[x51] Programming AT89S51 with another 8051 through ISP

The reason to do this is the trouble in usage of DIY or cheap programmers to program AT89S51 on modern computers. On the LPT era there was a cheapest and simpliest STK-200 programmer, which is supporting by all programmer's software and consists only of some wires at least, some resistors for more comprehensive, logical buffers and even led for most complete versions.
As the modern computers have poor IO capabilities reduced mostly to USB only, there is a problem appears. Some USB programmers made for Atmel supports AVR but not supports AT89S, other supports both but has only windows software.
So, when I met inexpectively that I can not program AT89S but I had to do this badly I decided to use my Olduino to solve this problem! So, it appeared not too complex as ISP protocol is described in each Atmel datasheet.

Here is a code to control MCU through ISP:

Code:

;=======================================================================
ISP_SCK     equ P3.5
ISP_MISO    equ P3.4
ISP_MOSI    equ P3.3
ISP_R       equ P3.2
;-----------------------------------------------------------------------
macro ISP51 cmdl&
  iterate cmdb,cmdl
    mov A,#cmdb
    call isp51.xbyte
  end iterate
end macro
;-----------------------------------------------------------------------
isp51:
.xbyte:
    push B
    mov  B,#8
    rlc  A
.xbyte_loop:
    mov  ISP_MOSI,C
    mov  R0,#50 ; SCK off delay
    djnz R0,$
    setb ISP_SCK
    mov  R0,#50 ; SCK on  delay
    djnz R0,$
    mov  C,ISP_MISO
    clr  ISP_SCK
    rlc  A
    djnz B,.xbyte_loop
    pop  B
    ret
;-----------------------------------------------------------------------
.prog_enable:
    clr  ISP_R
    clr  ISP_SCK
    setb ISP_R
    mov  R0,#30
    call .delay_ms
    ISP51 10101100b,01010011b,0b,0b
    add  A,#-01101001b
    ret 
.release:
    setb ISP_SCK
    clr  ISP_R
    ret
.read_sig: 
    push ACC
    ISP51 00101000b
    pop  ACC
    call .xbyte
    ISP51 0,0
    ret 
.read_byte: ; R7:R6 - MCU addr
    ISP51 00100000b 
    mov  A,R7
    call .xbyte
    mov  A,R6
    call .xbyte
    ISP51 0
    ret 
.write_byte: ; R7:R6 - MCU addr
    xch  A,B
    ISP51 01000000b 
    mov  A,R7
    call .xbyte
    mov  A,R6
    call .xbyte
    xch  A,B
    call .xbyte
    mov  R0,#2
    jmp  .delay_ms
.erase:
    ISP51 10101100b,10000000b,0b,0b
    mov  R0,#100
    jmp  .delay_10ms
.writex: ; R7:R6 - MCU addr, DPTR - mem addr, R5:R4 - block length
    mov  A,R5
    orl  A,R4
    jz   .writex_done
    clr  A
    movc A,@A+DPTR
    call .write_byte
;-----------------------------------------------------------------------
    call .read_byte
    xch  A,B
    clr  A
    movc A,@A+DPTR
    clr  C
    subb A,B
    jnz  .writex_done
;-----------------------------------------------------------------------
    inc  DPTR
    mov  A,R6
    add  A,#1
    mov  R6,A
    mov  A,R7
    addc A,#0
    mov  R7,A
    clr  C
    mov  A,R4
    subb A,#1
    mov  R4,A
    mov  A,R5
    subb A,#0
    mov  R5,A
    jmp  .writex
.writex_done:
    ret        
;=======================================================================
;   delays with no timer usage
;-----------------------------------------------------------------------
.delay_10ms:
    mov   R1,#9
.delay_loop:
    call  .delay_ms
    djnz  R1,.delay_loop
;-----------------------------------------------------------------------
.delay_ms:
    call .delay_x
    call .delay_x
    call .delay_x
;-----------------------------------------------------------------------
.delay_x:
    mov  R0,#0            
    djnz R0,$ ; 256 us
    ret     
;=======================================================================

.xbyte - sends/recieves byte through ISP
.prog_enable - enters programming mode. this procedure substracts proper MCU responce, so, if success then accumulator contains zero value on return
.release - sets reset into inactive level (zero), so MCU may execute internal program without disconnection from programmer
.read_sig - reads signature byte
.read_byte - reads single byte from flash
.write_byte - writes single byte to flash
.erase - clears entire flash of MCU and lock bits
.writex - writes block into the flash byte by byte

As this code was made as quick solution, there is no subroutines for block reading, verifying flash or writing lock bits. If necessary their implementation is a bit trivial with this code.

Here is a part of code from s51uploader to write data from command memory (RAM for Olduino) into the AT89S51 flash:

Code:

    mov  DPTR,#buffer
;-----------------------------------------------------------------------
    call isp51.prog_enable
    jnz  error
    call isp51.erase
    call isp51.writex
    jnz  error
;    jmp  ok
;=======================================================================
ok:   ; led: long on & same long off
    mov  A,#100
    mov  B,#100
    jmp  finish
;-----------------------------------------------------------------------
error: ; led: short on & long off
    mov  A,#5
    mov  B,#50
;-----------------------------------------------------------------------
finish:  ; A:LED ON, B:LED OFF (1..256) x 10 ms 
;-----------------------------------------------------------------------
    call isp51.release

NOTE: in-system programmer for AT89S51 must have strong pull-up to provide reset to target MCU as 8051 boards usually have capacitor and pull-down resistor for power-on reset, which may make reset impossible as internal pull-ups of x51 MCUs are weak (~40k). To be specific, my board has 10k pull-down resistor for RESET, thus I've been using 2k pull-up resistor on RESET wire for ISP programming.