By JPK Systems Limited
jpkyle@xtra.co.nz

1. Overview
Updated: August 24, 1998

The X-Programmer provides a simple and effective method for SPI programming of Atmel microcontrollers that support serial downloads. Table 1 below lists devices that can be programmed with the X-Programmer. So what is SPI programming and how does it work? What follows is a brief introduction to this topic allowing the reader to make a decision on whether or not it is right for their application.

The Serial Peripheral Interface (SPI for short) is a full-duplex 3-wire synchronous data transfer interface that is built into some of Atmel’s microcontrollers, more detailed information can be found on specific product data sheets. What is important to us is that it has been adapted for downloading programs and data into the device without the need to physically remove it from a circuit, unlike parallel programming where the device must be plugging into the programmer and then replaced into the target circuit. This means you can solder a device directly into the PCB right from the word go, no need for sockets and no need to even power the target circuit off to program it.
 
In order to program the target device it must be powered and have a crystal as is needed to make it work anyway, the reset line is then pulled into the active state to halt any program execution. A special command sequence is sent down to the SPI interface which enables serial programming and then reading or writing can take place via the 3-wire SPI. Once programming is complete the reset line is released and program execution starts as it would upon power up.
 
There are several simple, very low cost, DIY SPI programmers available in the public domain and they do work adequately for some jobs. Typically the target device must be removed from circuit and then hooked up to the parallel printer port of a PC and special bit bashing software run to program it. Accurate software timing on a PC running Microsoft Windows can be difficult to achieve and there are some doubts as to whether or not all timing requirements are actually met with these programmers. Also they are host dependent, that is they can only be used on an IBM compatible PC. The X-Programmer achieves host independence by using an RS-232 serial port for communications and no special software, all that is required operate an X-Programmer is a terminal program that supports X-Modem-CRC file transfers.
 
Optical isolation is provided by the X-Programmer so there is never any concern about destroying the serial port on that $3k laptop and both host and target connections can be hot-plugged, that is they can be connected whilst power is applied to the target. Figure 1 shows the X-Programmer unplugged from both host and target.
  Figure 1. X-Programmer  

Device Part Number	FLASH size	EEPROM size
AT89S8252	8kB	2kB
AT89S53	12kB	N/A
AT90S1200	1kB	64Bytes
AT90S2313	2kB	128Bytes
AT90S2323	2kB	128Bytes
AT90S2343 (with ext. clock)	2kB	128Bytes
AT90S4414	4kB	256Bytes
AT90S4434	4kB	256Bytes
AT90S8515	8kB	512Bytes
AT90S8535	8kB	512Bytes

Table 1. Devices supported by the X-Programmer

 As can be seen in figure 1 the X-Programmer is an in-line box that has a DB-9 socket to connect to a PC serial port and a DB-9 plug to connect to your target circuit. In some cases it may be desirable to make an adapter cable to go from an existing SPI connector on a target board to the X-Programmer’s DB-9 plug. It is intended that the designer incorporate a DB-9 socket on future board designs to allow direct connection of an X-Programmer, note that the DB-9 could also be mounted on the enclosure so there is no need to open up the product in order to re-program the microcontroller. Figure 2 shows a typical application of SPI programming, here an X-Programmer is being used to download new laser graphic pictures into a laser graphics console. Note the use of an Amiga computer to run the X-Programmer, no special software – just a terminal program, and the laser graphics console is switched on.
 

Figure 2. Programming laser graphics images into a console.

Another application is shown in figure 3, a simple LED test board for the AT90S1200 running the Atmel LEDTEST.ASM program that is distributed with AVR Assembler (download available from http://www.atmel.com). Here a palmtop computer is being used to download a binary file to the target device, this is very useful for on-site software updates as there is no need to take a bulky laptop. Once again, host independent and just a terminal program required. The X-Programmer is totally transparent after programming has been completed, SPI port pins can be used for other purposes (such as LED flashing) once released by the programmer.
 

Figure 3. LEDTEST program running on an AVR, target programmed with palmtop.

Power for the X-Programmer is derived from the target board and also from the serial port of the host computer (for isolated RS-232), there is no need to plug in a power supply of any kind. There are no parts or leads to loose as everything needed for programming devices is attached to the X-Programmer itself. The following chapters describe in detail how to connect and operate the X-Programmer.

 2. SPI Connections

Serial programming only requires the following SPI connections to be made; MISO, MOSI, and SCK. Note that SS need not be connected as the device automatically assumes slave SPI status when being programmed. A total of 6 connections must be made to the X-Programmer in order to successfully perform SPI programming, table 2 shows how to connect the various devices supported.
 

DB-9 Male from X-Programmer	AT89S8252 and AT89S53 (PDIP)	AT90S1200 and AT90S2313 (PDIP)	AT90S2323 and AT90S2343 (PDIP)	AT90S4414, AT90S4434, AT90S8515, and AT90S8535 (PDIP)
Pin 1 (MOSI)	Pin 6 (P1.5/MOSI)	Pin 17 (PB5/MOSI)	Pin 5 (PB0/MOSI)	Pin 6 (PB5/MOSI)
Pin 2 (MISO)	Pin 7 (P1.6/MISO)	Pin 18 (PB6/MISO)	Pin 6 (PB1/MISO)	Pin 7 (PB6/MISO)
Pin 3 (SCK)	Pin 8 (P1.7/SCK)	Pin 19 (PB7/SCK)	Pin 7 (PB2/SCK)	Pin 8 (PB7/SCK)
Pin 4 (RST)	Pin 9 (RST)	N/C	N/C	N/C
Pin 5 (GND)	Pin 20 (GND)	Pin 10 (GND)	Pin 5 (GND)	Pin 20 (GND)
Pin 6 (NOT RESET)	N/C	Pin 1 (NOT RESET)	Pin 1 (NOT RESET)	Pin 9 (NOT RESET)
Pin 7 (+5V)	Pin 40 (VCC)	Pin 20 (VCC)	Pin 8 (VCC)	Pin 40 (VCC)

 

Table 2. X-Programmer connections to a target device

 Note that there are 2 reset lines from the X-Programmer, this is due to differences in the way reset is handled by 89Sxxxx devices and the newer AVR devices. Power supply lines for the X-Programmer need not be connected directly to the target device’s power pins, this is just to make it clear exactly what to supply it. Power supply requirements are very low, typically 16mA at 5V, so there is no need to make significant allowances for times when the programmer is connected. It is intended that any new board designs include a DB-9 connector to allow direct connection of an X-Programmer, although any other connector can be used if desired and an adapter lead made up to connect to the programmer.

Special Note: Do not use capacitors larger than 1uF in your reset circuit, doing so increases the time-constant to a point that the X-Programmer will not accept.

 There are general purpose microcontroller boards available from several vendors, one of them is Dontronics (http://www.dontronics.com) who make the DT103 and DT104 boards. These boards have an 8 way SIL header for SPI connections which can be used to connect to an X-Programmer, table 3 has details on pin connections.
 

DB-9 Male from X-Programmer	DT103 with AT89S8252 or AT89S52 fitted	DT103 or DT104 with AT90Sxxxx AVR fitted
Pin 1 (MOSI)	Pin 4 (D5/MOSI)	Pin 4 (D5/MOSI)
Pin 2 (MISO)	Pin 5 (D6/MISO)	Pin 5 (D6/MISO)
Pin 3 (SCK)	Pin 6 (D7/SCK)	Pin 6 (D7/SCK)
Pin 4 (RST)	Pin 8 (RST)	N/C
Pin 5 (GND)	Pin 1 (GND)	Pin 1 (GND)
Pin 6 (NOT RESET)	N/C	Pin 8 (NOT RESET)
Pin 7 (+5V)	Pin 2 (+5V)	Pin 2 (+5V)

  Table 3. X-Programmer connections to a DT103 or DT104

 Once the X-Programmer is connected to a target device the transmit LED will immediately come on as serial data is being sent to the host computer. At this stage the programmer is completely transparent to your target and can do nothing unless instructed by keystrokes from a terminal program.

In order to preserve host independence there is no special software supplied to communicate with the X-Programmer, all user interfacing is done through a terminal program that supports X-Modem-CRC file transfers (note that X-Modem(1k) is not suitable, nor is X-Modem-Checksum). Nearly every computer on the planet has a terminal program available for it and most modern operating systems come with one, i.e. Windows 95 has Hyperterminal and Windows 3.1 also has a terminal program.

Your host’s RS-232 communication port must be set to the following;

• RS-232 port that the X-Programmer is connected to.
• 2400baud 8 data bits No parity 2 stop bits
• Hardware handshaking (RTS/CTS and DTR/DSR), no DCD and no XON/XOFF
• Local Echo ON
• Translate CR to CR+LF should be OFF
 
Although other settings may appear to work (such as 2400 8N1) there will be timing errors that can not be tolerated by the X-Programmer.

Once an X-Programmer is correctly wired to a target board it can be connected to the host computer and the main menu will be displayed in the terminal program. Note that you may have to request the menu again by pressing any key other than A to F (these are menu options) unless the target is powered up after the terminal program is run. If no response is seen on the screen then there is likely to be an error in your serial communications setup.

 Power for the isolated RS-232 interface is provided by the host’s RS-232 port in a similar manner to a serial port mouse. Although hardware handshaking is not actually used it must be switched on to ensure that the RTS and DTR lines stay in the high state so power can be drawn from them, RTS is linked to CTS and DTR is linked to DSR inside the X-Programmer. In some circumstances, particularly with handheld computers, there is insufficient power for the isolated interface although it has been designed and tested with Psion palmtops. If this is the case then a break-in lead can be made up that connects to the host using Txd and Rxd only while power (+5Vdc to +9Vdc) is externally injected into the X-Programmer’s RTS and/or DSR lines. Contact JPK Systems Limited if you require details on how this is done.

Upon successful connection to a target and host your terminal screen should look like figure 4 below.

 

Atmel 89S/90S SPI Programmer

 
Select Device:

(A)89S8252

(B)89S53

(C)90S1200

(D)90S2313

(E)90S4414

(F)90S8515

 

Figure 4. Device selection menu

 If this does not appear then press the SPACE key and it will be re-sent, you can verify your data is reaching the X-Programmer by observing the Rxd LED. Powering the target (and X-Programmer - same power supply) off then on again will also cause the menu to be re-sent, if the Txd LED does not come on then there is something wrong with the connection from the target to the programmer.

Select the device you want to program by sending the corresponding letter (press the key) in upper case only, then the programming menu will be sent in response as shown in figure 5.
 

Select Option:

(1)Erase

(2)Rd FLASH

(3)Wr FLASH

(4)Lock bits

(5)Rd EEPROM

(6)Wr EEPROM

 

Figure 5. Programming options menu Pressing the ‘1’ key will erase your target device and then report back DONE! to indicate that the operation is complete, the programming options menu is also re-sent for the next operation.

By keying ‘3’ the X-Programmer expects a file to be sent to it, this is indicated on screen. Select the send file option in your terminal program which will then prompt for a protocol and filename, be sure to select X-Modem-CRC (sometimes abbreviated to just X-Modem) and also ensure you have a binary file that contains your program starting from address 0000H. The terminal program will indicate file transfer progress, as each byte is received it is programmed into the target and verified in real time. Once completed the response will be either DONE! or * FAIL * in the case that there was a verify error. Any errors in the file transfer will be detected by the CRC-16 that X-Modem-CRC transfers use. If your assembler produces Intel hex files then you will need to convert them to binary before sending them to the programmer, the binary file is exactly what is programmed into the target.

Keying ‘2’ will tell the X-Programmer to read back the target device’s entire FLASH contents by X-Modem-CRC which requires your terminal program to receive a file. X-Modem does not specify filenames automatically so when you select this it will prompt for one, and then start receiving the file. Once the file transfer is complete the X-Programmer will respond with DONE!.

Options 5 and 6 behave in exactly the same manner as 2 and 3 except operations are performed on the EEPROM instead of the FLASH, this includes the auto-verify feature. Note that the EEPROM can be written over and does not need a chip erase, unlike the FLASH memory. You can use this feature for downloading new configuration information for example without having to re-load the main program.

Keying ‘4’ automatically programs all lock bits in the currently selected device.

If you are having difficulty coming to grips with X-Modem file transfers then it is suggested that you experiment with 2 computers hooked together with a null modem cable and both running terminal programs. It should be possible to type on one computer and see it on the other one, then try sending a file. Don’t forget that the receiving end must be told to expect an X-Modem file.

At any time the menu can be repeated by keying anything other than 1 to 6, this can be useful as a quick check if everything is still connected correctly.

Because the X-Programmer is transparent as soon as an operation has been completed there is no need to disconnect it from the target unless you have completed the job. During software development this means there is no need to even switch off your target as the X-Programmer has full control over the reset line, true In Circuit Programming. Simply erase the device then program it and repeat whenever necessary. No more chip swapping!

Programming speed is set by the serial communications byte rate (11 bits at 1/2400s per bit = 4.583ms) which is 14% longer than the maximum byte programming time of 4ms specified by Atmel in their AVR device data sheets. There is no way of making it go faster as byte programming is internally timed in the target device and is independent of external crystal frequency. The X-Programmer imposes minimum frequency limits on the external crystal, 2MHz for AT90Sxxxx and 8MHz for AT89Sxxxx devices. There are no upper frequency limits apart from those dictated by the target device itself. These limitations are associated with SPI timing which must be strictly adhered to for successful programming, it is possible to halve these limits by slowing down to 1200baud and changing the X-Programmer's internal crystal from 22.1184MHz to 11.0592MHz but is not recommended. Note that the AT90S2343 device is supported only when an external clock source of 2MHz or greater is provided on pin 2, the internal oscillator frequency is around 1MHz which is too slow for the X-Programmer.

Future devices are automatically supported provided they follow the same SPI programming instruction format as the current devices (quite likely), although new devices will of course be added to the device selection menu as they become available.

Due to the 128 byte block nature of the X-Modem protocol there are sometimes minor anomalies in what is written after the actual file has been sent, for example if the program is only 100 bytes long then 28 extra bytes must be added to fill the block. Usually these are 1AH characters and do not have any effect as they will only appear after the real data has been sent, just ignore them.

Enjoy the benefits of in-circuit programming!

JPK Systems Limited
November 15, 1997
Updated: August 24, 1998
 

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