Add "stc89a" protocol

This protocol variant is designed for newer STC89 series chips with BSL
version 7.x.x. The new firmware uses framing
with 16-bit checksum.

This protocol variant is currently untested because I don't have any
hardware at hand.

Addresses #50, #40.

stcgal/frontend.py

@@ -26,6 +26,7 @@ import stcgal
 import serial
 from stcgal.utils import BaudType
 from stcgal.protocols import Stc89Protocol
+from stcgal.protocols import Stc89AProtocol
 from stcgal.protocols import Stc12AProtocol
 from stcgal.protocols import Stc12BProtocol
 from stcgal.protocols import Stc12Protocol
@@ -49,6 +50,8 @@ class StcGal:
         """Initialize protocol backend"""
         if opts.protocol == "stc89":
             self.protocol = Stc89Protocol(opts.port, opts.handshake, opts.baud)
+        elif opts.protocol == "stc89a":
+            self.protocol = Stc89AProtocol(opts.port, opts.handshake, opts.baud)
         elif opts.protocol == "stc12a":
             self.protocol = Stc12AProtocol(opts.port, opts.handshake, opts.baud)
         elif opts.protocol == "stc12b":
@@ -228,7 +231,7 @@ def cli():
     parser.add_argument("-a", "--autoreset", help="cycle power automatically by asserting DTR", action="store_true")
     parser.add_argument("-r", "--resetcmd",  help="shell command for board power-cycling (instead of DTR assertion)", action="store")
     parser.add_argument("-P", "--protocol", help="protocol version (default: auto)",
-                        choices=["stc89", "stc12a", "stc12b", "stc12", "stc15a", "stc15", "stc8", "usb15", "auto"], default="auto")
+                        choices=["stc89", "stc89a", "stc12a", "stc12b", "stc12", "stc15a", "stc15", "stc8", "usb15", "auto"], default="auto")
     parser.add_argument("-p", "--port", help="serial port device", default="/dev/ttyUSB0")
     parser.add_argument("-b", "--baud", help="transfer baud rate (default: 19200)", type=BaudType(), default=19200)
     parser.add_argument("-l", "--handshake", help="handshake baud rate (default: 2400)", type=BaudType(), default=2400)

stcgal/protocols.py

@@ -225,6 +225,8 @@ class StcBaseProtocol(ABC):
             mcu_name += "E" if self.status_packet[17] < 0x70 else "W"
             self.model = self.model._replace(name = mcu_name)
 
+        self.bsl_version = self.status_packet[17]
+
     def get_status_packet(self):
         """Read and decode status packet"""
 
@@ -383,6 +385,10 @@ class StcAutoProtocol(StcBaseProtocol):
         else:
             self.protocol_name = None
 
+        # STC89 devices with BSL version 7.x.x have a slightly different protocol
+        if self.protocol_name == "stc89" and self.bsl_version >= 0x70:
+            self.protocol_name = "stc89a"
+
     def initialize_options(self, status_packet):
         raise NotImplementedError
 
@@ -607,6 +613,22 @@ class Stc89Protocol(StcBaseProtocol):
         print("done")
 
 
+class Stc89AProtocol(Stc89Protocol):
+    """STC89 protocol variant with different framing"""
+
+    def extract_payload(self, packet):
+        """Verify the checksum of packet and return its payload"""
+
+        packet_csum, = struct.unpack(">H", packet[-3:-1])
+        calc_csum = sum(packet[2:-3]) & 0xffff
+        if packet_csum != calc_csum:
+            self.dump_packet(packet)
+            raise StcFramingException("packet checksum mismatch")
+
+        payload = StcBaseProtocol.extract_payload(self, packet)
+        return payload[:-3]
+
+
 class Stc12AOptionsMixIn:
     def program_options(self):
         print("Setting options: ", end="")