FAQ : (Thanks for each mail from you ~~ ) - Q: Wichit Sirichote : I saw your bigger drawing in gif file, you didn't provide the reset signal to 8255, I've never been used with this circuit, "tie to GND". Does it work properly?
A: I have update something of the programmer page. U said that 8255 shall provide the reset signal to 8255. Is it necessary ? One of the reason is because parallel port don`t provide one more control line. I had try this circuit on the bread-board, and it works well.
- on the datasheet, it write " RESET: A high on this input clears the control register and all ports (A, B, C) are set to the input mode with the "Bus Hold" circuitry turned on."
- Q: Wichit Sirichote : Why don't try with simple RC reset, I suppose the reset signal is logic 'high' for a short period, and this will clear internal register. Please recheck with another PC. If it work fine. please provide me a description with your circuit.
A: The original ideal of this circuit was " As simple as possible." The portion of 82C55A, I referred to the book " eripheral Application with Parallel Port." It don`t take any reset portion for 82C55A,but just used a pull-down resister about 10K ohm. Although I don`t know why, I just take the same concept as the book. - Advice from Wichit Sirichote : Shiao, most of the peripheral chip with internal sequential logic (flip-flop function) must have such reset pin, the internal circuitry needs a short period to reset all internal not only control register, but also the latch of the port. In the past I have experience with 8035 single chip (father of the 8051) and interface with 8255 PPI, I forgot to connect reset pin of the 8255 to 8035, the 7-segment display and keypad didn't work. When I connect it, it works OK. I suppose with a simple pulldown resistor, and a stray or internal capacitance between reset pin and +Vcc, then would provide sufficient RC time to reset internal circuit when powering the chip with sudden change of DC supply. You may use the logic probe to measure the logic when turns the DC supply on, you should see a blib signal for a logic high then stay low. If this happen, the simple 10K pull down is OK. If can't see, try DSO with single short. Capture the transient, you will see the Reset pin go high then decay with RC constant.
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AT89C51 
还真是转的，整理一下嘛