STK1002/Reset circuitry
From AVRFreaks Wiki
STK1002 have added reset circuitry to delay the reset pulse created when pushing down the reset switch on STK1000. The following sections describe properties of the complete reset circuitry including that on STK1000.
- Power on reset (what happens when power is applied to STK1000/STK1002
- Reset behavior when using JTAGICE (How JTAGICE work together with the reset circuitry)
- Manual reset from STK1000. (behavior of RESET_N nets on STK1000 and STK1002)
- Schematics of reset circuitry
- Voltage curves after reset switch is released
Power on reset
When power is first applied to AP7000 the processor waits for 1 second before accessing the external memory on STK1000. Reset behavior when using JTAGICE
There are two things to notice about the behavior of the reset circuitry when using JTAGICE.
JTAG reset
When the JTAGICE resets AP7000, the RESET_N net on STK1000 is not affected. The diode separating the two reset nets prevent the JTAGICE from pulling the reset net on STK1000 low.
STK1000 manual reset
The reset switch will not function properly when a JTAGICE that is not turned on is connected to STK1002.
Manual reset from STK1000
When the external reset is pulled low and released then AP7000 access external memory immediately after the reset pin input is high. The reset signal on STK1002 is therefore delayed to avoid a situation where the external flash is not ready for access when the processor access memory after a reset. The schematics and the voltage curves show how this is implemented. Note that the schematic shows circuitry residing on STK1000 as well as on STK1002.
Both STK1000 and STK1002 have a reset net common for the whole board. Ie. the external flash, the external DAC etc. share the same reset signal on STK1000. AP7000 and the debug connectors share the same reset signal on STK1002.
Schematics of reset circuitry
When the reset switch on STK1000 is pushed both capacitors are pulled low through the 100 Ohm resistor on STK1000. The capacitor on STK1002 will have a voltage higher than GND due to the diode connecting the STK1002 reset net to the STK1000 reset net. When the reset button is released the two reset nets will rise with individual rise times. This is possible since the STK1000 reset net rise faster than the STK1002 reset net.
Voltage curves after reset switch is released
The following curve shapes show normal behavior when no hardware is added to STK1000 or STK1002. The curve shapes are an approximation only.
Please note that the internal pull up resistors on the components on both boards affect the rise time of the two capacitors. Adding components to STK1000 or a debug connector to STK1002 will also change the reset voltages.