Thrashbarg
Experienced Member
Hi,
I'm making an MC6845 video card for my 8080 computer and I'm having difficulty programming it. I've calculated the registers to what seem to be appropriate values but I'm getting some very odd frequencies on the HSync and VSync pins.
I'm certain that the chip is receiving the correct data. I wasn't disabling the video RAM at one stage but I am now. The wires all check out with my multimeter beeper and I've tried several chips from different manufacturers, with the same results.
The clock is derived from a TTL 14.31818MHz oscillator, and divided by 8 for each character then sent to the MC6845's CLK input. Each character has a period of 558.7nS.
Does anyone know the correct way to program the MC6845? The datasheet says to initialize it by sending the data for the 15 registers sequentially, starting at R0 and finishing at R15.
For example, I send 114 - 1 to R0 which determines the time for the horizontal sweep in characters. 114 x 558.7nS = 64.251uS which is pretty close to the PAL standard, but I'm getting 111.8kHz (8.945uS) on the HSync pin read from my multimeter.
Any comments or links to documentation will help.
Thanks.
I'm making an MC6845 video card for my 8080 computer and I'm having difficulty programming it. I've calculated the registers to what seem to be appropriate values but I'm getting some very odd frequencies on the HSync and VSync pins.
I'm certain that the chip is receiving the correct data. I wasn't disabling the video RAM at one stage but I am now. The wires all check out with my multimeter beeper and I've tried several chips from different manufacturers, with the same results.
The clock is derived from a TTL 14.31818MHz oscillator, and divided by 8 for each character then sent to the MC6845's CLK input. Each character has a period of 558.7nS.
Does anyone know the correct way to program the MC6845? The datasheet says to initialize it by sending the data for the 15 registers sequentially, starting at R0 and finishing at R15.
For example, I send 114 - 1 to R0 which determines the time for the horizontal sweep in characters. 114 x 558.7nS = 64.251uS which is pretty close to the PAL standard, but I'm getting 111.8kHz (8.945uS) on the HSync pin read from my multimeter.
Any comments or links to documentation will help.
Thanks.