As part of my video card project, i've been digging into the datasheets for some of the various Cathode Ray Tube Controler chips (CRTC) used on early video cards. the MC6845 is the "gold standard" for these chips. there are a bunch of variations on these chips, and while most websites and hackers will tell you they are identical, that isn't exactly true. The MC6845 and HD46505 (and HD46505R) having a clock limit of 3MHz. While the R6545 has 2.5MHz and the R6545E has a limit of 3.7MHz. Hitachi renamed their HD46505 to HD6845, but there was another change that happened at the exact same time. Hitachi made the HD6845S with 3.7MHz support! so why is this clock limit important? to support 640x400@70Hz, you need a pixel clock of 25.175MHz. that clock is then divided by 8 to provide the clock to the CRTC. in this case the divided clock would be 3.147MHz, outside of the maximum clock range for the standdard 6845 chip. so watch out for ebay sellers who have listings for HD6845S, but are selling you a HD46505, because the specifications are NOT the same. if you want to get 3.7MHz support, you will need a HD46505S, HD6845S, or R6545E.

http://www.6502.org/users/andre/hwinfo/crtc/crtc.html#horizontal
https://en.wikipedia.org/wiki/Motorola_6845