pin is set up to be an input
with no pull up. Only after the “Programming Enable” command has been correctly transmitted
to the target will the target AVR microcontroller set its MISO pin to become an output. During this
first time, the In-System programmer will apply its pull up to keep the MISO line stable until it is
driven by the target microcontroller. VCC When programming the target microcontroller, the programmer outputs need to stay within the
ranges specified in the DC Characteristics.
To easily adapt to any target voltage, the programmer can draw all power required from the target system. This is allowed as the In-System Programmer will draw very little power from the
target system, typically no more than 20 mA....
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- Three '12
- Atmel AVR, target avr