I have a CPU board that interfaces to a standard 16x2 chars display, based on HD44780 controller. I'd like to replace this display with a full-color big TFT display, maybe 10", but I can't change the firmware of the CPU (because I don't have the source code and I can't rewrite it, because of cost and time). The simple idea is to create an interface between the current CPU board and the TFT display. This interface should be able to detect the signals from the CPU and that was directed to HD44780, interpret the instructions (write CGRAM/DDRAM and so on) and write the same things to the big TFT display. Of course, exactly the same texts will be written on TFT display (of course with a bigger and visible font) and this is acceptable. So the MCU on the interface should be able to read 8 data bits, RS and R/W signals syncronous to signal E. I'm thinking to configure E signal as an external interrupt on the falling edge. In the interrupt, I read data bits, R/W and RS and decode the instruction or data. Any suggestions? Maybe a similar code is already online.
Convert 16x2 chars display to big TFT display
Started by ●September 21, 2021
Reply by ●September 21, 20212021-09-21
On Tue, 21 Sep 2021 12:31:54 +0200, pozz wrote:> I have a CPU board that interfaces to a standard 16x2 chars display, > based on HD44780 controller. > > I'd like to replace this display with a full-color big TFT display, > maybe 10", but I can't change the firmware of the CPU (because I don't > have the source code and I can't rewrite it, because of cost and time). > > The simple idea is to create an interface between the current CPU board > and the TFT display. This interface should be able to detect the signals > from the CPU and that was directed to HD44780, interpret the > instructions (write CGRAM/DDRAM and so on) and write the same things to > the big TFT display. > > Of course, exactly the same texts will be written on TFT display (of > course with a bigger and visible font) and this is acceptable. > > So the MCU on the interface should be able to read 8 data bits, RS and > R/W signals syncronous to signal E. > > I'm thinking to configure E signal as an external interrupt on the > falling edge. In the interrupt, I read data bits, R/W and RS and decode > the instruction or data. > > Any suggestions? Maybe a similar code is already online.I guess you don't need an interrupt. The signals to the 44780 are quite slow and well sequenced. So simple polling would do. Beware that the E signal is high when data is valid. So I would look for the rising edge. -- Reinhardt
Reply by ●September 21, 20212021-09-21
Il 21/09/2021 15:44, Reinhardt Behm ha scritto:> On Tue, 21 Sep 2021 12:31:54 +0200, pozz wrote: > >> I have a CPU board that interfaces to a standard 16x2 chars display, >> based on HD44780 controller. >> >> I'd like to replace this display with a full-color big TFT display, >> maybe 10", but I can't change the firmware of the CPU (because I don't >> have the source code and I can't rewrite it, because of cost and time). >> >> The simple idea is to create an interface between the current CPU board >> and the TFT display. This interface should be able to detect the signals >> from the CPU and that was directed to HD44780, interpret the >> instructions (write CGRAM/DDRAM and so on) and write the same things to >> the big TFT display. >> >> Of course, exactly the same texts will be written on TFT display (of >> course with a bigger and visible font) and this is acceptable. >> >> So the MCU on the interface should be able to read 8 data bits, RS and >> R/W signals syncronous to signal E. >> >> I'm thinking to configure E signal as an external interrupt on the >> falling edge. In the interrupt, I read data bits, R/W and RS and decode >> the instruction or data. >> >> Any suggestions? Maybe a similar code is already online. > > I guess you don't need an interrupt. The signals to the 44780 are quite > slow and well sequenced. So simple polling would do.Are you sure? Enable cycle time could be 500ns at 5V, look at page 52 of datasheet[1].> Beware that the E signal is high when data is valid. So I would look for > the rising edge.Are you sure? Look at Figure 25[1], data are valid at the falling edge of E signal. [1] https://www.sparkfun.com/datasheets/LCD/HD44780.pdf
Reply by ●September 21, 20212021-09-21
On Tue, 21 Sep 2021 16:01:42 +0200, pozz wrote:> Il 21/09/2021 15:44, Reinhardt Behm ha scritto: >> On Tue, 21 Sep 2021 12:31:54 +0200, pozz wrote: >> >>> I have a CPU board that interfaces to a standard 16x2 chars display, >>> based on HD44780 controller. >>> >>> I'd like to replace this display with a full-color big TFT display, >>> maybe 10", but I can't change the firmware of the CPU (because I don't >>> have the source code and I can't rewrite it, because of cost and >>> time). >>> >>> The simple idea is to create an interface between the current CPU >>> board and the TFT display. This interface should be able to detect the >>> signals from the CPU and that was directed to HD44780, interpret the >>> instructions (write CGRAM/DDRAM and so on) and write the same things >>> to the big TFT display. >>> >>> Of course, exactly the same texts will be written on TFT display (of >>> course with a bigger and visible font) and this is acceptable. >>> >>> So the MCU on the interface should be able to read 8 data bits, RS and >>> R/W signals syncronous to signal E. >>> >>> I'm thinking to configure E signal as an external interrupt on the >>> falling edge. In the interrupt, I read data bits, R/W and RS and >>> decode the instruction or data. >>> >>> Any suggestions? Maybe a similar code is already online. >> >> I guess you don't need an interrupt. The signals to the 44780 are quite >> slow and well sequenced. So simple polling would do. > > Are you sure? Enable cycle time could be 500ns at 5V, look at page 52 of > datasheet[1].An eternity.>> Beware that the E signal is high when data is valid. So I would look >> for the rising edge. > > Are you sure? Look at Figure 25[1], data are valid at the falling edge > of E signal.Data should be valid for the whole E time. Look for the rising edge and sample the other signals. Otherwise you rely on the z´unspecified hold time of your signal source. -- Reinhardt
Reply by ●September 21, 20212021-09-21
On 2021-09-21 pozz wrote in comp.arch.embedded:> Il 21/09/2021 15:44, Reinhardt Behm ha scritto: >> On Tue, 21 Sep 2021 12:31:54 +0200, pozz wrote: >> >>> >>> So the MCU on the interface should be able to read 8 data bits, RS and >>> R/W signals syncronous to signal E. >>> >>> I'm thinking to configure E signal as an external interrupt on the >>> falling edge. In the interrupt, I read data bits, R/W and RS and decode >>> the instruction or data. >>> >>> Any suggestions? Maybe a similar code is already online. >> >> I guess you don't need an interrupt. The signals to the 44780 are quite >> slow and well sequenced. So simple polling would do. > > Are you sure? Enable cycle time could be 500ns at 5V, look at page 52 of > datasheet[1].Datasheet indeed says 500ns minimum. It ofcourse depends on the mcu what the actual timing is. 500ns is 'slow' for todays controllers, so I think you could do polling as long as you don't do too much else (depending on the speed of your mcu). But an interrupt may be a safer choice and makes other processing (outside irq) easier. But even with using an interrupt you have to make sure latency is small enough, especially if there are other interrupts.>> Beware that the E signal is high when data is valid. So I would look for >> the rising edge. > > Are you sure? Look at Figure 25[1], data are valid at the falling edge > of E signal. > > [1] https://www.sparkfun.com/datasheets/LCD/HD44780.pdfBoth is true: data is valid when E is high, but with a small delay (Tddr and Tdhr), so data is still valid on the falling edge. But you must be very fast in your response, minimum Tdhr is only 5 ns. This would require an awfully fast interrupt response. Setting an interrupt on the rising edge gives you another challenge: Tddr is 160ns max, so you may not actually read the data within 160ns of the rising edge of E. If you have a fast interrupt response, you may need to wait a little before reading. Maybe an FPGA solution is easier for this problem. ;-) have you already thought about the other end of the interface? How do you plan to get data to the TFT panel? And don't forget about the part in between, the frame buffer etc. -- Stef Yow! Those people look exactly like Donnie and Marie Osmond!!
Reply by ●September 21, 20212021-09-21
Il 21/09/2021 17:09, Stef ha scritto:> On 2021-09-21 pozz wrote in comp.arch.embedded: >> Il 21/09/2021 15:44, Reinhardt Behm ha scritto: >>> On Tue, 21 Sep 2021 12:31:54 +0200, pozz wrote: >>> >>>> >>>> So the MCU on the interface should be able to read 8 data bits, RS and >>>> R/W signals syncronous to signal E. >>>> >>>> I'm thinking to configure E signal as an external interrupt on the >>>> falling edge. In the interrupt, I read data bits, R/W and RS and decode >>>> the instruction or data. >>>> >>>> Any suggestions? Maybe a similar code is already online. >>> >>> I guess you don't need an interrupt. The signals to the 44780 are quite >>> slow and well sequenced. So simple polling would do. >> >> Are you sure? Enable cycle time could be 500ns at 5V, look at page 52 of >> datasheet[1]. > > Datasheet indeed says 500ns minimum. It ofcourse depends on the mcu what > the actual timing is. 500ns is 'slow' for todays controllers, so I think > you could do polling as long as you don't do too much else (depending on > the speed of your mcu). But an interrupt may be a safer choice and makes > other processing (outside irq) easier. But even with using an interrupt > you have to make sure latency is small enough, especially if there are > other interrupts. > > >>> Beware that the E signal is high when data is valid. So I would look for >>> the rising edge. >> >> Are you sure? Look at Figure 25[1], data are valid at the falling edge >> of E signal. >> >> [1] https://www.sparkfun.com/datasheets/LCD/HD44780.pdf > > Both is true: data is valid when E is high,Immediately before the falling edge data must be valid, but they aren't valid immediately after the rising edge. So I don't think the sentence "data are valid when E is high" is correct.> but with a small delay (Tddr > and Tdhr), so data is still valid on the falling edge.So I think it's much more robust to read on the falling edge.> But you must be > very fast in your response, minimum Tdhr is only 5 ns. This would > require an awfully fast interrupt response.I got the point and indeed is critical. Maybe rising edge is better, because most probably MCU set data bits and after make a pulse on E. >Setting an interrupt on the> rising edge gives you another challenge: Tddr is 160ns max, so you may > not actually read the data within 160ns of the rising edge of E. If you > have a fast interrupt response, you may need to wait a little before > reading.Consider that the read operation for HD44780 will be a write operation for my interface. In other words, when a read operation is send from MCU, my interface should write on data bits. Howevere your point is valid: my interface should write on data bits before Tddr, 160ns maximum, and this could be difficult. Anyway read operations aren't usually used on this kind of applications.> > Maybe an FPGA solution is easier for this problem. ;-) > > have you already thought about the other end of the interface? How do > you plan to get data to the TFT panel? And don't forget about the part > in between, the frame buffer etc.I will send data to a serial port.
Reply by ●September 21, 20212021-09-21
On 9/21/2021 3:31 AM, pozz wrote:> I have a CPU board that interfaces to a standard 16x2 chars display, based on > HD44780 controller. > > I'd like to replace this display with a full-color big TFT display, maybe 10", > but I can't change the firmware of the CPU (because I don't have the source > code and I can't rewrite it, because of cost and time). > > The simple idea is to create an interface between the current CPU board and the > TFT display. This interface should be able to detect the signals from the CPU > and that was directed to HD44780, interpret the instructions (write CGRAM/DDRAM > and so on) and write the same things to the big TFT display. > > Of course, exactly the same texts will be written on TFT display (of course > with a bigger and visible font) and this is acceptable. > > So the MCU on the interface should be able to read 8 data bits, RS and R/W > signals syncronous to signal E. > > I'm thinking to configure E signal as an external interrupt on the falling > edge. In the interrupt, I read data bits, R/W and RS and decode the instruction > or data. > > Any suggestions? Maybe a similar code is already online.You will likely find there are issues that you've not even considered that the original software relies on (directly or indirectly). Can your emulation handle a *steady* stream of reads/writes? Or, are you expecting the host to be gentle with the display interface? Keep in mind, that you have to run a fair bit of code to paint pels onto the display from "character codes". Does the host rely on the timing of the display's "busy" indication in any way? Are there any bug in the display controller that the host code works around -- that you'd have to emulate (but aren't disclosed in the datasheet because they *are* bugs)? Are the visual characteristics of your display similar to those of the original display? Cursor/character blink and other attributes? Put a pair of back-to-back latches on the data bus and some steering logic and don't sweat the fine-grained timing issues.
Reply by ●September 22, 20212021-09-22
Don Y <blockedofcourse@foo.invalid> wrote: >You will likely find there are issues that you've not even considered >that the original software relies on (directly or indirectly). Of course, but in these day much is possible. https://hackaday.com/2021/09/02/pi-pico-emulates-rom-for-speedy-retro-hacking/ But it feels strange to do this. :-) Olaf
Reply by ●September 22, 20212021-09-22
On 9/21/2021 9:04 PM, olaf wrote:> Don Y <blockedofcourse@foo.invalid> wrote: > > > >You will likely find there are issues that you've not even considered > >that the original software relies on (directly or indirectly). > > Of course, but in these day much is possible. > > https://hackaday.com/2021/09/02/pi-pico-emulates-rom-for-speedy-retro-hacking/ > > But it feels strange to do this. :-)It used to feel strange to only partially decode address spaces for devices -- so, a single 8b latch would occupy 128KB of address space. <shrug> Times change. By comparison, a ROM is highly predictable. You don't, for example, expect a read of address X to take longer when it follows a read of address A vs. B. Or, when the datum returned is Q vs. R. A display controller, OTOH, is a serial machine and the time it takes to perform an action is typically related to the amount of "effort" that must be expended. It's likely that the original code took this into consideration (implicitly or explicitly). and, without delving into that code, it's hard to decide how you can reasonably expect to emulate it without unexpected consequences in the host. [E.g., the host may expect to have to wait when it does X. But, will expect X to have completed before some particular future time. If you fail to do so, there's no easy way of knowing what might "misfire".]
Reply by ●September 22, 20212021-09-22
Il 22/09/2021 04:34, Don Y ha scritto:> On 9/21/2021 3:31 AM, pozz wrote: >> I have a CPU board that interfaces to a standard 16x2 chars display, >> based on HD44780 controller. >> >> I'd like to replace this display with a full-color big TFT display, >> maybe 10", but I can't change the firmware of the CPU (because I don't >> have the source code and I can't rewrite it, because of cost and time). >> >> The simple idea is to create an interface between the current CPU >> board and the TFT display. This interface should be able to detect the >> signals from the CPU and that was directed to HD44780, interpret the >> instructions (write CGRAM/DDRAM and so on) and write the same things >> to the big TFT display. >> >> Of course, exactly the same texts will be written on TFT display (of >> course with a bigger and visible font) and this is acceptable. >> >> So the MCU on the interface should be able to read 8 data bits, RS and >> R/W signals syncronous to signal E. >> >> I'm thinking to configure E signal as an external interrupt on the >> falling edge. In the interrupt, I read data bits, R/W and RS and >> decode the instruction or data. >> >> Any suggestions? Maybe a similar code is already online. > > You will likely find there are issues that you've not even considered > that the original software relies on (directly or indirectly).Yes, it could be possible.> Can your emulation handle a *steady* stream of reads/writes?� Or, > are you expecting the host to be gentle with the display interface?The host is a 16-bits MCU from Fujitsu. I will measure timings by an oscilloscope, but I don't think it writes so fast. And I can implement a busy flag trick: after any instruction, I can simulate a busy state... if the host really use busy flag check (I suspect yes, because I saw R/W signal goes high and low often).> Keep in mind, that you have to run a fair bit of code to paint > pels onto the display from "character codes".I will send sniffed character codes by a serial line to a "smart TFT".> Does the host rely on the timing of the display's "busy" indication > in any way?Most probably the host polls for busy flag and I think I can replicate this.> Are there any bug in the display controller that > the host code works around -- that you'd have to emulate (but > aren't disclosed in the datasheet because they *are* bugs)?I wrote some code for HD44780 in the past and I don't remember of any workaround. Of course it depends on how many features of controller the host uses.> Are the visual characteristics of your display similar to those > of the original display?� Cursor/character blink and other > attributes?If I can sniff these things, I think I can replicate on the TFT.> Put a pair of back-to-back latches on the data bus and some > steering logic and don't sweat the fine-grained timing issues.Yes, it could be done in this way. And I was thinking to leave the original display connected and put the interface as a passive sniffer. In this way I don't need to replicate the exact behaviour of the controller, but only detect the stream of data and commands. Of course, in this case I have to be fast enough as the controller (I can't stop communication by busy flag).
