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I. Bootloader
These are modifications of Mikobu's boot loader circuit. Each will function in the same way as the original circuit does. However, the SN7407 has been replaced with NPN transistors. Version 1of the circuit uses two transistors and is logically equivalent to the original BS circuit. Therefore, the existing BS Atmel code should work with this circuit. Version 2 is the single transistor version of the circuit. This version will require a change to the Atmel code to reverse the logic behavior of PB4. These circuit offers two advantages. First, the overall current draw of this circuit should be about 35mA less. Second, since the 7407s are difficult to find, the small signal transistors used here should be easier to obtain. Note that this circuit will still draw high current (80mA) during the glitch.

2nd Update 3/8/01

The Code
All of the program files below have been loaded and tested. The BL.ASM is a disassembly of the original BL.HEX file that was included in the simplefull last package. BL_SMALL is a revised version that significantly reduces the size of the program. The original BL program used 573 Words (or just over 1kB) of flash memory on the Atmel 90S2313. This modified small version uses just 35 Words of flash memory. The real advantage to having such a small program is that it will fit into the lower end Atmel 90S1200 which has 1kB flash memory. Therefore, the pin compatible 90S1200 can be a drop in replacement for the 90S2313.

The BL_INVRT file contains a special version of the small BL code that has reversed control logic on the glitch control line (pin 16). This inverted control logic is useful for controlling the single transistor circuit. Below are the links to these program files.

BL.ASM - Disassembly of Mikobu original BL Code
BL_SMALL - Small version of BL. Works just like the original but only takes up 35 Words of memory
BL_INVRT - Inverted glitch control logic. Use with single transistor circuit below. This is also small code.

Success
That's right! The single transistor version of the BL circuit has been successfully tested! It does work a little differently than the original 7407 version of the circuit. Also, a few changes were made to achieve reliable operation. This circuit was tested using a 2N2222 which has an Hfe of about 50. Therefore, the bias resistor on the Base need to be reduced to about 1.5K. Also, the total resistance of the pull down circuit had to be reduced from the 18 Ohms (total) that usually works with 7407. For most BS cards, the useful resistance range for this circuit was 13 to 16 Ohms. A value of 14 or 15 ohms proved to be pretty reliable. Below is a photo of the test circuit. Obviously it is a little ugly but it works beautifully!

Single Transistor Test Circuit

Version 1, Dual transistor versions. Use with existing Atmel BS code.

Version 2, Single transistor version. Use with BS_INVERT.HEX Atmel code.