cpc664 and cpc6128 have resistors R157, R158 at one multiplexer 153.
cpc464 sometimes not.
Who knows what these resistors do?
They are definitely part of the memory management circuit, but I'm not exactly sure what they do.
I suspect it's a propagation time aid or a TTL high state load aid. The 74153 outputs are loaded with 16 RAM chips.
I must count the voltage resulting from typical PWM on the RA1,2 and MA4,5 lines.
R157 seems to be pulsing an address line and R157 is pulling an address line high independent of the actual MUX state. I've never read up on the refresh method for these RAM chips, but both resistors seem to be related to that. Possibly because the RAM refresh function is carried out by the GA instead of the CPU due to the CPU and GA both reading from the same RAM.
Bryce.
Quote from: Bryce on 16:11, 07 March 24Possibly because the RAM refresh function is carried out by the GA instead of the CPU due to the CPU and GA both reading from the same RAM.
The GA cannot do any RAM refresh as it does not produce the address bits to the RAM chips.
The refresh can only be done by either the CPU or the Video Chip.
In my opinion, it would be the CPU since AMSTRAD would take the cheapest option and that would be let the CPU do it as it is there already.
Hmm maybe Its like terminator in long line? This 153 is far last.
First present this resistors is in Rev2C.
Quote from: rpalmer on 22:23, 07 March 24Quote from: Bryce on 16:11, 07 March 24Possibly because the RAM refresh function is carried out by the GA instead of the CPU due to the CPU and GA both reading from the same RAM.
The GA cannot do any RAM refresh as it does not produce the address bits to the RAM chips.
The refresh can only be done by either the CPU or the Video Chip.
In my opinion, it would be the CPU since AMSTRAD would take the cheapest option and that would be let the CPU do it as it is there already.
I would have to look into it again, but as far as I can remember, the CPC uses a very complicated method to refresh the RAM. The CRTC sets the address, but the refresh trigger comes from the GA. The built-in refresh feature of the z80 wasn't used at all.
Bryce.
DRAM manual: "any normal memory cycle will perform the refresh operation". read for screen is a lot of refresh.
Quote from: rpalmer on 22:23, 07 March 24Quote from: Bryce on 16:11, 07 March 24Possibly because the RAM refresh function is carried out by the GA instead of the CPU due to the CPU and GA both reading from the same RAM.
The GA cannot do any RAM refresh as it does not produce the address bits to the RAM chips.
The refresh can only be done by either the CPU or the Video Chip.
In my opinion, it would be the CPU since AMSTRAD would take the cheapest option and that would be let the CPU do it as it is there already.
Actually it is a combination of GateArray and CRTC.
What is required for a refresh is a address and a RAS strobe.
The CRTC is generating the Address (and is connected for that reason in a weird way), but the GateArray is instructing the Multiplexers to switch to the CRTC Address and then strobes RAS.
CPU is not doing anything in refresh game.