The sound board is assembled and working. For tests I`ve written a simple program that generates a different tone for each of the three channels. There was a small problem with cap value (the one used in CLK signal generator). Measurment indicates 2.4Mhz frequency, acceptable value, but near the top 2.5Mhz value from the datasheet. If the cap value isn`t properly choosen, the signal fill rate gets out of specs (40-60% range nominal) and the AY chip doesn`t work. I`ve managed to pick a value of 560pF, which gave me a stable signal of about 2Mhz frequency and acceptable fill rate.
Whats more – LM386 amplifier refuses to work. I`ve tested the card in line-out mode this time. In meantime I`ll check the cause of the problem.
Photos of the assembled module:
For now I have to lay aside the plans of building more expansion boards (RS232, Z80 CTC board) and focus on the development of the ROM Monitor program. The design must be carefully planned, as the decisions made here will have impact on the future form of the system. For start I`ll use partialy written routines for keyboard readout. I have to decide on some variable handling procedures, stack usage, etc. The program and all it`s subroutines code should of course be heavily commented.