This design example shows a new way to drive six LEDs using only two MCU I/O lines. This method is especially useful for any chip with limited pins. This method uses two I/O lines and a pair of complementary bipolar transistors. More than one LED light can be illuminated by multiplexing.
Note: VDD must be between 2V and 3V. For example, if LED 2 is lit (Table 1), then LED 4 and LED 6 will also be illuminated if VDD is too high. You can choose to use any general purpose low power transistor, but if you want to keep all LEDs at the same brightness, you need to choose a low VCE (sat) transistor.
Figure 1: Test Selection PIC10F200, firmware is provided in two separate, simple assembler source code.
Table 1: LED drive logic table. Avoid (0,0) and (1,1).
The PIC10F200 was chosen for testing because the PIC10F200 is the smallest and simplest six-pin MCU. For demonstration purposes, the firmware is provided in two separate, simple assembler source code.
Note: In either mode, only one minimum current (high-brightness LED down to 2mA) and periodic short-pulse burst mode are used to drive one LED. The assembler source code is fully annotated, optimized for length, no more than 256 words, allowing the user to select any other PIC with limited pins.
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