Difference between revisions of "8251 USART chip"
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=== Control Register (W) === | === Control Register (W) === | ||
The first write after internal or external Reset is the "Mode Instruction": | The first write after internal or external Reset is the "Mode Instruction": | ||
| + | Bit Expl. | ||
0-1 Baud Rate Divider (0=Sync Mode, 1=Div1, 2=Div16, 3=Div64) | 0-1 Baud Rate Divider (0=Sync Mode, 1=Div1, 2=Div16, 3=Div64) | ||
2-3 Character Length (0=5bit, 1=6bit, 2=7bit, 3=8bit) | 2-3 Character Length (0=5bit, 1=6bit, 2=7bit, 3=8bit) | ||
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3rd write - Sync Character 2 | 3rd write - Sync Character 2 | ||
All further writes to the Control Register are "Command Instructions": | All further writes to the Control Register are "Command Instructions": | ||
| + | Bit Expl. | ||
0 Transmit Enable (0=Disable, 1=Enable) | 0 Transmit Enable (0=Disable, 1=Enable) | ||
1 Data Terminal Ready (0=No, 1=Yes) (DTR Pin) | 1 Data Terminal Ready (0=No, 1=Yes) (DTR Pin) | ||
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=== Status Register (R) === | === Status Register (R) === | ||
| + | Bit Expl. | ||
0 Transmit Ready (0=Busy, 1=Ready; "DB Buffer Empty") | 0 Transmit Ready (0=Busy, 1=Ready; "DB Buffer Empty") | ||
1 Receive Ready | 1 Receive Ready | ||
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=== Data Register (R/W) === | === Data Register (R/W) === | ||
| + | Bit Expl. | ||
0-7 Data | 0-7 Data | ||
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== 8251 vs 8251A == | == 8251 vs 8251A == | ||
Revision as of 07:23, 5 August 2010
Contents
Usage in CPCs
Used by Aleste 520EX:
EEXXh Aleste 8251 USART chip (RS232/Mouse) Data (R/W) EFXXh Aleste 8251 USART chip (RS232/Mouse) Control/Status (R/W) Uses a KR580WW51A (russian clone of the 8251). Clock source comes from a 8253 chip.
Used by Cirkit serial interface (for use with the included Prestel Link modem)):
FADFh - Cirkit 8251 UART Data register FBDFh - Cirkit 8251 UART Status/Control register
TX clock source is fixed (19.2KHz), RX clock source can be selected via DTR bit (1200Hz or 19.2KHz).
Registers
Control Register (W)
The first write after internal or external Reset is the "Mode Instruction":
Bit Expl. 0-1 Baud Rate Divider (0=Sync Mode, 1=Div1, 2=Div16, 3=Div64) 2-3 Character Length (0=5bit, 1=6bit, 2=7bit, 3=8bit) 4 Parity Enable (0=Disable, 1=Enable) 5 Parity Type (0=Odd, 1=Even) In Async mode: 6-7 Stop Bits (0=Reserved, 1=1bit, 2=1.5bit, 3=2bit) In Sync mode: 6 External Sync Detect (0=Syndet is an output, 1=Syndet is an input) 7 Single Character Sync (0=Double Character, 1=Single Character)
If (only if) the above 1st write selected Sync Mode, then 2nd/3rd write are:
2nd write - Sync Character 1 3rd write - Sync Character 2
All further writes to the Control Register are "Command Instructions":
Bit Expl. 0 Transmit Enable (0=Disable, 1=Enable) 1 Data Terminal Ready (0=No, 1=Yes) (DTR Pin) 2 Receive Enable (0=Disable, 1=Enable) 3 Send Break Character (0=Normal, 1=Force TxD "low") 4 Error Reset (0=No change, 1=Reset Error Flags) 5 Request to Send (0=No, 1=Yes) (RTS Pin) 6 Internal Reset (0=No change, 1=Reset; expect new Mode Instruction) 7 Enter Hunt Mode (0=No, 1=Search Sync Characters; Sync Mode only)
Status Register (R)
Bit Expl. 0 Transmit Ready (0=Busy, 1=Ready; "DB Buffer Empty") 1 Receive Ready 2 Transmit Empty 3 Parity Error 4 Overrun Error 5 Framing Error 6 Syndet/Brkdet 7 Data Set Ready (DSR Pin)
Data Register (R/W)
Bit Expl. 0-7 Data
8251 vs 8251A
The original 8251 seems to be very rare (there's no programming info found in the internet). The 8251A is backwards compatible, but includes some additional features. The 8251A gives some details on the additional features, so one could eventually use the 8251A datasheet to reconstruct the functionality of the original 8251.
However, since the 8251 is so rare, it's rather doubtworthy that the Aleste or Circit actually uses that chips, probably they use 8251A chips.
- The Circit advert says 8251 (which is maybe meant to be an abbreviation for 8251A).
- Russian to Western part number translation tables say KR580WW51A is 8251 (probably an abbreviation for 8251A, too) (especially as the ending "A" in "KR580WW51A" implies that it is an 8251A).
Datasheets
- Media:8251A.pdf - newer 8251A datasheet (detailed info)
- Media:8251.pdf - older 8251 datasheet (pin-outs only, without programming info)
